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Geology of the Devizes district. Sheet description for the British Geological Survey 1:50 000 Series Sheet 282 (England and Wales)

By 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

Bibliographical 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. 2011. Geology of Devizes district. Sheet description of the British Geological Survey, 1:50 000 Series Sheet 282 (England and Wales).

Geology of Devizes district. Sheet description of the British Geological Survey, 1:50 000 Series Sheet 282 (England and Wales)

Authors: 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

Keyworth, Nottingham: British Geological Survey. ISBN 978 0 85272 662 4. © NERC 2011. All rights reserved

The National Grid and other Ordnance Survey data are used with the permission of the Controller of Her Majesty’s Stationery Office. Licence No: 1000017897/2011. Maps and diagrams in this book use topography based on Ordnance Survey mapping.

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(Front cover) Front cover Caen Hill Locks on the Kennet and Avon Canal, Wiltshire. View looking east up the Upper Greensand scarp from the lower basin. (P698535).

(Back cover)

Geology of the Devizes district—summary

This Sheet Description provides a summary of the geology for the area of 1:50 000 Sheet 282 Devizes. The Devizes district extends over approximately 600 km² of north-east Wiltshire, covering much of Salisbury Plain in the south and most of the Vale of Pewsey in the north.

Jurassic, Cretaceous and Palaeogene strata crop out at surface and Quaternary deposits include alluvium, peat, river terraces and head. A full account of the stratigraphy is given in this report, based on recent mapping and also drawing from an extensive archive and previous publications.

Concealed strata that have been proved in deep boreholes are also included in the description. The Oxford Clay Formation and the Corallian Group, of Jurassic age, crop out in the extreme north-west of the district, beyond a major fault. The uppermost part of the Jurassic; the Kimmeridge Clay formation, Portland and Purbeck groups, crop out in the north-west near Devizes. However, Cretaceous rocks underlie most of the district: the Weald Clay, Lower Greensand, Gault and Upper Greensand formations of the Lower Cretaceous and the Grey Chalk and White Chalk subgroups of the Upper Cretaceous. Palaeogene strata are rare, preserved only as a single isolated outlier capping Sidbury Hill, near Tidworth.

The Quaternary deposits are described in relation to their mode of origin and they include the residual deposits, fluvial and organic deposits, and artificial ground. A section is devoted to applied geological issues such as geotechnical factors that should be taken into consideration in any land development, for example, landsliding has affected the Upper Greensand escarpment in the north-west of the district. The chalk is a major aquifer in the district and an account of its hydrogeology is given. Other resources described include sand and gravel, building stones and brick clays. The Information Sources lists all the BGS publications relevant to the district and gives information on how to gain access to BGS collections and databases, including borehole records, geophysical, geochemical and geotechnical data.

(Table 1) Lithostratigraphy of the Chalk Group.

(Table 2) Geological succession and bed thicknesses of the Devizes district.

Acknowledgements

The following past and present colleagues contributed to original survey, the accompanying 1:50 000 scale map and/or parts of this Sheet Description: L B Bateson, K A Booth, C R Bristow, D J Evans, A R Farrant, P M Hopson, R J Marks, A J Newell, N J Smith, I P Wilkinson and M A Woods.

The new 1:10 000 scale survey, commenced in 1999, was completed in 2005 by L Bateson, K A Booth, A R Farrant, P M Hopson, R J Marks, C R Bristow and A J Newell, with support from N J Smith, D J Evans (deep geology), I P Wilkinson (microbiostratigraphy) and M A Woods (macrobiostratigraphy).

Professor R N Mortimore of Brighton University is especially acknowledged for his enthusiastic contribution to the study of the Chalk in the Devizes and southern England area.

The landowners, farm managers and land agents of the district area are thanked for their co-operation in providing access to private land for the purposes of this survey. We acknowledge the Environment Agency (South-west Region) for their support of the field survey in the south-east of the district (Bourne/Nine Mile River Catchments). Landowners, tenants and quarry companies are thanked for permitting access to their lands.

Special mention must be made in acknowledging the contribution of the Ministry of Defence (MoD) in permitting access throughout the Salisbury Plain Training Area (SPTA). Considerable assistance and guidance was forthcoming from Mr Chris Waldren of Defence Estates, Lt. Col. Nigel de Foubert (Public liaison) and the team in Range Control (West Down Camp) without whose help the survey would could not have been undertaken.

This document has drawn heavily, and quoted extensively from the individual 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. Additional notes and observations are included from the authors stated on the title page.

Notes

The area covered by Sheet 282 Devizes is referred to as the district. National Grid References are given in the form [SU 1234 1234] or [ST 1234 1234]. Symbols in brackets, for example (SCk) refer to the symbols used on the 1:50 000 scale map. National Grid References quoted in this report, given in the form [SU 1234 5678], are all within Grid Zones SU and ST, unless otherwise stated. All the boreholes mentioned in the text are identified by a BGS Borehole Registration Number in the form (SU13SE/23). Details of described sections and boreholes a generally given in order of occurrence on the ground starting from north to south and west to east of the constituent 1:10 000 scale maps. Boreholes are indicated thus (SU12NW/10), this being the tenth registered borehole within the SU12NW 1:10 000 scale OS sheet. The number given with plate captions is the registration number in the Imagebase photographic collection of the British Geological Survey.

Fossil specimen number designations are provided according to the registration code of the collector e.g. WMD 1234 and registered in the BGS archives.

Chapter 1 Introduction

Topographical setting

The Devizes Sheet district extends over about 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 (Hampshire). From Upavon, where these minor streams converge, the river commonly 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 into eastern and central/western portions. In the east, the Nine Mile River and River Bourne dissect the Plain. The River Bourne approximates to the eastern extremity of the Salisbury Plain Training Area (SPTA) controlled by the MoD. To the centre and west of the plain the area is drained by the River Till and Chitterne Brook, both southward flowing tributaries of the River Wylye that flows west to east on the Salisbury district to the south.

In the north-west of the Devizes district numerous small generally westward flowing streams form the headwaters of the Gloucestershire River Avon.

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 formations outcroping 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, the scarp is made up of the Upper Greensand and Gault formations and has been subject to landsliding. Beneath this scarp 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.

History of research

The district was mapped by W T Aveline and published on ‘Old Series’ One-inch Geological Sheet 14 as Sheet 14 [Old series] at 1:63 360 (one in to one mile) in 1857. The Devizes district was resurveyed on the new series six-inch scale at 1:10 000 (six inches to one mile) by F J Bennett and A J Jukes-Browne between 1885 and 1898, and published at the one-inch scale 1:63 360 as Sheet 282 [New Series], with superficial deposits, in 1899. The sheet was revised in the west at the 1:10 10 560 scale by W Edwards (1924) and G A Kellaway (1943) and reprinted with minor amendments in 1959.

The district was substantially resurveyed in 2002 and 2005 at the 1:10 000 scale. The Devizes map incorporates surveying at the same scale completed as part of the surrounding Wincanton (Sheet 297), Frome (Sheet 281), Winchester (Sheet 299) and Salisbury (Sheet 298) districts in, respectively, 1994, 1997, 2001 and 2004. In addition, 1:10 000 scale mapping covering the district to the east of the River Avon northwards from Salisbury to the Vale of Pewsey, was completed in 1999 during a contract partly funded by the Environment Agency. This survey was, in part, incorporated into the recently published Winchester and Salisbury sheet areas and the forthcoming Andover sheet (283).

The new 1:10 000 scale survey, commenced in 1999, was completed in 2005 by L Bateson, K A Booth, A R Farrant, P Hopson, R J Marks and A J Newell, with support from D J Evans (deep geology) I P Wilkinson (microbiostratigraphy) and M A Woods (macrobiostratigraphy).

The BGS memoir of this district to accompany the New Series Geological Sheet (Jukes-Browne, 1905) is out of print.

A J Jukes-Browne and W Hill recorded some observations on the Lower and Upper Cretaceous rocks of the district in the memoirs on The Cretaceous Rocks of Britain (1900, 1903, 1904). These are now out of print. Brydone’s (1912; 1942) descriptions of the Chalk of Hampshire include biostratigraphical details for many individual localities in Hampshire, including some within the district. Correlation of the Lower Chalk (now the Grey Chalk Subgroup) of south-east England was expounded by Kennedy (1969), whilst the petrology, environment of deposition, and diagenesis of the Chalk Group were published by Hancock (1975). Bromley and Gale (1982) considered the lithostratigraphy of the Chalk Rock at many sites across southern England; a number of these sites fall within the district. Robinson (1986) described in detail the stratigraphy of the Chalk of the North Downs and Mortimore (1986; 1987) did the same for the Middle Chalk and Upper Chalk (now the White Chalk Subgroup) of Sussex. Bristow et al. (1995) proposed a new scheme of lithostratigraphical nomenclature for the Chalk Group in the Shaftesbury district of Dorset. The units recognised in Dorset and in Sussex can be traced into Hampshire and Wiltshire (Bristow et al., 1997), and form the basis of the description of the Chalk in this report. The lithostratigraphy of the Chalk Group was the subject of a Geological Society Stratigraphical Commission review in 1999 that broadly endorsed the units put forward by Bristow et al. (1997), and recommended adoption of the scheme shown in (Table 1) (Rawson et al., 2001). The full lithostratigraphical framework for the Chalk Group is available as a free download from the BGS website (Hopson, 2005) [www.bgs.ac.uk] (Hopson, 2005). The Palaeogene strata of the Hampshire Basin are discussed by Edwards and Freshney (1987).

There are a number of papers that describe the local geology of the Devizes district and its immediate hinterland. Wimbledon (1976) discussed the ‘Portland Beds’ of Wiltshire and various Jurassic exposures in the district formed part of Woodward’s treatise (1895) on The Jurassic Rocks of Britain.

Geological sequence

The bedrock and superficial deposits mapped during the survey are listed in (Table 2).

Chapter 2 Structure

Structurally, the Devizes district lies along the north-western margin of the Channel–Wessex Basin (Figure 2), a major structural element of southern England. The formation of the Channel–Wessex Basin, comprising a system of post-Variscan extensional sedimentary basins and ‘highs’ that covered much of southern England south of the London Platform and Mendip Hills during Permian to Mesozoic times, influenced the deposition of strata from Permo-Triassic times through to the Palaeogene. Deposition within this basin reflects expansion and contraction in response to major earth movements associated with the opening of the Atlantic and English Channel and later contraction during Alpine compression. At greater depths are Palaeozoic strata that were deformed during the Variscan Orogeny, a period of tectonic upheaval and 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 sequence.

The rocks of the ‘Variscan Basement’ are weakly metamorphosed shales, sandstones and limestones of Ordovician to Carboniferous age. Several major southward-dipping low-angle thrust zones and steeper north-west-orientated wrench faults, thought to have originated during the Variscan Orogeny, have been tentatively identified in the basement rocks. The general sediment palaeoflow direction in the region was from the south or south-east, with possible local variation, and the thrusts controlled the development of later important extensional structures in their hanging-wall blocks (Chadwick et al., 1983; Chadwick, 1993; Chadwick et al., 2005). The present district lies close to the northern limits of the concealed thrusts defining the Variscan Front that lie beneath and to the north of the district (Busby and Smith, 2001, fig.32). The Variscan Front thrust formed in latest Carboniferous times, postdating deposition of the late upper Westphalian strata and probably underlies the Mesozoic cover to the north of the district (e.g. Chadwick, 1986).

Subsidence associated with the onset of crustal extension began to affect southern England from Permian times. This process was facilitated by the reactivation of the low low-angle Variscan thrusts that controlled the location and development of the wider Channel–Wessex Basin and smaller fault-bounded grabens or sedimentary basins and Tertiary structural elements within the area. Large syn-depositional normal faults were initiated, generally dipping and downthrowing to the south, and with these, less important structures developed within the Channel–Wessex Basin dividing the region into a series of structural provinces (Chadwick, 1986) such as the Weald, Dorset, Pewsey, Mere and Portland–Wight sub-basins with the Hampshire–Dieppe High (in part known as the Cranborne–Fordingbridge High), forming a major intrabasinal fault-bounded high. The basin-bounding normal faults suffered a number of phases of reactivation throughout Jurassic and Cretaceous times, with the final ‘Alpine’ reactivation being in a reverse sense during basin inversion. Periods of significant syn-depositional extensional movement on major basin-bounding faults resulted in thicker Triassic, Jurassic and Early Cretaceous (‘Wealden Group’) sequences being laid down on the downthrown (hanging-wall) fault blocks. During periods of relative tectonic quiescence, rates of subsidence and sedimentation increased evenly towards the depocentre of the Weald Basin towards the east of this district.

Permo-Triassic sedimentation across the Channel–Wessex Basin was controlled by the formation of a graben during the onset of crustal extension, within which variable thicknesses of continental red beds were deposited. Coarse-grained basal Permian deposits (conglomerates and breccias) are overlain by sandstones which in turn pass upwards into siltstones and mudstones of late Permian age. These are succeeded by coarsely arenaceous Lower Triassic strata of the Sherwood Sandstone Group and mudstones (with halite and anhydrite beds to the south) of the Sidmouth Mudstone Formation (Mercia Mudstone Group). The sea began to flood the Wessex Basin in Rhaetian (Late Triassic) times, depositing the Penarth Group. The area of deposition increased gradually throughout the Jurassic, although minor periods of erosion occurred, mainly at the basin margins. By Late Oxfordian to Kimmeridgian times, the London Platform was probably entirely submerged. Towards the end of Kimmeridgian times, the London Platform began to re-emerge, probably due to a combination of global sea-level fall and a reduction in the rate of tectonic subsidence. This resulted in erosion on the margins of the Wessex Basin and the beginning of the development of the Late Cimmerian unconformity. This marine regression continued into Cretaceous times, the environment of deposition changing from offshore marine (Kimmeridge Clay Formation) through shallow marine (Portland Group), to brackish water and deposition of evaporites (Purbeck Group), and fluviatile sediments (‘Wealden Group’). The final period of extensional fault movement, marked by normal faulting, resulted in the accumulation of thick sequences of ‘Wealden Group’ sediments in the main fault-bounded troughs in the Wessex Basin, whereas to the west, beneath this district, and on the intervening exposed highs, erosion occurred.

A period of regional subsidence followed (associated with sporadic tectonic movements along pre-existing fault lines) and, combined with eustatic rise in sea level, led to a renewed marine transgression of the Wessex Basin. The ensuing deposition of the Lower Greensand Group, Gault Formation, Upper Greensand Formation, and eventually the Chalk Group covered all the surrounding high areas, including the London Platform. This continued into Late Cretaceous times, when minor episodes of uplift affected the Wessex–Channel Basin and, combined with 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-Cenozoic unconformity. The Late Cretaceous uplifting was a precursor to the main Alpine Orogeny, caused by the convergence of the African and European plates and closure of the Tethys Ocean in the early to mid Cenozoic (Miocene). Later, deposition in Palaeocene times was followed by the onset of the compressive tectonic regime during mid Neogene ‘Alpine’ earth movements, during which the main basin inversion took place. In areas like southern England, which were peripheral to the mountain chains created by this continental collision, the effect of these compressive tectonic stresses caused the inversion of the Mesozoic sedimentary basins and their controlling faults. Consequently, Mesozoic basin-fills now occur beneath regional upwarps and, in contrast, former structural highs now underlie the Cenozoic basins (Chadwick, 1986; Hamblin et al., 1992). Superimposed upon and delimiting these general regional upwarps are more or less linear-trending, en échelon inversion structures (typically monoclinal or periclinal flexures) formed above the main basin-controlling normal faults that were reactivated and suffered reversal of movement during Cenozoic compression (see below). Uplift in general is estimated at about 1500 m (Simpson et al., 1989) for both the former Weald and Channel depocentres. Subsequently, erosion during the Neogene and Quaternary has unroofed these inverted basins and removed most of the Palaeogene strata. Erosion occurred first in subtropical, and later in periglacial environments associated with considerable sea-level variation eventually giving rise to the present-day landscape.

In detail, the Devizes sheet district lies between two complex folded and faulted structural elements in the north of the Channel–Wessex Basin. It straddles the northern margins of the Pewsey Sub-basin, lying above the Variscan Front thrusts that controlled the development of the Pewsey Fault and Pewsey Basin and ultimately, the Pewsey Anticline inversion structure during Alpine compression. The latter asymmetric east–west structure runs through the Vale of Pewsey within the north of the Devizes district (Chadwick, 1986). To the south of Salisbury, the similarly asymmetric Wardour anticlinal structure, which manifests itself partly as the complex Mere Fault (see Chadwick et al., 2005), runs east–west through the Vale of Wardour, passing to the east into the offset Dean Hill Anticline.

The geological resurvey of this district has revealed the presence of a number of relatively small synclines and anticlines that affect the Chalk outcrop and the distribution of the Lower Cretaceous and youngest Jurassic strata in the Vale of Pewsey. Regionally, 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 shallows. A broad low amplitude east–west-oriented syncline extends just south of the district, through Cholderton and plunges east towards Thruxton. To the west, this broad syncline splits into two, one branch extending northwards through Bulford Camp, on the south-eastern part of the Devizes district, the other, the Amesbury Syncline, lies just north of Boscombe Down Airfield in the Salisbury district.

Faulting

There are a number of recognisable, mappable faults at surface in the area, and most of those occur within the Jurassic and Lower Cretaceous strata in the Vale of Pewsey.

The main structure in the district is the zone of arcuate east–west trending faults forming the Pewsey Fault Zone, which seismic reflection data illustrate represents a complex series of en échelon, down-south normal faults (Chadwick, 1986, 1993). These structures formed the boundary between the London Platform (to the north) and the Pewsey Basin, the latter having formed over the northerly dipping hanging-wall block to the Pewsey Fault Zone and across which Variscan basement is stepped down so that the Permian–Mesozoic sequences thicken into the Pewsey Basin. Southwards across the Pewsey Basin, sequences thin onto the Bruton–Norton Ferris High, which forms the footwall block to the southerly dipping and downthrowing Mere Fault that controlled the development and evolution of the Mere Basin to the south (Barton et al., 1998; Chadwick et al., 2005).

Post-Cretaceous reverse movement on the major fault zones during basin contraction led to reversal of movement that, in part, cancels, and at shallower levels, reverses, the earlier normal displacement. A null point is recognised at depth where the displacement is essentially zero. This point lies generally within the lowermost Jurassic and Permo-Triassic strata over much of the length of the fault but may well be considerably higher stratigraphically in some parts.

Medium-scale faulting is commonly seen where the Cretaceous strata are thin due to erosion. These faults probably reflect Early Cretaceous reactivation of deep-seated normal faulting and perhaps attest to the continued expansion of the Wessex Basin at this time. The more significant faults are named where they can be detected, but their detection in the higher formations of the Chalk is commonly not possible due either to the similarity of the lithologies adjacent to the fault, or a simple reduction in throw or a change in the ‘style’ (the different way in which faults propagate through differing lithologies) of the faulting.

Many smaller faults with throws of less than 1 m were seen in several of the disused pits, but could not be traced outside the pits. 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. On seismic profiles, faults that exist within the harder successions beneath the Chalk apparently tend to become attenuated as they propagate upwards. Instead of a discrete fault, displacement can be dispersed into zones of numerous relatively minor faults. Such zones could be several tens of metres wide, and can themselves die out upwards, passing up into anticlines or synclines.

Tectonic activity during deposition can affect Chalk lithology on a basin-wide scale. There is growing evidence that tectonic and eustatic movement occurred in phases throughout the Late Cretaceous (Mortimore and Pomerol, 1987; Mortimore and Pomerol, 1991; Mortimore et al., 1998, Evans and Hopson, 2000). Four major tectonic phases (demonstrated in Germany and the eastern Anglo–Paris Basin) caused local channelling and slumping, and the local formation of hardgrounds and phosphatic chalks, as well as variations in marl development throughout southern England. Some characteristics of the Chalk in the present area may be a result of these continued movements, for example the presence of a bed of very hard chalk near the top of the Seaford Chalk and the thin marl seams in the New Pit and Newhaven Chalk. In the Salisbury district to the south, phosphatic chalks have been seen associated with a channel cut into the Seaford Chalk, but the attenuation of the Tarrant and Spetisbury Chalk members within the Culver Chalk Formation may also be due to channelling or thinning northwards towards the London Platform.

Folding

Across the north of the district a number of small en échelon northerly verging anticlinal structures are developed, including the Pewsey Anticline. These are recognisable at surface in the area affecting Jurassic and Lower Cretaceous strata. They represent relatively small inversion structures along the southerly dipping and downthrowing faults that define the southern margin of the London Platform.

Details

In the Devizes and Rowde (Seend) area the anticlinal structure of the Vale of Pewsey is subdued and mapping suggests a number of minor synclinal flexures that help preserve the Chalk outliers. An example of this is the gentle synclinal feature trending west-south-west to east-north-east that preserves the Chalk of Etchilhampton Hill and a similar sub-parallel syncline that preserves the Zig Zag Chalk on the hill south of Coate Grove House. This situation is complicated by faulting which is more prevalent than the old, 1959, map suggests. In general the dip of the strata beneath the Chalk escarpment is gently towards the north whilst to the south of the scarp the dip is gentle and southerly. These structures may be a result of flexures associated with a similarly trending fault that separates the two hills. This fault throws strata by between 2 and 5 m to the south. A third shallow syncline, this time truncated to the north-east by a south-westward- throwing fault, preserves the very basal beds of the Chalk in and around Roundway Park.

There is a major set of four faults that cross-cut the Chalk scarp between Roundway and Oliver’s Castle to the north-west that have been identified on the basis of the disruption of the Upper Greensand/West Melbury Chalk boundary and the displacement of the Melbourn Rock at the scarp top. All trend between north and north-east and have a throw of between 5 and 10 m. Generally the throw is to the east or south-east with the exception of the southernmost of a pair that form the coombe south of Oliver’s Castle. This throws towards the north-west. The most easterly fault trending north–south forms the very steep-sided and narrow coombe to the west of Home Covert [SU 0085 6320] and cross cuts a fifth fault that trends north-west–south-east and limits the apparent thickness of the Upper Greensand along the foot of the chalk escarpment. This fifth fault is truncated by the fault south of Oliver’s Castle.

Immediately to the north-west of this district, near Martinslade, (sheet ST96SE) the general dip is to the south. A large fault (or possibly a pair of faults) traverses the district from the vicinity of Sells Green [ST 953 620] towards Wick Farm [ST 9705 6293] where it apparently becomes hidden beneath the Lower Greensand unconformity. Its exact orientation is uncertain. The fault(s) juxtapose the Kimmeridge Clay and Oxford Clay formations around Sells Green demonstrating that up to 120 m of throw to the south-east may be possible along this disruption. Deep sections confirm the presence of this major structure at depth, and it is probably one of the bounding structures associated with the Pewsey Anticline.

A shallow syncline brings the Seaford Chalk Formation to the base of the Berril valley, in its middle course between [ST 9930 4675] and [ST 9995 4535]. This syncline trends east-north-east across the area. The general dip of the Seaford Chalk Formation on the northern limb of this syncline is about 3° to the south-south-east.

In the Chitterne area the outcrop demonstrates a low dip towards the south-east, but this is disrupted by three faults all trending roughly north-west–south-east and throwing to the north-east. The two faults in the east are followed by minor valleys and may well be the same structure although they could not be traced through the village of Chitterne in the bottom of the valley. The fault to the west cuts across an interfluve separating the Knook valley from the tributary valley of the Chitterne Brook.

Within the Downs of Westdown Artillary Range, south of Urchfont, the beds dip gently to the south-south-east at an angle of little more than 2° but towards the north and east the beds appear almost horizontal and the unit boundaries follow the contours for considerable distances.

Chapter 3 Concealed strata

There are five deep boreholes, within or close-by the district, that penetrate below the base of the exposed strata, and a further number that prove extended successions of the Chalk (Figure 3).

The five deepest wells at Devizes (ST95NE) [ST 96026 56987], Urchfont (SU05NW) [SU 40444 15816], Netherhampton (SU12NW/6) [SU 11315 28766], Shrewton (SU04SW/1) [SU 03137 41989] and Yarnbury (SU04SW/5) [SU03357 41053] provide data on the buried successions (Table 3).

The Upper Ordovician, Silurian and Devonian strata are thought to be present (and in some areas imaged on seismic profiles), at depth beneath parts of the district, but they are not penetrated by boreholes within or close to this district. A digest of the available data on the concealed strata is given below based on the lithologies described in the five principal boreholes, the Pre-Permian Geology Map of the United Kingdom (Smith, 1985) and numerous other papers and publications. The degree of detail generally reflects the volume of data available.

Tremadoc

Proved in the Shrewton Borehole, the Tremadocian strata are preserved in the core of an anticlinal structure, at depth, in the north of the district (the Tremadoc was formerly considered as the youngest part of the Cambrian, and is shown as such on the map referenced above; this stage is now considered as the oldest within the Ordovician Period). The rocks encountered in this borehole are grey to very dark grey, siltstone with interbedded mudstone and fine-grained sandstone. All are micaceous with calcite veining in part, and there are subordinate beds that are calcareous or slightly calcareous throughout. At depth, in the Shrewton Borehole the rocks are generally pyritic. Dipmeter logs run in the Shrewton Borehole suggest a dip in a generally southerly or south-westerly direction of between 20? to 48? throughout this succession.

Within the Shrewton Borehole (1743.5 to 1783.1 m [5720 to 5850 feet]) the highest part of the Tremadocian strata have high ‘dips’ to the north indicated on the dipmeter log. These are also described as calcareous, micaceous siltstone and mudstone, but they are of a generally grey colour with distinct reddish brown and greenish grey interbeds, some of which are mottled. The lithology, colour contrasts, high contrary dips, the higher stratal velocities measured and the more ‘ratty’ appearance to the gamma-ray log suggest that these are questionably basal Permian dune cross-bedded units and the ‘dip’ is therefore considered to indicate dune foresets showing a south to north wind direction (see Permian description below).

Ordovician to Silurian

The Pre-Permian geology map of the southern UK (Smith, 1985) shows undivided Ordovician and Silurian (and Devonian, see below) strata having an arcuate subcrop around the Tremadocian strata previously described. There is very little evidence as to the lithological character of these rocks in nearby boreholes although it is believed that a full succession is present.

The nearest analogues for this succession are the Silurian strata brought to surface in the Beacon Hill Pericline, one of the en échelon structures of the Mendip Hills about 35 km due west of the Shrewton Borehole. Here the approximately 350 m of exposed Silurian comprises tuffs with fossiliferous mudstone of Wenlockian age overlain by 200 m of andesitic lavas (Bristow, et al., 1999). Other nearby Silurian strata are described in the Bristol Special Sheet Memoir (Kellaway and Welch, 1993). The nearest exposed and similar Ordovician strata are probably in south Wales.

Devonian

Devonian strata are postulated to the south of the district, south of the Shrewton Borehole, and proven again further south (in the Salisbury district) of the complex Mere Fault beneath the Vale of Wardour, and the postulated extension of the Mere Fault towards the east. Here, the descriptions of the limited thicknesses penetrated show sandstone with subordinate siltstone and mudstone in fining-upward cycles of fluviatile origin. These continental successions were deposited on the Brabant Massif to the north of the Cornwall Basin (Ziegler, 1982) that was itself a part of the Variscan Foredeep Basin. The subcrop patterns on the Pre-Permian map (Smith, 1985) would suggest that these rocks dip towards the south and south-east and probably at similar angles to the older strata. There are no boreholes that penetrate this succession within the Devizes district.

Carboniferous

Carboniferous strata underlie the Devizes district at depth. They form the basement beneath the southern part of the Pewsey Basin, an element of the larger Wessex Basin. The Pewsey Basin is bounded in the south, by the complex Mere structure, and to the north by the Pewsey structure. Evidence from the Stockbridge and Goodworth boreholes suggest that the Carboniferous is separated from the Lower Devonian by the post-Devonian unconformity in the Pewsey Basin. It is not known whether younger Devonian strata are present away from these boreholes

The rocks encountered in the district are lower Carboniferous (Tournaisian–Visean) platform carbonates. They 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. The succession is generally light to dark grey with light brown, pink and red tones being associated with the dolomite-rich units.

These platform carbonates formed in a shelf sea on the southern margin of the emergent London–Brabant Massif.

Permian to Triassic

The thickness variation of the Permian–Triassic strata (including the Penarth Group, see below) is shown in (Table 4).

The Permian strata encountered within the Shrewton and Yarnbury boreholes comprise interbedded, red, micaceous mudstone and siltstone with rare thin sandstone interbeds. Towards the base within the Yarnbury Borehole a white to reddish brown, microcrystalline, ‘chalky’ limestone is described from chippings but a sidewall core failed to make any recovery.

As mentioned above, the lithological and geophysical log characteristics of the uppermost (?) Tremadoc strata, beneath the identified Permian, suggest that this unit may also be of Permian age. These log characteristics are displayed in the Shrewton Borehole.

The Triassic strata of the Wessex Basin are broadly divided into four lithostratigraphical units. A basal Aylesbeare Mudstone Group, intermediate Sherwood Sandstone and Mercia Mudstone groups and a thin uppermost Penarth Group. The division between the Sherwood Sandstone and Mercia Mudstone groups is known to be diachronous within the individual depositional basins that make up the British Isles Triassic, with this boundary being younger in the south (in the Wessex, Avon/South Wales and Worcester basins) than in the north (e.g. the East Midlands Basin).

To the west in the Wincanton district (Bristow et al., 1999) the thickest Triassic strata are postulated south of the Mere Fault (the Mere Basin) with a thinner succession to the north (the Pewsey Basin) and to the south (on the Cranborne–Fordingbridge High). The lowest 350 m (beneath the Mercia Mudstone Group) of this thicker succession is thought to comprise a lower mudstone unit, the Aylesbeare Mudstone Group, overlain by an interval having the typical seismic signature of the Sherwood Sandstone Group. Seismic reflection data suggest that the Mercia Mudstone Group is up to 400 m thick in the Mere Basin, thinning, by onlap, both to the Mendips northward and onto the Cranborne–Fordingbridge High to the south. In the Avon Basin (Kellaway and Welch, 1993), to the north-west, of this district, there is was a longer period of post-Carboniferous nondeposition or erosion and the strata of the Triassic basin ‘lap’ against older strata. This means that calcareous, ferruginous sandstones equivalent to the Redcliffe Sandstone ‘Formation’ (most recently proposed as a member of the Eldersfield Mudstone Formation of the Mercia Mudstone Group (Howard et al., 2008) rest, on older strata.

Within the local area, the thickness of Triassic strata is proved in all five deep boreholes. Boreholes to the south of the district demonstrate a thinner succession immediately to the north of the Mere Fault, thickening northwards into the Pewsey Basin. Further to the south, outside the district, the Cranborne Borehole proves a thin succession associated with the Cranborne–Fordingbridge High. There is no indication of the Aylesbeare Mudstone Group north of the Mere Fault, presumably lost through onlap against the faulted Mere Basin and only the upper three groups of the Triassic are positively identified from boreholes in this district.

As defined within the Shrewton and Yarnbury boreholes, the Sherwood Sandstone Group is divided into older Bunter Sandstone ‘equivalent’ and younger Keuper Sandstone ‘equivalent’ (essentially following the traditional scheme of Hull, 1869). Thus defined, the Sherwood Sandstone Group comprises interbedded, red and grey-green mudstone and siltstone (the ‘Keuper Sandstone’) overlying very fine- to medium-grained, calcareous sandstone with interbeds of siltstone (the ‘Bunter Sandstone’). In terms of the British Triassic succession these terms are no longer meaningful. However, if this boundary between older and younger beds is correctly defined within these boreholes then there is an erosional event between the two ‘equivalents’ that can be related to the extensive and well-known Hardegson Disconformity. Modern terminology based on distant outcrops in the South-west England Basin (SWB) and in the Worcester Basin (WB) would suggest that the lower beds beneath the disconformity are equivalent to the Budleigh Salterton Pebble Beds (SWB) or the Wildmoor Sandstone Formation (WB). The upper beds are equivalent to the Otter Sandstone Formation (SWB) or the Bromsgrove Sandstone Formation (WB).

However it could be postulated that the calcareous nature of the lower sandstone (the ‘Bunter Sandstone equivalent’) and its generally siltstone/mudstone lithology of the overlying ‘Keuper Sandstone’, may indicate that this unit is a marginal facies of the Triassic perhaps even equivalent to the Redcliffe Formation of the Bristol district and thus the whole Sherwood Sandstone Group succession as defined in the boreholes could be considered as entirely within the Mercia Mudstone Group.

The highest beds (1496.6 m to 1505.7 m) of the Mercia Mudstone Group are attributed to the Blue Anchor Formation in the Shrewton Borehole. They are described as dark grey and greenish grey calcareous mudstone. This unit has not been identified within the other deep boreholes in the district but may still be present in these borehole successions as there is only a slight non-sequential contact with the overlying Penarth Group.

The Penarth Group, of Rhaetian age, is believed to underlie the whole district since the massive limestone (Langport Member or ‘White Lias’) at the top of the succession forms a prominent widespread seismic reflector. The group was deposited in marginal marine and lagoonal conditions as a precursor to the fully marine conditions prevalent in the Jurassic. As such the group brings an end to the long period of continental red-bed sedimentation characteristic of the Permian and much of the Triassic periods.

The group is now divided into the Westbury Formation overlain by the Lilstock Formation. The Westbury Formation is equivalent to the Westbury Beds, Black Shales and the Rhaetic Bone Beds of the older literature. The Lilstock Formation includes the Cotham Member (formerly Cotham Beds), and the Langport Member (formerly the White Lias). Whilst the upper part of the succession is identified as the White Lias in four deep boreholes and can therefore be attributed to the Langport Member, the succession below is described as the Cotham and Westbury Beds. The lithological descriptions are insufficient to place the Westbury Formation/Lilstock Formation boundary with any certainty.

In areas to the south of the Devizes district, the Westbury Formation is known to comprise dark grey to black laminated to finely bedded micaceous, carbonaceous and pyriteous mudstone (or shale). The Cotham Member is described as white to pale greenish grey calcareous siltstone with traces of pyrite and shell fragments. The Langport Member is hard microcrystalline white to grey sparsely bioclastic limestone, argillaceous in part, with carbonaceous material and pyrite.

The greater part of the Jurassic succession is encountered at depth beneath the Devizes district. In general the successions described are insufficiently detailed to allow direct comparisons with strata exposed to the west and south-west and readers are recommended to consult the Wincanton (Bristow et al. 1999) and Shaftsbury (Bristow et al., 1995) memoirs for detailed lithostratigraphical descriptions. Where possible, correlatives are discussed in the sections below.

Lower Jurassic (Lias Group)

Traditionally the Lias was divided into Lower, Middle and Upper units and it is these that appear on the logs for the five deep boreholes. The Lias Group has recently been divided into a number of formations and members based on the regional basins across the British Isles (Cox, et al., 1999). This paper brings together a multitude of quasiformal and informal names (some used to classify the strata in this district) into a UK-wide hierarchy. Many of the names are heavily entrenched in the literature but commonly poorly defined. The scheme for the Wessex and Worcester basins is shown in (Table 5).

In general, the formations can be identified within the described successions in the five deep boreholes although the identification of individual members is more difficult due to the inevitable gaps in the described strata in these un-cored sections. The Yarnbury Borehole described below, gives the most comprehensive section of the lithologies encountered in the district.

To the south-west, the Wincanton Memoir (Bristow et al., 1999) describes the traditional terms Lower, Middle and Upper Lias with locally named units separated by non-sequences. In that area the top of the Lower Lias is placed at the top of the Ditcheat Clay (a lateral equivalent of the Green Ammonite ‘Beds’) and the top of the Middle Lias at the disconformity at the top of a lower Marlstone Rock Bed. It is not known whether the identified gaps in the succession can be traced north-eastward, beneath the Salisbury district, into the Devizes district.

In the Yarnbury Borehole the Lower Lias is described as claystone, medium to dark grey, hard blocky, silty, pyritic, very calcareous and interbedded with hard brittle, blocky, microcrystalline argillaceous light to medium grey limestone grading to calcareous claystone. This lower part of the succession is regarded as Hettangian in age. Similar lithologies are described for the Sinemurian and Lower Pliensbachian parts of the succession above but these are generally micaceous and shelly. They grade in colour from dark greyish brown to very dark grey black. Taken together, the Lower Lias amounts to some 122.5 m (402 feet) of strata. It is not possible from the descriptions to determine the horizon at which the change from Blue Lias Formation to Charmouth Mudstone Formation occurs. The Middle Lias of Late Pliensbachian age comprises 132.9 m (436 feet) of clean, white to pale grey, very fine- to fine-grained sandstone, calcareous, micaceous and with a trace of glauconite, which grades downwards into calcareous and pyritic siltstone and claystone with thin interbeds of soft ‘chalky’ limestone.

The Marlstone Rock Bed (equivalent) is described as 1.5 m (5 feet) of white to buff (pale brown) firm to hard microcrystalline and sparry limestone and is considered to be of Late Pliensbachian age. Above is 8.8 m (29 feet) of a ‘Junction Bed’ comprising light grey, calcareous very fine-grained sandstone with thin interbeds of claystone and limestone overlain by a cream microcrystalline slightly dolomitic hard limestone both of which are of Toarcian age. These beds can be considered as equivalent to the Dyrham Formation. The Upper Lias, of Toarcian to Bajocian age, is 107.6 m (353 feet) thick and comprises a basal unit of grey and greyish brown siltstone and claystone with thin limestone interbeds which grades up into greenish grey, glauconitic, calcareous, pyritic, very fine-grained sandstone possibly equivalent to the Bridport Sand Formation of the Dorset coast.

In the Devizes No. 1 Borehole the Blue Lias is distinguished at the base of the group and the remainder of the Lias is divided into the members characteristic of the Wessex Basin. A summary of the group is given below in (Table 6).

Middle Jurassic

Inferior Oolite Group (InO)

To the south-west in the Wincanton District, the Inferior Oolite Group (there termed the Inferior Oolite Formation) is known to contain appreciable breaks in the succession. The group spans the latest Aalenian, Bajocian and earliest Bathonian stages. In general the descriptions available from the five deep boreholes in and around the Devizes district are insufficient to determine whether these breaks in deposition extend beneath this area.

In the Yarnbury Borehole the Inferior Oolite is described as 15.5 m (51 feet) of interbedded limestone and claystone. The limestone is pale to medium grey, friable to firm, sucrosic, with some fine- to medium-grained ooids, and is argillaceous and sandy in places. The claystone is medium grey to dark greenish grey, slightly calcareous with mica and a trace of pyrite.

In the Devizes No. 1 Borehole the Inferior Oolite is described as 19.2 m of cream, ooidal and cryptocrystalline limestone that is locally shelly.

Great Oolite Group (GtO)

This group comprises four formations each divided into a number of members. In ascending order the formations are the Fuller’s Earth Formation, Frome Clay Formation, Forest Marble Formation and the Cornbrash Formation. Each of these is identified at outcrop to the south-west in the Wincanton sheet district where they are known to span the Bathonian and earliest Callovian stages. With the exception of the Frome Clay Formation (a newer term adopted since the publication of the five completion logs where this unit is called the Great Oolite undifferentiated without the formal formation suffix), all are tentatively identified within the five deep boreholes in and around this district. (Table 7) shows the relative thicknesses determined within these boreholes for each of the units.

The most complete descriptions of the group are given in the Yarnbury Borehole. The Fuller’s Earth Formation is described as 39.3 m (129 feet) of limestone overlying sandstone and claystone. The limestone is light grey to dark brownish grey, fine to coarsely crystalline with silty laminations. Beneath is very fine- to fine-grained light brownish grey calcareous sandstone over grey to greenish grey calcareous claystone. To the south-west, in the Wincanton district the formation is divided in part by the Fuller’s Earth Rock ‘Member’ but it is also noted that the upper part of the formation is probably incorporated within the Frome Clay Formation. The same may be true beneath this district.

The Frome Clay Formation was formerly included within the upper Fuller’s Earth and it is not entirely clear within the five deep boreholes as to whether this formation is included within the descriptions for the thicker Fuller’s Earth successions in the Yarnbury and Shrewton boreholes or within the undifferentiated Great Oolite in the Netherhampton Borehole for example. It may well be that this part of the succession is absent in the south of the Salisbury district as the Mere Fault complex is approached.

Throughout these boreholes the Great Oolite (undifferentiated) strata are described as limestone. In the Yarnbury Borehole this unit is described as cream to buff, hard, ooidal limestone. Ooids vary from fine- to coarse-grained and are present within a sparry to microcrystalline argillaceous matrix. In the north and east of the Wincanton District, the memoir states that the Frome Clay passes from a principally argillaceous succession into one dominated by shelf limestones and it is tempting to make this association for these limestones (Bristow et al., 1999).

The Forest Marble Formation succession is highly variable. The unit is principally of sandy calcareous mudstone with subordinate argillaceous, calcareous sandstone and argillaceous limestone to the south-west in the Wincanton district. Similar descriptions are known from the five boreholes in this district. In Wincanton the formation is between 35 and 40 m thick at outcrop. The unit ascribed to the Forest Marble within the five boreholes is considerably thicker being around 60 to 65 m to the south in the Salisbury district and between 95 and 100 m in the north of the Devizes district.

The Cornbrash Formation at outcrop to the south-west is traditionally divided on lithological and faunal grounds into lower and upper units. The lower part, is of Bathonian age, and the upper of Callovian age. The lower comprises pale cream ooidal and biomicritic limestone with thin shelly mudstone partings whilst the upper unit is sparsely sandy, peloidal, biomicritic limestone overlying fine-grained calcareous sandstone and sandy biosparite limestone. Descriptions in the five deep boreholes in and around the Devizes district are dominated by limestone but there is insufficient detail to say whether the upper and lower units are present.

Kellaways Formation (Kys)

The formation, of lower early Callovian age, is traditionally divided into an upper Kellaways Sand Member (Kellaways Rock in some literature) and a lower Kellaways Clay Member. These two members have only locally been identified at outcrop to the south-west in the Wincanton district where the succession is dominated by medium grey sandy mudstone.

In the south of the area, the Kellaways Formation is recognised within the Yarnbury Borehole with Kellaways Rock or Sand overlying Kellaways Clay being recognised in the other four wells. In the Yarnbury Borehole the formation is described as unconsolidated very fine-grained sandstone overlying pale grey well-cemented, calcareous pyriteous and carbonaceous, very fine-grained sandstone and moderately calcareous, medium grey claystone. In the other boreholes the lower unit is described as grey mudstone with calcareous siltstone.

Chapter 4 Middle to Upper Jurassic

Oxford Clay Formation (OxC)

Traditionally the Oxford Clay Formation was subdivided into three units, and these lower, middle and upper divisions have now been formalised as the Peterborough, Stewartby and Weymouth members respectively. The Peterborough and Stewartby members are of Callovian age (Middle Jurassic) whilst the Weymouth Member is of Oxfordian age.

In the five deep wells, the formation is described as a monotonous succession, light to dark grey, in part calcareous, pyritic, micaceous and bituminous (carbonaceous) mudstone and claystone with some thin limestone and sandstone beds. In the Urchfont Borehole, it is only divided along traditional lines. To the south-west within the Wincanton district the Peterborough Member is described as brown fissile mudstone and is the most fossiliferous part of the formation. The Stewartby and Weymouth members are undivided and comprise calcareous medium grey, variably silty, shelly mudstone with some thin, very fine-grained, poorly cemented sandstones.

Details

The Oxford Clay crops out in the north-western corner of the district beyond a major fault. Jukes-Browne (1905), noted ‘Gryphea dilatata [now called Gryphea (Bilobissa) dilobotes Duff], which characterises the upper beds of Oxford Clay, has been obtained in this area, and the beds were formally worked in a brick yard north-east of Seend and east of Sells Green (Woodward, 1895)’. This brickyard is thought to be that indicated on historic maps as the Seend Brick Works located at [ST 9567 6209] to the north of both the railway and canal and very close to the bounding fault mapped to the south-east of the working face. The site is now a small industrial estate, has been substantially landscaped and there is no exposure.

In the Urchfont Borehole, the Oxford Clay Formation is divided into lower, middle and upper units of respectively 20.6 m, 50.6 m and 74.7 m thickness. It is not known with certainty whether these equate directly to the members. The succession is described briefly with a lower brown, slightly calcareous, carbonaceous, siltstone passing up into light, becoming dark grey, carbonaceous mudstone. All is variably pyritic 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 deep-water argillaceous shelf sedimentation represented by the Oxford Clay Formation and Kimmeridge Clay Formation.

In the Yarnbury Borehole the Corallian is described as a grey silty mudstone passing up into calcareous and glauconitic sandstone and into ooidal and pisolitic limestone. In other wells, a lower sandstone unit is described. In general the descriptions are too poor to permit attribution to the thin complicated formal nomenclature described at outcrop to the west in the Frome district.

Details

The group is represented by two small outcrops in the north-west of the district but the whole succession is not seen. To the north of the major fault through Sells Green a small outcrop [ST 948 616], closely associated with a landslide, is classified as the Hazelbury Bryan Formation (HyB) and rests on the Oxford Clay. A general description for the formation in this region is muddy sandstone and yellow, brown and grey very fine- to medium-grained quartzose sand, with thin sandy siltstone beds in parts; however, very little of the formation is seen in the outcrop available. The second outcrop in a shallow valley south-east of Seend [ST 948 605], is substantially covered by head and is attributed to the Newton Clay Member (NeC) of the Stour Formation. The member comprises mudstone, sandy clay and fine-grained sand.

Kimmeridge Clay Formation (KC)

The Kimmeridge Clay Formation forms low-lying ground in the far north-west within the small valleys to the south of Potterne and west of Worton and Poulshot. There is virtually no exposure and the outcrop is partially obscured by recent fluviatile and head deposits. The succession of the Kimmeridge Clay Formation to the south of the district is known from the Tisbury Borehole (ST92NW/2) [ST 9359 2907] in the north-east of the Wincanton district, and from the nearby Westbury Quarry section to the west within the Frome district. A generalised section of the Tisbury Borehole and other data from that district was published in the Wincanton memoir (Bristow et al., 1999, fig. 25) and again in the Salisbury Sheet Description (Hopson et al., 2008). In general this succession comprises a variable, small-scale rhythmically bedded, calcareous, kerogen-rich, bituminous, dark grey to black mudstone and oilshale with silty and sandy mudstone. The rhythms are identified by the presence of thin siltstone and cementstone beds.

Details

The Kimmeridge Clay Formation crops out around Poulshot, Worton and Marston and runs eastward along the valley towards Potterne Wick. The outcrop extends over much of the lowland west of Devizes. Much of the area is a low-lying flat plain, mostly covered in pasture. There are only very limited sections in the area. The clay was proved mostly by augering. It consists of stiff, waxy, dark grey mudstones, locally with small pale grey calcareous concretions (known as ‘race’), calcareous pale grey mudstones, oil shale and silty dark grey mudstone. In many areas, especially in the extreme west of the district area around Bulkington, the mudstone is quite fossiliferous, with much shell debris, commonly with a nacreous lustre, being brought up in auger samples.

The top of the Kimmeridge Clay is taken at the base of the ‘Portland Sand’ herein referred to as the Wardour Formation. In this area, the top of the Kimmeridge Clay is quite silty and transitional, so the boundary is taken where silty/sandy mudstones pass up into argillaceous sandstones/siltstones, rarely marked by a spring line, or by a series of small knoll-like features.

There are very few exposures of the Kimmeridge Clay. Several disused brickpits exist in the Potterne area. The Larborough (or Potterne) Brick Works consisted of two small pits, one north of Heron Bridge at [ST 993 569] and one south of the stream at [ST 993 566]. Both are in the uppermost part of the Kimmeridge Clay. The northern pit was in existence in 1888 and disused by 1926. The southern pit was probably already disused by 1888. Jukes-Browne (1905, p. 5) recorded a section in the pit (probably the northern pit) in 1888, consisting of 0.6 m of brown loamy soil, passing down into 2 m of yellow mottled loamy clay overlying 3 m of grey and black laminated clay. He recorded several large doggers of dark grey argillaceous limestone with poorly preserved fossils. The yellow mottled clay may well be a thin head deposit. No sections remain today and the northern pit has been completely infilled and little trace on the ground remains. The southern pit is now a hummocky field used as a sheep pasture.

Another old brickpit dug in the uppermost Kimmeridge Clay occurs south of Worton at [ST 9828 5704]. Woodward (in Jukes Browne, 1905, p. 6) recorded about 5 m of slate-grey loam overlain by blue-grey loamy clay, with ferruginous and sandy layers near the top. These beds represent the uppermost part of the Kimmeridge Clay, which passes up into the Portland Sand less than 5 m above the top of the pit. The pit was disused in 1888. No section exists today and the old pit is now a private garden. A small riverside exposure near Hurst Mill [ST 9810 5628] showed a few metres of dark grey silty sandstone.

A micropalaeontological sample collected from the Kimmeridge Clay at Lower Baynton farm near Coulston [ST 9448 5470] including Lenticulina subalata, Marginulina undulate, Citharina serratocostata and Marginulina ‘fragraria’ suggests an age, no higher than Pallasioides Zone. Another sample from near Larborough Farm [ST 9898 5743] included Lenticulina subalata, Lenticulina ‘muensteri’ and Lenticulina ponderosa also indicates an age, again probably no younger than Pallasioides Zone.

Portland Group (PL)

Traditionally the Portland Group is divided into a lower arenaceous unit, the informal Lower Portland Beds, and the Upper Portland Beds that are principally limestones (Woodward, 1895). The Upper Portland Beds have been further divided into five units named after quarryman’s terms based largely on their building stone potential. Wimbledon (1976) formalised this nomenclature, dividing the Portland Group into two formations; a lower Portland Sand Formation and an upper Portland Stone Formation, each divided further into members. This designation is essentially biostratigraphical and was modified in by Bristow (1995) on the basis of a ‘mappable’ lithostratigraphy in the Vale of Wardour in the Wincanton District (Table 8). This formal lithostratigraphical terminology was carried eastward into the Salisbury District (Hopson et al., 2007 and, 2008).

The Portland Group of the Devizes district is only divided at formation level since the outcrop and exposures of the Portland Stone Formation are not divisible into the members seen around Chilmark in the Salisbury district to the south. It may well be that the member-level terminology developed in the south for the Vale of Wardour is inappropriate in this district.

The Wardour Formation corresponds to the Lower Portland Beds of Woodward (1895) and to the Wardour Member of Wimbledon (1976). The term Wardour Member term was introduced for the basal, dominantly sandy, part of Wimbledon’s Portland Sand Formation (which also includes the dominantly limestone successions of his Chicksgrove and Tisbury members). Bristow (1995) considered these younger two members to be more appropriately part of the overlying Portland Stone Formation and re-designated the Wardour Member as the Wardour Formation.

The memoir for Devizes (Jukes-Browne, 1905) describes the Portland Group as: ‘Portland Beds consist of an upper portion known as the Portland Stone and a lower portion called the Portland Sands’. He noted that the Portland Sands emerge from beneath the Gault north of Earl Stoke and form a continuous outcrop as far as Potterne Park. They also crop out on the northern side of Stert Brook by Potterne Wick, and there is an outlier of the sand at Worton’.

Wardour Formation (War)

The Wardour Formation crops out over much of the south-eastern part of the district, and forms a continuous outcrop from Coulston in the south-west, north-east 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 beneath the Lower Greensand along the escarpment west of Poulton.

The base of the Formation is transitional with the top of the Kimmeridge Clay. 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. It forms the low hills with a steep facing scarp and gentle ‘dip slope’, which in fact is the plane of unconformity with the overlying Lower Greensand Group. To the west, the typical topographical features associated with the Wardour Formation die out and the outcrop can only be traced by augering before it is overstepped by the Gault Formation just north-east of Coulston. The top of the unit is not seen in this area as it is overstepped by the Lower Greensand Group or the Gault Formation.

Dinoflagellates recovered from the basal five metres of the Wardour Formation in the Tisbury Borehole to the south of the district in the Vale of Wardour, indicate that the basal glauconitic sandstone falls within the youngest zones of the Kimmeridgian Stage. The succeeding beds span the Albani and basal Glaucolithus zones.

Details

There is a small section in a sandpit in Cheverell Wood at [ST 9715 5555]. Here, 3 m of massively bedded, bioturbated well- to very well-sorted, fine- to medium-grained, weakly indurated ‘sugary’ quartz sand can be seen. No sedimentary structures were evident, partly because of the massive nature of the unit and the pervasive bioturbation.

A small section can be seen in the footpath 600 m east of Worton Church at [ST 9765 5733]. Here a few metres of massively bedded, bioturbated well-sorted, fine- to medium-grained, weakly indurated ‘sugary’ quartz sand can be seen.

Up to 2 m of the Wardour Formation can be seen in a small lay-by at the east end of Worton [ST 9827 5740]. The West Lavington Borehole near Hurst Farm (ST95NE/2), [ST 9898 5633] recorded 7.6 m of Wardour Formation (Gallois, 1976). However, this thickness is rather anomalous compared with the approximately 20 m estimated thickness of the Wardour Formation based on the recent survey.

Portland Stone Formation (PoSt)

In the Vale of Wardour to the south of the district, the Portland Stone Formation consists of three distinct members defined by Bristow and Lott (1994, 1995, 1996) and in the Wincanton Memoir by Bristow et al. (1999). The succession commences with the Tisbury Member that includes the Chicksgrove Member of Wimbledon (1976) at the base and the Ragstone as defined by Woodward (1895) at the top. This member is succeeded by the Wockley Member (which locally includes significant chert beds) and the overlying Chilmark Member. However, in the Vale of Pewsey, there are very few exposures and thus the sequence is not well defined and it is has not been possible to identify the individual members as seen in the Vale of Wardour.

The upward change from well-sorted fine- to medium-grained glauconitic sand of 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. These doggers were noted on the south-facing slope south of Cheverell Wood around [ST 9748 5522]. Locally these sandstone doggers are packed with fossils, the majority of which are preserved as moulds. The fossils are mostly fragments of bivalves, gastropods and serpulids, including the trigonids Laevitrigonia gibbosa? and L. wightensis.

Details

The sandstone at the base of the Portland Stone Formation has been dug from pits near Hurst Mill [ST 985 561] and on the hill north-east of Greenlands Farm [ST 9884 5577] (Jukes-Browne, 1905, p. 6). The stone from this latter pit was used for flooring barns and road stone, and specimens of Astarte and Trigonia were identified. Casey and Bristow (1964) re-examined the fossils found at the Greenlands Farm Pit and identified specimens of Myrene fittoni and Laevitrigonia gibbosa (J Sowerby) and L. wrightensis (Strand), to which they ascribed a Purbeck age. However, new material (WMD 11077–11099) collected by A R Farrant in May 2005 from a field [ST 9748 5522] at approximately the same stratigraphical level as the Greenlands Farm Pit, included ammonites, corals, gastropods, trigonid bivalves and serpulids including Galbanities, ?Mactromya circulare, Laevitrigonia gibbosa, L. wightensis and Aptyxiella portlandica, indicating a Okusensis Zone, and thus suggested the Portland Stone Formation (sensu Bristow et al., 1999).

A borehole (SU05NW/40) drilled at Stroud Hill Farm [SU 0150 5833] provided an 8.7 m section through the Portland Stone Formation shown in (Figure 4). The base of the succession consists of 3.5 m of locally very shelly, dark grey-green, highly glauconitic moderately well-sorted, medium-grained sand and sandy limestone. Locally this sand is well cemented and hard, but other parts consist of loose sand giving a ‘ragstone’ appearance. The abundant fauna includes bivalves, Modiolus boloniensis, Ostrea expansa? and Camptonectes lamellosus?, gastropods, oysters and serpulids. Towards the base of the sequence, the sand becomes increasingly clay-rich and less well cemented.

The upper part of the Portland Stone Formation succession is separated from the glauconitic sands by a marked burrowed erosion surface. Above this surface, this unit consists of a 2 m-thick fining-up sequence of pale brown fine- to medium-grained sandy packstone, with some glauconite and occasional scattered black chert fragments (lydite). This unit is fossiliferous, with a fauna dominated by bivalves, serpulids and oysters which decrease in number towards the top. The fauna also contains the claws of the ghost shrimp Callianassa. Above this is a sharp change to a pale yellow-brown mudstone with small clasts or peloids of pale brown carbonate with some shell debris and bivalves. This is overlain by the Lower Greensand Group.

A small exposure of the hard, glauconitic calcareous sandstone was seen on a bend in the road, around 200 m north-west of Crookwood Farm at [SU 01345 58127]. The sandstone yielded a fauna (WMD 10217–10346) including the bivalves Modiolus boloniensis, Ostrea expansa? and Camptonectes lamellosus. A few small black chert pebbles (lydite), 5 to 10 mm in diameter, are also present. The bivalve fauna is characteristic of the Portland Group, and probably represents the lower part of the Portland Stone Formation (sensu Bristow et al., 1999), since the underlying Wardour Formation typically comprises sparsely shelly siltstone and fine-grained sandstone. Bristow et al. (1999) noted that the base of the Portland Stone Formation is taken at the abrupt lithological change from fine-grained, clayey sand of the Wardour Formation to calcareous sandstones and sandy limestones.

The deep borehole at Urchfont records 29.5 m of loose translucent fine- to medium-grained, glauconitic fossiliferous quartz sand (Wardour Formation) overlain by 17 m of buff-white hard microcrystalline limestone with sparry calcite and fossil fragments (Portland Stone Formation).

Chapter 5 Lower Cretaceous

The Lower Cretaceous of the district is principally represented by the outcrop of the Selborne Group formations, with a small outcrop of the Lower Greensand Group in the north-west corner of the map. The Purbeck Group is interpreted at depth on seismic sections, and has been proved in the Yarnbury Borehole. This group is now considered as principally of Cretaceous age and a brief description is included here. A succession representative of the ‘Wealden Group’ is suspected at depth in the east of the district from seismic data but has not been proved by boreholes within the district. Chadwick and Kirby (1982) show an arcuate subcrop of the Purbeck Group and a wider-spread subcrop of the Wealden Group derived from an early interpretation of seismic data for the whole of the Salisbury and Winchester districts that also includes a small part of the southern Devizes district (see also Hopson et al., 2008).

Purbeck Group (PB)

The Purbeck Group is only positively identified in the Yarnbury Borehole marginally to the south of the district and again in the Netheravon Borehole further to the south, both boreholes lying within the Salisbury district. Its presence is interpreted from seismic sections and is considered as being more widespread at depth in the south and east of the district but is indicated together with the Portland Group and Wealden Group on the vertical sections on the map. The group is not known at outcrop being concealed beneath the unconformity at the base of the Lower Greensand Group.

In the Yarnbury Borehole the group is described as 36.9 m of interbedded limestones and sandstones with shell fragments, chert and glauconite. The whole succession being sandier within its middle part.

Wealden Group (W)

The Wealden Group is interpreted as being present beneath the east of the district from seismic data. The nearest outcrop is described from the Vale of Wardour within the Salisbury district (Hopson et al., 2007 and 2008). A thin representative of the group has been identified in the Farley South Borehole (SU22NW/2) [SU 23589 28529] close to the common border of the Salisbury and Winchester districts and about 20 km to the south of this district. To the east within the Andover and northern Winchester districts a thicker succession of the Group, up to approximately 200 m, was proved in the Goodworth 1, Upper Enham 1 and Egbury 1 boreholes (see Booth, 2002). This thicker succession has been divided using the Weald region lithostratigraphy. These beds are preserved in the extension of the Wealden Basin that connects through into the Pewsey Basin. Overstep by the Lower Greensand unconformity progressively cuts out the Wealden Group to the west into this district; but thin representatives of these beds are likely to be present at depth as far west as a line represented by the north–south portion of the River Avon (see Section 2 on the Devizes sheet).

Lower Greensand Group (LGS)

The Lower Greensand of the Vale of Pewsey consists usually 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; this is the Seend Ironstone Formation.

The age of the group in this area is open to debate but is considered to be equivalent to the Folkestone Formation of the Weald, and by implication therefore of youngest Late Aptian to earliest Early Albian in age.

Details

The Lower Greensand crops out in the north-western area of the district and comprises a sequence of ferruginous sands and ironstones. These beds rest unconformably on the Jurassic and lowest Cretaceous strata in this area. They lie on the Wardour Formation and overstep onto the Kimmeridge Clay at outcrop; but must overlie the Wealden Group and the Purbeck Group in the subcrop. The Lower Greensand Group is itself overstepped by the Gault Formation, producing a discontinuous outcrop. The Lower Greensand occurs in five discrete areas in this district. The largest outcrop occurs in the Poulshot area where it forms a well-developed dip slope, which also approximates to the erosional plane of the Albian unconformity. Three 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, not recorded on the old Devizes sheet published in 1959, occurs on the hill just east of Greenlands Farm at [ST 985 554].

A further outcrop lies around Seend, at the eastern end of the ridge, where sandy ironstones occur and from which the Seend Ironstone Formation takes its name. The type sections are to be found in old degraded quarries to the north and west of the village within the Frome district. They are however described in Jukes-Browne (1905) and in Cunnington (1850). These pits show successions between 3.65 and 6.1 m thick (12 to 20 feet) of variable sands and iron-rich sands becoming gravelly and fossiliferous with depth.

There are no sections in this area, but augering and field brash prove that the Lower Greensand consists of orange-red, ferruginous, very poorly sorted, coarse sandstone, which is quite distinct from the well-sorted sand of the underlying Portland Sand. The sand contains many well-polished, rounded quartz grains and granules, with some glauconite and angular chert clasts. Locally the unit is almost entirely composed of sandy concretionary ironstone, elsewhere it is a very ferruginous medium- to coarse-grained sand with concretions of ironstone, especially in the north of the area around Poulshot. Fragments of fossil wood have been found, notably at [ST 9938 4610].

A pit, dug in 1888 near Larborough Farm [ST 9913 5719] showed coarse yellowish white quartz sand with rounded chert pebbles (lydite) at the base (Jukes-Browne, 1905). This pit is no longer visible (and not indicated on the historic-maps of the area) but is thought to occur somewhere on the ridge north-east of the farm at approximately [ST 9926 5733]. The thickness of the formation is generally less than 5 m and in many places it is a thin skim on the Portland Sand. The old map (1959) exaggerated the extent of the outcrop because of the large amount of ironstone and Lower Greensand debris in slope-wash surrounding the outliers. The sub-Lower Greensand unconformity cuts down quite strongly into the underlying Jurassic strata and in places may be channelled or possibly fault controlled.

Selborne Group

Gault Formation (G)

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 commonly obscured by landslides and slumped masses below the Upper Greensand scarp, particularly around the town of Devizes. Jukes-Browne (1905) noted a horizon of springs, marked by a line of landslides north-eastwards by Fiddington Farm and Forest Farm.

Lithologically 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 when 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 older literature places the so so-called ‘Basement Bed’ of ferruginous fine- to medium-grained sand and fine pebbles interbedded with argillaceous sand within the Gault Formation although this may well locally be considered as part of the Lower Greensand. No recent core material is available to resolve this.

Away from the outcrop towards the east, the Gault Formation is believed to underlie the remainder of the district and is has been proved in the five deep boreholes mentioned above. Its thickness is between 33 and 58 m. At crop the thickness is estimated at between 25 and 45 m during recent mapping but there are no boreholes that penetrate the whole succession near to the crop.

Details

The Gault Formation forms a continuous, low lying outcrop around the margin of the Upper Greensand escarpment around Potterne. However, there are very few exposures and the area was surveyed by augering; diagnostic features of the formation include the presence of heavy clay soils with rushy vegetation, the presence of a spring line and small feature at the contact with the overlying Upper Greensand. In the Potterne area, the Gault Formation oversteps onto the Lower Greensand, the Wardour Formation and the Kimmeridge Clay. In weathered auger samples, the Gault Formation is very difficult to distinguish from the Kimmeridge Clay, although the latter is usually darker grey and is not micaceous. The Gault Formation has a maximum thickness of about 45 m. Locally the Gault Formation is subject to landslides. A former brick and tile works on Broadway, near West Lavington [ST 9998 5532] worked the Gault Formation for the production of tiles and flower pots, but no section was recorded (Jukes-Browne, 1905, p. 14).

Jukes-Browne (1905) also recorded a large brick and tile works on the road to Lavington near West Farm where the grey micaceous clay was apparently well suited to making tiles and flower-pots but was almost destitute of organic remains. He also noted an abandoned brickyard in the valley south of Stert where similar grey and lilac clay was dug.

Jukes-Browne and Hill (1900) noted ‘a better section’ of the Gault Formation in the Caen Hill Brickyard [ST 9806 6139] west of Devizes. This pit has been much expanded since but has been disused degraded and overgrown for a number of years. It is currently the site of a car breakers yard and no exposures were noted during this survey. Jukes-Browne noted the following section in 1888.

Jukes-Browne and Hill (1900) noted that fossils were not common but included a short faunal list derived mainly from the collecting of Mr W Cunnington.

Upper Greensand Formation (UGS)

The Upper Greensand Formation forms a significant scarp above the lower ground occupied by the Gault. In this district the outcrop runs from the Coulstons eastward by Great and Little Cheverell to Market Lavington and Urchfont. To the north, it crops out around Potterne–Devizes and forms much of the landslide prone scarp from here, eastwards, towards Stert. The higher part of the formation has an extensive outcrop across the floor of the Vale of Pewsey, eastward to Burbage and beyond, outside the district to the east.

Traditionally the Upper Greensand in Wiltshire was divided into five subdivisions based on gross lithology (Jukes-Browne and Hill, 1900). These authors gave a general description of the Upper Greensand of the Vale of Wardour divided into two units that they associated with the characteristic fossils of Ammonites rostratus, below, and zone of Pecten asper and Cardiaster fossarius, above (Table 9).

During the surveys for of the Shaftesbury and Wincanton Sheets districts a new scheme was adopted based similarly on gross lithology. (Table 10) gives a correlation of the old and new schemes as determined by Bristow (1995) for the Tisbury area. He gave a slightly different interpretation to that of Jukes-Browne and Hill (1900).

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. It is described in Chapter 6.

However, during the recent survey of the Devizes district the members 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, an absence of the chert beds within the upper part of the formation.

Woods (2005) produced the following discussion: ‘Two important points have arisen from this work in the Devizes area. Firstly, there appears to be a significant thickness of Upper Greensand belonging to the C. auritus Subzone in the Devizes area. Although Spath (1943) and Owen (1975) stated that the ‘Malmstone’ and Potterne Rock of the Devizes district were chiefly of C. auritus Subzone age, the thickness of this interval and its implications for regional lithostratigraphical correlations has only become apparent during the recent survey. In the Shaftesbury district, the thickness of the auritus Subzone is inferred to be very limited and restricted to a few metres of sediment forming the ‘Ragstone’ at the top of the Shaftesbury Sandstone Formation (Bristow et al., 1995). By implication, the age-equivalents of the combined Cann Sand and Shaftesbury Sandstone of the Shaftesbury district must form the lower part of the Upper Greensand succession in the Devizes district; they are perhaps no more than 10 m in thickness, compared to their 30–50 m in the Shaftesbury district and 15–40 m in the Wincanton District (Bristow et al., 1995, 1999). Owen (1975) observed that a phase of broad, low amplitude folding occurred at the end of the C. auritusSubzone, leading to current erosion and lateral variability in the thickness of preserved sediments of this age. The second key observation is that the Potterne Rock forms a widely recognisable horizon at outcrop in the Upper Greensand of the Devizes district, and that the Upper Greensand has a consistent succession of lithologies at outcrop. The Potterne Rock is typically a hard, nodular, well-burrowed horizon, usually with phosphatic clasts, and always underlain by a few metres of grey-green glauconitic sand. The latter sands commonly contain small, scattered phosphatic clasts and conspicuous pale grey clay-lined burrows, and outcrops of this interval usually appear poorly bedded, forming smooth-faced cliffs a few metres in height. Overall, a distinct three-fold subdivision of Upper Greensand lithologies can be recognised: grey-weathering, poorly fossiliferous, clayey, micaceous sandstone, overlain by pale orangey-yellow or buff-weathering, micaceous sandstone, with a common and diverse mollusc fauna, capped by grey-green-weathering, distinctly glauconitic sandstone; the change to glauconitic sandstone is at the base of the phosphate-bearing glauconitic sands that occur a few metres below the Potterne Rock’.

Details

In the north-west of the district the Upper Greensand Formation forms the high ground around Potterne where several sections can be seen. One of the best sections occurs on the main A360 Devizes road though the village. A section in the grounds of a private house at the northern end of the road cutting [ST 9944 5830] is thought to occur just below the Potterne Rock Bed. This cutting exposes a few minor exposures of micaceous sandstone from which a few large ammonites were found (Jukes-Browne, 1905, p. 21).

Several small sections in the public footpath at [ST 9982 5845] show up to 2 to 3 m of very glauconitic fine micaceous sand with large doggers of hard green micaceous sandstone. The Potterne Rock is exposed in the cutting at the upper end of Coxhill Lane (on sheet [SU05NW] ) and was formerly dug in the fields north of Blounts Court Farm, approximately [ST 9998 5850] for building stone (Jukes-Browne, 1905). No trace of this quarry now remains.

A small bluff about 500 west-south-west of Furzehill Farm, near Devizes [ST 98868 59993] comprises about 1 m of grey-weathering, clayey, micaceous sparsely glauconitic sandstone, passing fairly rapidly up into very pale weathering, much less clayey, micaceous glauconitic sandstone. The sparse fauna from this locality comprises the bivalves Neithea quinquecostata and Entolium orbiculare and the serpulid Rotularia. This fauna is biozonally undiagnostic, but is inferred to be from within 10 m of the base of the Upper Greensand Formation.

A limited exposure north-east of Whistley Farm [ST 98857 59981] appears to show the junction of grey, clayey, micaceous sandstone and much paler weathering, buff-coloured micaceous sandstone. The fauna includes the bivalve Pinna robinaldina and the ammonite Epihoplites?. The species of Epihoplites occur in the orbignyi and varicosum subzones, in the lower M. (M.) inflatum Zone, but some are questionably ascribed to the C. auritus Subzone (Spath, 1942). This is the only tentative evidence for a pre-C. auritus Subzone age for an exposure of Upper Greensand seen in the course of the current work in the Devizes district.

A second exposure in this area [ST 98834 59950] reveals the fauna of the heteromorph ammonite Idiohamites turgidus? in an indurated horizon within bioturbated clayey, micaceous sandstone. Idiohamites turgidus is indicative of the H. varicosum and C. auritus subzones of the M. (M.) inflatum Zone (Spath, 1942).

An old quarry, south-west of Potterne church [ST 99475 58308] exposes about 6 m thick succession of greenish-grey and orangey-grey, glauconitic and micaceous sandstone (Figure 5). There are some indurated sandstone beds and locally common phosphatic clasts. The fauna consists of the bivalve Nanonavis carinata and two specimens of the ammonite Mortoniceras sp. The fauna is biozonally undiagnostic, but the lithological details suggest that the section may include the equivalent of the Potterne Rock Bed, at the top of the C. auritus Subzone.

A roadside section running south-south-east of Coulston to the intersection with B3098 [ST 95355 53947] to [ST 95376 53796] exposed about 13.4 m thick of Upper Greensand succession (Figure 6), comprising weathered, micaceous greensand in the lower part, with soft, greenish grey glauconitic sand in the upper part. Indurated sandstone horizons occur throughout, including a massive, 0.6 m-thick sandy limestone which caps the succession.

The fauna collected includes the ammonite Hysteroceras subbinum, which ranges through all but the lowest part of the M. (M.) inflatum Zone. The conspicuously glauconitic lithology of the upper part of the Coulston section is analogous to the lithology seen at Westbury. The hard bed that caps the Coulston section may relate to the hard beds at the top of the Westbury section, but it lacks phosphatic clasts or the association of Pycnodonte (Phygraea) vesiculosum, and may instead represent a slightly lower hard bed that is not developed at Westbury.

A cutting adjacent to a footpath, about 650 m north-north-west of Devizes Castle, [ST 99860 61897] to [ST 99830 61940] exposes about 5.2 m of the Upper Greensand Formation, comprising locally clayey, micaceous glauconitic sandstone, with horizons and blocks of more cemented sandstone in the upper part (Figure 7). The co-occurrence of Callihoplites and Mortoniceras potternense suggests the C. auritus Subzone of the M. (M.) inflatum Zone. The lithology and biozonation suggest a level below the Potterne Rock Bed.

An exposure, about 300 m north-west of Devizes Castle, [ST 99949 61536] shows a 7 m section of Upper Greensand, comprising clayey micaceous glauconitic sandstone (Plate 1). Large, flattened ‘doggers’, occur approximately 2 m above the base of the section (Figure 8).

The fauna collected by M A Woods comprises Callihoplites and Mortoniceras potternense? Which suggests the C. auritus Subzone of the M. (M.) inflatum Zone. The lithology and biozonation suggest a level below the Potterne Rock Bed.

A further section (Figure 9) on western side of the A360 descending the hill to the south of Devizes [SU 00360 60400] to [SU 00240 60060] is crucial for the identification of the Potterne Rock and understanding how this horizon relates to other Upper Greensand successions seen in the district. The section was described by Jukes-Browne and Hill (1900) who state ‘At the beginning of the road cutting sandy malmstone is seen, and a little above there is a continuous section showing first soft yellowish micaceous sandstone containing large rounded calcareous ‘burrstones’. The upper part of this sandstone has a yellowish green tint, and rapidly passes up into soft greenish grey sand containing more glauconite and less mica than the sandstone below: of this sand there is about 4 feet (about 1.2 m), and it is succeeded by a hard dark grey calcareous rock which forms a continuous course from 18 to 20 inches (0.45–0.5 m) in thickness. This is the Potterne rock.’

(Figure 9) clearly shows the micaceous sands with large, rounded ‘doggers’ in the lower part which are referred to by Jukes-Browne and Hill (1900). The sudden increase in glauconite and decrease in mica also matches the interval slightly higher in the section, immediately below a 0.4 m-thick, laterally continuous hard bed, seen 6.2 m below the section top; this is almost certainly the Potterne Rock of Jukes-Browne and Hill. The only feature that they failed to note is the occurrence of small scattered phosphatic clasts in the glauconitic sandstone below the Potterne Rock and within the Potterne Rock itself. These serve to further distinguish this bed at other localities.

An exposure in the bank of Coxhill Lane, west-south-west of Stroud Hill Farm, near Potterne [SU 0012 5840] to [SU 0062 5842] exposes an estimated 15 m thick succession of Upper Greensand (Figure 10) and (Figure 11), comprising grey-weathering, micaceous sandstone at the base, passing up into pale yellowish orange-weathering micaceous sandstone. In the higher part of the section there is a change to greenish grey, conspicuously glauconitic, micaceous sandstone containing many scattered phosphatic clasts, capped by a succession of three indurated sandy limestones.

A variety of fauna was collected from this locality including ammonites, bivalves and gastropods. The hard limestones at the top of the succession and underlying glauconitic sands with scattered phosphatic clasts are analogous to the Potterne Rock and immediately underlying succession seen in a road cutting south of Devizes. The three limestones, perhaps, represent a locally expanded thicker development of the Potterne Rock; they are conspicuously burrowed and the lowest and middle limestone beds contain phosphatic clasts.

The contact of the Upper Greensand and basal Chalk Group is exposed in a lane leading up to Urchfont Hill [SU 03982 56592]. The Upper Greensand comprises soft, bright green to greenish grey glauconitic sand with an intensely hard indurated bed, ‘B’ of (Figure 12) 0.6 m from the top. The base of the Chalk Group is marked by a 0.16 m-thick bed of very coarse-grained, nodular and iron-stained glauconitic sand. The latter is overlain by marly glauconitic sand with phosphatic clasts becoming increasingly common towards the top of the exposure.

The increasing concentration of phosphatic nodules towards the top of the exposure suggests that there may have been two discrete erosion events; the first at the base of the Chalk Group, producing the bed of nodular, glauconitic sandstone, and the second perhaps within the mantelli Zone.

A cutting 900 m north-north-east from Sands Farm, Easterton Sands [SU 0195 5683] reveals a 14.5 m section of Upper Greensand (Figure 13), and can be divided into three broad intervals. The lowest 7 m are distinctly micaceous, glauconitic sandstone; this is the most fossiliferous part of the section, containing an ammonite-bearing phosphatic nodule bed about 1 m above the base. The sandstone is bioturbated throughout, with pale grey, clay-lined burrows and there are horizons of more indurated sandstone. Overlying the micaceous sandstone are about 3 m thick interval of very fine-grained, soft, glauconitic sandstone. Glauconite is more abundant and mica less abundant than in the lower part of the section, and there are small scattered phosphatic nodules throughout. This interval has a distinctly unbedded appearance, typically weathering as a smooth-faced ‘cliff’. Pale grey, clay-lined burrows are more conspicuous in the darker, grey-green lithology of this interval. A 0.43 m-thick, laterally continuous indurated greensand bed caps this interval, containing occasional scattered shell fragments, phosphatic fragments and sand-filled burrows that weather proud of the rock surface, caps the interval. An estimated 3.75 m of strata forms the highest part of the section, and although largely inaccessible, appears to comprise soft, glauconitic sandstone with a bed of indurated sandstone about 1 m below the top.

Callihoplites auritus is the index fossil of the auritus Subzone at the top of the M. (M.) inflatum Zone. The lithology of the laterally continuous hard bed and of the immediately underlying strata (with scattered phosphatic clasts in glauconite-rich sandstone) seen in the above section, compares closely with an interval currently exposed in a road cutting south of Devizes, and also recognised at several other localities during the recent survey (e.g. Urchfont [SU 0426 5717] (Figure 14); and Littleton Panell [SU 0025 5430] (Figure 15)). The Devizes cutting was described by Jukes-Browne and Hill (1900), who appear to identify the hard bed as the Potterne Rock. Previously, the Potterne Rock has been regarded as equivalent to the ‘Ragstone’ at the top of the Shaftesbury Sandstone of the Shaftesbury district (Bristow et al., 1999, p 72); this is about 3 m thick, indurated and oyster-rich interval with an inferred C. auritus Subzone age in the Shaftesbury district. However, the above findings suggest that at least 9.5 m of poorly consolidated sandstone below the Potterne Rock also correlate with the ‘Ragstone’. Evidence from elsewhere in the UK shows that there are dramatic lateral variations in the thickness of preserved C. auritus Subzone sediments. The base of the youngest Albian, S. dispar Zone, has been placed immediately above the ‘Ragstone’ in the Shaftesbury district (Bristow et al., 1995, p. 102), and therefore might be inferred to occur immediately above the Potterne Rock.

An exposure adjacent to a footpath, about 400 m at south-east of Wick Farm, near Littleton Panell, ([SU 0025 5430] SU05SW) revealed a 8.5 m section that includes a laterally continuous hard bed, 0.55 m thick, with phosphatic fragments and conspicuous burrows (Figure 15). This bed is believed to be correlative with that seen near the top of the succession elsewhere in the district and with that identified as the Potterne Rock near Devizes. Scattered black phosphatic nodules occur in fine-grained glauconitic sands 1 to 2 m below the hard bed, and pass downwards into more micaceous glauconitic sands with a large rounded ‘dogger’ about 1.5 m above the base. Grey-green, conspicuously glauconitic sands occur in the 2 m interval exposed above the Potterne Rock. A nearby road cutting [SU 0008 5411] did not yield a fauna, but appears to contain the equivalent of the large rounded ‘dogger’ horizon seen near the base of the above section. Thickness comparisons suggest that the top of this section is probably just below the horizon of the Potterne Rock.

A cutting in Peppercombe Lane (Plate 2), north-west of the church at Urchfont, [SU 0395 5738] reveals an approximately 7 m section of Upper Greensand, comprising dark grey, clayey glauconitic sandstone in the lower part and becoming increasingly less clayey and more micaceous towards the top. A 0.4 m-thick bed of sandy clay occurs around the middle of the succession and a phosphatic nodule bed occurs a little higher at about 4.2 m above the section base (Figure 16). The fauna collected, ammonites Anahoplites planus, Callihoplites sp. (crushed specimen, but possibly C. auritus), Hysteroceras bucklandi?, and Mortoniceras potternense? suggests assignment to the C. auritus Subzone, and indicates the development of a much thicker interval of C. auritus Subzone sediment strata compared to with the Wincanton district to the south-west. The absence of the bivalve Actinoceramus concentricus in the above fauna supports assignment to a level above the H. varicosum Subzone. The distinctly micaceous character of the sediment bed at the top of the section suggests a level below the Potterne Rock, and this is confirmed by a nearby locality [SU 0426 5717]. In the latter section, the presumed equivalent of the Potterne Rock comprises three more-or-less laterally continuous hard beds, about 0.95 m thick, underlain (as elsewhere) by fine-grained glauconitic sandstone with scattered phosphate nodules (Figure 14).

In the very north of the district, around All Cannings and Etchilhampton, exposures are found within the youngest parts of the Upper Greensand. Spoil from a swimming pool excavation, about 100 m west-north-west of Heath Knapp Farm [SU 05818 60407] includes the bivalves Aucellina gryphaeoides, Entolium orbiculare and Neithea quinquecostata. Field data suggest that material is likely to have come from the higher part of the Upper Greensand, between the Potterne Rock and the base of the Chalk Group. This inference is supported by the lithology seen in the excavation (Figure 17), which comprises 2.2 m of soft, dark green, glauconitic sandstone, analagous to that seen below the base of the Chalk Group near Urchfont. There are two horizons of hard, pale-weathering sandstone with scattered grains of dark glauconite, not seen elsewhere during this survey. The pale-weathering hard beds seen in the section may relate to the hard, speckled sandstone reported by Jukes-Browne and Hill (1900) about 9 m above the Potterne Rock, however, there is no sign of Merklinia aspera, the characteristic fossil associated with this marker horizon.

In summary, it was recognised that the Upper Greensand Formation has a consistent succession of lithologies across the Devizes district and that within this succession, the Potterne Rock forms a widely recognisable feature at outcrop (Figure 18).

Chapter 6 Upper Cretaceous

Rocks of Late Cretaceous age, overwhelmingly in Chalk facies, underlie the majority of the Devizes district. The nomenclature for the Upper Cretaceous utilised in this district is shown in (Table 11), together with its relationship to the traditional scheme. The current nomenclature is a development of the schemes devised by Mortimore (1983, 1986), by Bristow et al. (1995, 1997) and adopted by the Geological Society Stratigraphy Committee in 1999 (Rawson et al., 2001).

Traditionally in this district the Chalk has been discussed in terms of its biostratigraphical zonation related to two conspicuous hard bands in the succession that divide the Chalk into three unequal parts. The zones and their estimated thicknesses are given in Jukes-Browne (1905) and in (Table 12). The thickness estimate of Jukes-Browne (1905) for the Chalk is about 280 m.

Inceptions and extinctions of microfossils (foraminifera), and a zonal scheme based on those fossils is given in (Table 13), which also shows the lithostratigraphical correlation and that includes the known markers horizons.

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 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 this district the West Melbury Marly Chalk is 15 m to 30 m in thickness 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].

Elsewhere in the south of the district the formation forms the base of the outliers at Etchilhampton Hill [SU 03396 6021], All Cannings [SU 0713 6164], and around Woodborough Hill [SU 1185 6145].

The West Melbury Chalk consists predominantly of rhythmically bedded, pale to medium grey marly chalks with thin grey to brown limestones. The base of the succession is marked by grey marl with variable proportions of glauconite and glauconitic calcareous sandstone, the Melbury Sandstone Member. The base may be transitional with the Upper Greensand Formation in this district but elsewhere in southern England the boundary is placed at a strongly burrowed surface associated with a development of phosphatic nodules. Glauconite grains are locally common in the lower 3 m to 5 m of the chalk above the basal unit locally. The top of the formation is taken as the top of the Tenuis Limestone where that bed is present otherwise it is taken at the base of the ‘Cast Bed’ (Bristow et al., 1995, 1997) which is a distinctive pale brown silty chalk containing abundant small brachiopods. The limestones in the basal part of the succession may be spongiferous and occasionally rarely contain glauconite grains. A limestone rich in Schloenbachia occurs in the middle of the succession and is thought to be equivalent to the M3 limestone (sensu Gale, 1989) at Folkestone. The upper limestones of the West Melbury Chalk are generally poorly fossiliferous and sponge free. The glauconite sandstone and glauconite-rich argillaceous chalk in the basal few metres of the sequence in this district, is called the Melbury Sandstone Member as described in the Shaftsbury district (Bristow, et al., 1995). It is of lowest Cenomanian age and is directly equivalent to the Glauconitic Marl Member further east in Hampshire and Sussex.

The West Melbury Marly Chalk includes all the chalk of the Cenomanian M. mantelli, M. dixoni and C. inerme zones and the basal part of the T. costatus Subzone (A. rhotomagense Zone). Biostratigraphically the Tenuis Limestone is placed at the base of the Turrilites costatus Subzone of the Acanthoceras rhotomagense Zone. The West Melbury Marly Chalk generally forms an aquitard between the Upper Greensand and the Zig Zag Chalk due to its high clay content.

Details

The West Melbury Marly Chalk Formation crops out along the lower part of the escarpment to the south of Erlestoke and eastward into the West Lavington valley. Field brash consists of cream- to grey-coloured marls and grey-buff limestones. Ploughed fields are commonly waterlogged after heavy rainfall and general drainage is slow. The slopes are well-featured, concave with well-developed buttressing. A well-developed negative feature marks the base of the West Melbury Chalk, which is approximately 20 to 25 m thick in this area.

A number of small exposures occur in the Urchfont area and around The Lavingtons. An old pit, now a depot yard at Market Lavington [SU 0165 5390] exposes about 2–3 m of uppermost West Melbury Marly Chalk. The fauna here includes several specimens of the bivalve Inoceramus virgatus and the ammonites Turrilites scheuchzerianus and T. aff. costatus, collected from about 2 m of marly chalk. This indicates a position within the Lower Cenomanian, M. dixoni Zone of the West Melbury Marly Chalk Formation.

Another old pit on the western side of the lane to Urchfont Hill, [SU 0398 5659] exposes about 1.4 m of marly glauconitic chalk, representing the Glauconitic Marl Member, passing up into marly chalk with scattered grains of glauconite. Phosphatic nodules and phosphatised fossils occur in the sandier lower part of the section.

The fauna from the more sparsely glauconitic upper part of the section includes the bivalve Inoceramus virgatus, Inoceramus crippsi, Mantelliceras sp. and phosphatised Schloenbachia also occur in the section. (Locality 23, Woods, 2005). This is interpreted as the Lower Cenomanian, M. dixoni Zone of the West Melbury Marly Chalk Formation.

Following the base of the scarp eastwards, past Urchfont, exposures in the side of a footpath, south-east of Manor Farm, Conock, near Chirton [SU 07498 56327] contained the bivalve Inoceramus virgatus?, Inoceramus crippsi? and the ammonite Schloenbachia varians. (Locality 35 and 36, Woods, 2005). Confirming the Lower Cenomanian? M. dixoni Zone of the West Melbury Marly Chalk.

A temporary excavation about 1 km east of Stoke Farm, Beechingstoke, [SU 09707 59137] revealed a 1.8 m section, comprising sandy, glauconitic chalk with clay-infilled burrows and common phosphatic clasts, overlain by grey, marly chalk with scattered grains of glauconite. The fauna from the section included the bivalves Inoceramus crippsi and Inoceramus virgatus and the ammonites Hypoturrilites tuberculatus, Mantelliceras dixoni?, Mariella sp. and common Schloenbachia varians. (Locality 30, Woods, 2005). The lithology is typical of the Glauconitic Marl at the base of the Chalk Group, however, I. virgatus and M. dixoni? are indicative of the M. dixoni Zone, which in basinal successions typically occurs some distance above the Glauconitic Marl. The M. mantelli Zone, which is the usual horizon of the Glauconitic Marl, is probably represented by the many, mostly phosphatised, fragments of Schloenbachia varians, and the single fragment of Hypoturrilites tuberculatus. The evidence from this and similar exposures in the district is that the M. mantelli Zone has been condensed down into a phosphate-rich remanié deposit in the lower part of the Glauconitic Marl. In the Wincanton district to the south-west, the M. mantelli Zone is represented by the Melbury Sandstone (Bristow, 1999), and the base of the West Melbury Marly Chalk is in the M. dixoni Zone. The Melbury Sandstone is typically a fine-grained sandstone (Bristow et al., 1999), unlike the coarse, glauconitic lithology seen in the current exposure.

In the far north-west of this district, the West Melbury Marly Chalk is up to 25 m in thickness and crops out along the base of the scarp below Roundway Hill and as outliers separate from the scarp around Roundway Park, centred around [SU 012 626] and around Etchilhampton Hill [SU 033 601] extending northward towards the hill [SU 024 617] south of Coate Grove House [SU 024 620]. A further outlier lies to the south of Devizes at Potterne Field [006 590]. Under optimum conditions the formation is delimited by a weak negative feature from the underlying Upper Greensand, and by a somewhat stronger negative feature from the overlying Zig Zag Chalk Formation. Within this area there are numerous weak features that may show a brash consisting either of pale grey to white, hackly fractured, moderately hard to hard, marly limestone or grey-brown calc-arenitic limestone brash. The base is marked by a brash of phosphatised fossils and phosphatic nodules within a marly soil that also contains glauconite grains. Augering confirms a calcareous (marly) fine- to medium-grained glauconitic sand with phosphate nodules and fossils overlying green, glauconitic, fine- to medium-grained sand (stone) of the Upper Greensand Formation. The top of the formation includes a least two beds of pale brown calc-arenitic limestone with an intervening soft sandy marl a short distance below a hard brown calcarenite with frequent numerous Entolium. This fossil-rich limestone is associated with a break of slope and herein equated with the Cast Bed or lateral equivalent of the Totternhoe Stone Member of the Chilterns and forms the basal bed of the succeeding formation. It appears from field evidence in this district that the lower part of the formation is absent or more likely condensed as there is little evidence for the spongiferous limestones found elsewhere.

The basal Glauconitic Marl Member has been positively identified, either in brash or by augering at near Roundway, north of Devizes [SU 0014 6413], [SU 0083 6348], [SU 0115 6351]. It is seen again, commonly, over the outlier to the west of Roundway Park House, (for example by augering at [SU 0106 6259], [SU 0170 6361]) and as a copious surface brash containing many phosphatic fossils at [SU 0112 6238], [SU 0137 6247]. These surface brash localities contain a fauna indicating the Mantelliceras mantelli and M. dixoni zones and thereby suggest that the lower part of the West Melbury Marly Chalk Formation is considerably condensed in this area. The faunas are detailed in Woods (2005, localities 47, 48, 49, and 50).

The lower boundary of the West Melbury Marly Chalk Formation within the outlier around Etchilhampton Hill northward to Coate Grove House is commonly identified in auger holes proving strongly glauconitic marls and is also associated with the weak negative feature but there is no evidence from brash of the basal phosphatic fossil bed seen to the west. The fauna on the east flank of Etchilhapton Hill [SU 04039 60670] includes the brachiopod Terebratulina sp. and the bivalves Inoceramus schoendorfi?, Limaria elongata, Oxytoma seminudum? and Plagiostoma globosum (Locality 42, Woods, 2005). The record of I. schoendorfi? suggests assignment to the C. inerme Zone or basal A. rhotomagense Zone (below the Cast Bed), from which the presence of the highest West Melbury Marly Chalk might be inferred.

Elsewhere in this outlier the higher beds of the formation are confirmed by fossils found at localities 46, 58 and 59 detailed in Woods (2005). The extensive fauna from the park area was collected by Mr Cunnington and given on pages 28 and 29 in the memoir (Jukes-Browne, 1905).

There are no current exposures of the formation in this north-western area. The old pits at Coate Grove House [SU 024 620] (see reference to Clay Hole below) and 900 m south [SU 0426 6106] of the crossroads in Coate on the east side of the road to Etchilhampton, are both degraded and the former is also now the landscaped garden of the house. Gypsy Path track and the track southward from Coate Grove House towards Etchilhampton Hill both show alternating limestones and marls in their worn track beds. In the memoir (Jukes-Browne, 1905) the lower beds of the Lower Chalk are seen at a number of localities. The text is given below:

‘Tough blocky chalk marl has been dug from several pits on the slope east of Nursteed, but these did not give clear sections in 1888. Fragments of Ammonites [Schloenbachia] varians occurred.’ The last of these pits is north-east of Broadway Farm [SU 0239 6117] but also at [SU 0242 6086].

‘The lowest beds of the Chalk Marl have been dug at Clay Hole [SU 0246 6198] (Coate Grove House) half a mile north of Broadway Farm.’

To the north-east of the district near Burbage, an exposure within an old chalk pit, about 800 m at 220° from Harepath Farm, [SU 22311 60172] revealed about 0.4 m of marly, glauconitic chalk with phosphatic clasts. The fauna included the bivalves Inoceramus virgatus (several specimens) and Plicatula inflata (large specimens) and the ammonites Mantelliceras? and Schloenbachia varians. (Locality 71, Woods, 2005). As seen elsewhere in the district, the Glauconitic Marl includes the M. dixoni Zone, the relative frequency of I. virgatus perhaps suggesting a level at or slightly above the middle of that zone.

Zig Zag Chalk Formation (ZCk)

The Zig Zag Chalk Formation is typically composed of 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 contains some thin limestones. Some distance above the base of the formation, 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, I. pictus and the echinoid Holaster subglobosus. No flints are recorded in this area. The upper limit of the Zig Zag Chalk is taken at the base of the Plenus Marls Member. The base of the formation falls within the basal Turrilites costatus Subzone and the top is coincident with the top of the Calycoceras guerangeri Zone.

In this district the formation is exposed as a narrow outcrop along the Chalk escarpment forming the southern flank of the Vale of Pewsey and as outliers around Etchilhampton, Alton Priors, and just to the south of Burbage.

The base of formation is commonly at a strong negative slope break at the base of the Chalk escarpment. This abrupt change in slope appears to correspond with the incoming first occurrence 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 commonly identified as a weak negative slope break beneath the strong bluff developed on the Melbourn Rock Member.

Details

In the western part of the district, the formation makes up the steep convex slopes of the escarpment to south of Erlestoke and Little Cheverell. In this area the formation is quite thick at between 40 and 50 m. Few outcrops are seen in this area, but fragments of smooth, blocky grey chalk can be seen around rabbit burrows and badger scrapes. Towards the base of the formation an increased amount of harder grey limestone is seen as bands within the chalk. On this escarpment a negative feature marks the base of the formation.

A small (1 m) exposure was seen near Great Cheverell Hill [ST 9672 5197]; this exposed the weathered grey blocky chalk with overlying slope wash and soil horizons (Plate 3).

Eastwards around Urchfont and the Lavingtons the Zig Zag Chalk Formation continues to form the lower part of the escarpment and the typically buttressed slopes can be observed to the east of Market Lavington. Field brash consists of soft to crisp grey chalks, in marly soils and an occasional some nodules of limestone are seen in brash where this coincides with a limestone band in the sequence. No exposures were noted in this area at the time of survey.

Towards the centre of the Devizes district, a section adjacent to the track to Wilsford Hill, [SU 0965 5587] reveals large terebratulid brachiopods (possibly including Concinnithyris) and thin-tested echinoid fragments from a chalk horizon below the Plenus Marls.

In the Avon valley, around Upavon, the Zig Zag Chalk Formation continues at outcrop around the foot of the primary Chalk escarpment. A small exposure of massive, soft, blocky, grey marly chalk occurs in the side of a tank track at [SU 1329 5400]. This section must be near the top of the formation as hard nodular Holywell Nodular Chalk occurs a short distance up the track to the south-west. The formation forms the floor of the broad valley to the west of Upavon (Plate 4).

In the far north-west of the district around Devizes, the Zig Zag Chalk Formation outcrops in the lower face of the steep scarp beneath Roundway Hill and as two outliers, at Etchilhampton Hill, and forming the cap of the hill south of Coate Grove House. In this area the formation gives rise to a grey-brown marly but friable soil containing a variable brash of small flaggy to equant blocks of hard grey limestone and softer off-white chalks. The outcrop has a number of positive features each with a different limestone/chalk brash. A conspicuous positive feature some 25 to 30 m below the top of the formation is associated with an intensely hard, hackly fractured limestone brash that has incipient flints (hard, entirely white, siliceous burrow-form fragments) associated with it. This bed is seen around Etchilhampton Hill and again, just north of the district, in the steep scarp slopes 600 m west-north-west of Roundway Farm [SU 00914 63699]. Here, deep subsoiling has brought large blocks (up to 80 x 40 x 30 cms) to the surface that contain a fauna indicative of the upper part of the Turrilites costatus Subzone of the Acanthoceras rhotomagense Zone (see Woods, 2005, locality 54). Woods (2005) recorded several specimens of Sciponoceras, associated with Schloenbachia, which suggests possible assignment to the middle part of the A. rhotomagense Zone. Towards the top of the T. costatus Subzone, at and slightly above the highest of the three widely occurring acmes of O. mantelliana, there is a group of limestones containing abundant heteromorph ammonites, including Sciponoceras (Mortimore et al., 2001).

Part of the Zig Zag Chalk Formation is seen in the small exposure [SU 03162 60017] 180 m south-west of the triangulation point on Etchilhampton Hill (Plate 5) and (Plate 6). Here, about 7 m of large blocky chalk, weakly calcarenitic in parts is attributed to the T. acutus Subzone of the A. rhotomagense Zone (see Woods, 2005, locality 56). This is probably the same locality mentioned in the memoir (Jukes-Browne, 1905) and copied in full below.

‘Where the new road crosses the old cart road is another and larger pit showing hard whitish chalk with minute green grains, breaking in massive blocks. The bedding is obscure, but seems to dip toward the north-west; a large Ammonites [Acanth.] rotomagensis, Am. [Schloenbachia] varians and Pecten orbicularis were found here.’

At this same locality Woods (2005) also recorded brachiopods: Concinnithyris subundata (large), terebratulids (possibly including Concinnithyris), bivalvia: Inoceramus atlanticus, I. atlanticus (5), and ammonoides: Schloenbachia sp. from a 6.5 m-thick succession of marly Zig Zag Chalk with scattered marcasite nodules.

During the recent survey parts of the Salisbury Plain Training Area, just outside the district to the west, were also mapped. Due to their close proximity to the Devizes district, the important notes and sections have been included below.

Several small sections in the upper part of the Zig Zag Chalk Formation occur along the Southern Transit Route of the Salisbury Plain Training Area just outside the district to the south-west. A cutting at East Hill Farm [ST 9374 4415], 300 m at 097° from East Hill Farm, north of Heytesbury shows approximately 1.2 m of blocky grey smooth marly chalk near the top of the Zig Zag Chalk Formation, overlain by 0.9 m of Plenus Marls Member (basal Holywell Nodular Chalk Formation). At the eastern end of the section the outcrop of the Plenus Marls is truncated by a fault of unknown displacement. The brachiopod Terebratulina nodulosa was found in the Zig Zag Chalk Formation. The fauna from the Plenus Marls includes a macroconch of the ammonite Calycoceras (C.) naviculare, the brachiopod Orbirhynchia multicostata and large specimens of the oyster Pycnodonte vesiculare, all typical of this horizon. The Plenus Marls belong to the M. geslinianum Zone, and the underlying Zig Zag Chalk Formation can be inferred to represent the C. guerangeri Zone.

A cutting 1 km to the north-west [ST 9312 4493] exposes the contact between the Zig Zag Chalk Formation and the Holywell Nodular Chalk Formation (Plate 7). The Plenus Marls are poorly exposed as the contact is cut by many minor faults and is highly weathered.

Small outcrops of blocky, smooth, grey, unfossiliferous marly chalk were seen in an old quarry just north of the A360 roundabout at the eastern end of the Warminster Bypass [ST 9178 4298].

The Zig Zag Chalk Formation forms the steep buttressed slopes to the north of and beneath Bratton Castle, of Combe Bottom and of Longcombe Bottom (Plate 8). There are few exposures and the ground is principally in managed or rough pasture. There are a few roadside exposures of variably hard grey-brown to off-white chalk on the eastern part of Castle Road (Port Way) leading from Bratton village to Bratton Castle. The trackway leading from Bratton up the scarp to White Hill passes through a narrow defile that might formerly have been a quarry [ST 9161 5182] and numerous small brash outcrops can be seen including a soft grey-brown arenitic limestone some 25 m up from the base of the formation (possibly equivalent to the Jukes-Browne Bed 7).

Within Longcombe Bottom [ST 920 514] numerous animal tracks that contour the coombe show variable pale grey and off-white chalk and limestone exposures. At [ST 9226 5150] a small exposure [ST 9226 5150] of grey brown arenitic and soft marly chalk, no more than 0.3 m, thick is seen between two pale grey limestone beds. On the east side of Imber Road that climbs the scarp in the valley that Longcombe Bottom joins there a many small scrapings and bluffs in animal and man-made terracing (Strip Lychetts?) that show a variety of soft to hard marly chalks and limestones.

White Chalk Subgroup (WCK)

The White Chalk Subgroup is essentially the combined Upper and Middle Chalk formations of Bristow et al. (1997). 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 (Table 11). In general the subgroup is characterised by white chalk with numerous flint seams, with nodular chalks in the lower part. The White Chalk is divided into seven formations (Table 11), of which, the lower six occur in this district. Up to a maximum 260 m of the White Chalk is estimated to crop out in this district. The Netheravon Borehole [SU 169 483] proves an estimated 138 m of the White Chalk Subgroup and is known to commence in the highest Seaford or lowest Newhaven Chalk.

Holywell Nodular Chalk Formation (HCk)

The Holywell Nodular Chalk Formation comprises generally 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. It crops out along the face of the primary Chalk escarpment, in re-entrant valleys and as an inlier in Water Dean Bottom. It is between 15 m and 25 m thick. The formation is also present around Knook in the south-west of the district and more extensively outside the area towards Warminster. The Melbourn Rock Member often forms a strong positive feature in places and characteristic brash that can be traced around the outcrop.

The Plenus Marls Member is rarely well exposed but is present along the whole outcrop of the Holywell Nodular Chalk Formation. The Plenus Marl 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), can be up to 50 cm thick (Bristow et al., 1995). The Plenus Marls, coextensive with the greater part of the Metoicoceras geslinianum Zone, contain common Inoceramus pictus, as well as the eponymous belemnite Praeactinocamax plenus.

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 a feature-forming unit up to 3 m thick. The overlying shell detrital 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 was taken at the highest recognisable shell detrital chalk during surveying.

Biostratigraphically, the Holywell Nodular Chalk Formation spans the Cenomanian–Turonian boundary, which lies close to the top of the Melbourn Rock Member. The greater part of the Holywell Nodular Chalk is in the Mytiloides spp. Zone (this usage follows Hopson et al., 1996). The base of Gaster’s (1924) ‘I. labiatus Zone’ coincides with base of the Melbourn Rock. The formation covers the Metoicoceras geslinianum Zone, the Neocardioceras juddii Zone (entirely in Melbourn Rock facies) and much of the Mytiloides spp. Zone.

Details

In the far south-west of the district the Holywell Nodular Chalk Formation is seen at outcrop on the steep lower to middle flanks of the upper Berril valley. Its base is defined below the strong positive feature of the Melbourn Rock Member. Its upper boundary is defined where the change from hard, nodular grainy chalks to firm to moderately hard, smooth, white chalks of the New Pit Chalk Formation occurs. There are numerous good exposures of the base of the Holywell Nodular Chalk Formation and the Plenus Marl Member–Melbourn Rock Member contact in this area. The remainder of the formation is well represented in worn tracks rising up the eastern flank of the valley. The change from grey, soft chalk of the Zig Zag Chalk Formation to the nodular hard chalk of the Melbourn Rock Member occurs over a short distance at [ST 9543 5104] and the remainder of the Holywell Nodular Chalk Formation is well exposed in the track rising to the north-east [ST 9056 5114]. The same Plenus Marl–Melbourn Rock contact is seen farther north-east at [ST 9518 6109]. The Plenus Marls have been excavated by a badger nearby at [ST 9511 5204].

A very good exposure of the Holywell Nodular Chalk Formation occurs on the track that rises from the Brouncker’s Well area along a track rising up the spur to the south-east of Brouncker’s Down. This exposure is shown in (Plate 9) and (Plate 10).

The Melbourn Rock Member forms the strong positive at the top of the escarpment to the south of Erlestoke, and is underlain by the Plenus Marls which mark the base of the Holywell Nodular Chalk Formation. In this area the Holwell Nodular Chalk Formation is about 15 m thick. It caps the ridge on Pear Tree Hill on the top of the escarpment and here is probably limited to just the Melbourn Rock as the overlying Holywell has been eroded.

Continuing eastwards past The Lavingtons and towards Urchfont small exposures can be seen in the sunken trackways descending the scarp slopes, e.g. [SU 04330 55850]. The Holywell Chalk underlies the broad valley south of West Lavington, and here, the brash consists of intensely hard nodules of grainy white chalk with abundant shell debris including Mytiloides. No section exposures were noted at the time of survey.

Continuing eastwards towards the centre of the Devizes district, a section in the track up to Wilsford Hill, [SU 0964 5586] exposed the Plenus Marls Member at the base of the Holywell Nodular Chalk Formation, and the fauna present included the inoceramid bivalve Inoceramus pictus. The Plenus Marls is the probable stratum typicum of Inoceramus pictus.

An old chalk pit near Marden Copse, south-east of Chirton [SU 0849 5540] exposed the Plenus Marls Member with large specimens of the oyster Pycnodonte vesiculare and the bivalve Inoceramus pictus. The Plenus Marls are 0.8 m thick at this locality, and underlain by 1.4 m of Zig Zag Chalk Formation containing burrows infilled with greenish grey marly chalk. About 6.5 m of hard, nodular chalk occurs above the Plenus Marls at this locality. This relatively thin Plenus Marls succession may be a consequence of shearing, since a fault that significantly offsets the Plenus Marls and overlying Melbourn Rock on the western side of the quarry appears to become subhorizontal as the main (southern) face is approached. The presence of large Pycnodonte vesiculare characterises the lower part of the Plenus Marls (Jefferies, 1963), and I. pictus is common at several horizons in this unit. Brash from burrows in the slopes above this old pit [SU 0852 5540] contains the brachiopod Orbirhynchia sp. and the bivalves Mytiloides mytiloides and Mytiloides ex gr. hercynicus-subhercynicus. This fauna is indicative of the uppermost Holywell Nodular Chalk and basal New Pit Chalk formations — lower Turonian, Mytiloides spp. Zone and T. lata Zone.

Another exposure in the side of a footpath, near Manor Farm, Conock (near Chirton) [SU 0708 5570] comprises specimens of the brachiopod Orbirhynchia, including O. cuvieri? and the bivalve Mytiloides? confirming the identification of upper Holywell Nodular Chalk Formation.

An old chalk pit about 650 m at 231° from the church at Charlton, [SU 1120 5565] exposes a 3.7 m-thick succession of weakly nodular shell-rich chalk in the lower 0.7 m overlain by poorly shelly, non-nodular chalk with regularly developed marls. The lowest of these marls is a conspicuous, 60 mm-thick seam, 0.8 m above the top of the shell-rich chalk. The fauna from the shelly chalk in the lower part of the succession includes fragments of the inoceramid bivalve Mytiloides. Large terebratulid brachiopods and a possible fragment of Mytiloides ex gr. hercynicus-subhercynicus occur in the overlying succession. This lithology and fauna suggest that the section exposes the junction of the Holywell Nodular Chalk and New Pit Chalk formations. The conspicuous marl is probably the Lulworth Marl of Gale (1996) (equivalent to the Gun Gardens Main Marl of Mortimore, 1986).

An old chalk pit about 1.05 km at 054° from the church at Upavon [SU 1439 5564] exposes 3.1 m of hard, nodular chalk with plexus marls and contains specimens of Mytiloides spp., including large M. mytiloides encrusted with serpulids. The chalk is spectacularly shelly, with many large, semi-complete shells of Mytiloides. The fauna is indicative of the Mytiloides spp. Zone and the middle to upper part of the Holywell Nodular Chalk Formation. The serpulid-encrusted Mytiloides might represent the Filograna avita event, recognised at many southern England localities in the Holywell Nodular Chalk Formation (Gale, 1996, fig. 4).

In the Avon valley, around Enford, the Holywell Nodular Chalk Formation is exposed in two sections along the tank track between [SU 1329 5400] and the military Northern Transit Route at [SU 1164 5287]. Good hard nodular shelly chalk with fragments of Mytolides can be seen in a track rut at [SU 1282 5343], and passes up into less nodular hard shelly chalk. About 450 m to the north-east, another small cutting exposes hard, nodular shelly chalk at the base of the Holywell Nodular Chalk Formation. A small inlier of Holywell Nodular Chalk occurs in the floor of the Water Dean valley. Fragments of hard nodular chalk with Mytolides fragments are exposed at several places in tank tracks, rabbit scrapes and earthworks, including [SU 1076 5226], [SU 1015 5285], [SU 1018 5274].

In the Devizes and Rowde (Seend) area in the north-west of the district, the formation forms the upper slopes and immediate hinterland on the Roundway Hill scarp from Oliver’s Castle [SU 0020 647] to Roundway Hill itself [SU 023 647], and forms an outlier on the top of Etchilhampton Hill [SU 0330 6015] where only the lowest part of the formation is present. The Plenus Marls are not seen at outcrop in this area being masked by brash from the overlying Melbourn Rock. Typical field brash consists of abundant nodular buff-coloured intensely hard chalk passing up into nodular and grainy hard chalks containing pink fragments of Inoceramid bivalves, most notably Mytiloides. On Etchilhampton Hill only the lowest non-shelly brash is present.

In the extreme south-west of the district, west of Chitterne, a narrow outcrop around the valley that runs southward to Knook Camp carries brash with Mytiloides but the basal Melbourn Rock feature is not seen in this area.

To the west, just outside this district, there are a number of significant exposures of Holywell Nodular Chalk Formation on the Southern Transit Route. The first is about 2 km north of Heytesbury at [ST 9316 4494] to [ST 9346 4500]. Here, approximately 2 to 3 m of blocky, hard, nodular shelly chalk with common shell fragments of Mytiloides is exposed. Many minor faults cut the section. Another section occurs 450 m to the east at [ST 9360 4493] which exposes the Holywell Nodular Chalk–New Pit Chalk boundary, and is described in the New Pit Chalk section below.

A section occurs about 600 m at 343° from West Hill Farm [ST 9246 4510] to [ST 9302 4494]. This exposes up to 1.2 m of marly chalk, overlain by 5.7 m of hard, nodular chalk. The lowest 0.7 m of succession immediately above the marly chalk is very strongly indurated. The fauna comprises shell fragments of Mytiloides, collected from near the top of the section. The marly chalk at the base of the section is presumed to be the Plenus Marls, immediately overlain by the Melbourn Rock.

Good specimens of Mytolides mytolides were found at around West Hill Farm [ST 9244 4401], [ST 9418 4425]. The Plenus Marl and the contact with the underlying Zig Zag Chalk Formation was noted just north of Knook Camp at [ST 9350 4287]. Here, hard, weakly shelly, nodular chalk and fragments of grey marl can be seen in the side of the track leading to the northern gate.

The Hollywell Nodular Chalk Formation forms a strong feature from Bratton Castle in the west, within the scarp above Combe Bottom (Plate 11), and around Longcombe Bottom in the east. It appears again in the deep valley from Farm Down Plantation [ST 5090 5025] southwards and eastwards towards Stoke Hill. The Plenus Marls Member is seen in brash only at White Cliff near Bratton [ST 9223 5156] where grey soft marly chalk is seen immediately beneath the feature formed by the Melbourn Rock Member feature. The very hard to intensely hard, commonly strongly nodular, pale grey to white Melbourn Rock Member chalk brash is seen at a number of localities closely associated with a positive feature along the outcrop to the south of Bratton at [ST 9042 5180], [ST 9158 5172], [ST 9213 5131], [ST 9220 5135].

On the track and footpath from the barn [ST 9153 5096] on the Northern Transit Route to White Hill Cliff [ST 9155 5153] and onward to the north to the top of the scarp at [ST 9154 5173] the higher beds within the formation are well represented in a copious brash that includes hard, nodular, grainy chalk, intensely hard, nodular, smooth chalk and most characteristically hard to very hard, off-white grainy and nodular chalks packed with three dimensional Mytiloides mytiloides. These same beds are seen around Farm Down Plantation [ST 5090 5025] in the valley to the south.

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. The top of the New Pit Chalk Formation is marked by the incoming appearance of nodularity that generally occurs between Glynde Marl 1 and Southerham Marl in the standard Sussex succession. The New Pit Chalk Formation forms sloping ground within the primary Chalk escarpment, above the first positive feature. It is usually softer than both the underlying Holywell Nodular Chalk Formation and the overlying Lewes Nodular Chalk Formation, and generally forms a slight negative feature in the scarp.

The base of the New Pit Chalk Formation is marked by the disappearance of inoceramid-rich nodular chalk. The upper limit is marked by the incoming of hard nodular chalks and flints. Flints in the New Pit Chalk Formation are rare. Where present they are small and occur in the uppermost beds. The fauna is much sparser than in the Holywell Nodular Chalk Formation and mostly comprises 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.

The New Pit Chalk Formation belongs to the uppermost part of the Mytiloides spp. Zone and all but the highest part of the Terebratulina lata Zone. In the standard sections in Sussex, the formation covers the interval between the Gun Garden Main Marl up to the base of Glynde Marl 1 (Mortimore, 1986) but following Mortimore and Pomerol (1996), BGS has mapped strata above Glynde Marl 1 as upper New Pit Chalk.

Details

The New Pit Chalk Formation crops out within the middle to upper valley slopes of the upper Berril valley and its tributary valleys and is also seen in the upper valley around New Zealand Farm [ST 973 508]. There are very few exposures throughout the outcrop and the formation is generally mapped by reference to the features bounding the Holywell Chalk, below, and the Lewes Nodular Chalk Formation, above.

Many exposures in the area are only afforded by the deeply worn tank tracks such as the one that rises from the Berril valley floor. Here soft to moderately hard, white, poorly fossiliferous, blocky chalks with few flints and only thin grey marls are seen over 80 to 100 m of track from [ST 9555 5113] to [ST 9563 5117]. A small, weathered and degraded exposure near a tank crossing on the south side of the Berril valley east-south-east of Imber village and 400 m at 110° from Jeapes Coppice, [ST 9818 4794] shows a few metres of soft–crisp weathered chalk. The fauna comprises the bivalve Inoceramus cuvieri in firm and hard, predominantly non-nodular chalk, and suggests assignment to the T. lata Zone and the upper New Pit Chalk Formation. The presence of rare nodular chalk suggests proximity to the base of the Lewes Nodular Chalk Formation, which occurs on the other side of the valley. Elsewhere smooth chalks of varying hardness are found in the soil brash but the heavier and more robust brash from the overlying Lewes Nodular Chalk Formation commonly disguises the New Pit Chalk Formation outcrop.

This formation also has an outcrop around the Knook valley and in the bottom of the Chitterne valley on the southern sheet boundary of the district. There are no good exposures in the Knook valley, only one small exposure at [ST 9849 4306] that shows a few metres of moderately hard, flint-free, smooth, white chalk in a degraded pit.

Towards the central part of the district the New Pit Chalk Formation floors the valley from north to south across the Westdown Artillary Range and has narrow extensions into tributary valleys. There are no good exposures and the formation is principally delimited by the base of the overlying Lewes Nodular Chalk Formation. A small pit spanning the New Pit–Lewes Nodular Chalk Formation contact at West Down Artillary Range [SU 0435 5033] is now heavily degraded and only weakly glauconitic, intensely hard, nodular to porcellanous chalk of the basal Lewes Nodular Chalk Formation is currently visible. The major worn tracks over the valley [SU 0431 5053] to [SU 0444 5036] and [SU 0445 5064] to [SU 0465 5065] show firm to moderately hard, smooth chalk in places but the overlying hard nodular chalk of the Lewes Nodular Chalk Formation Lewes commonly masks the exposures.

Northwards towards Urchfont and The Lavingtons, the New Pit Chalk Formation is relatively thin and much harder than typically. It is however smoother and less grainy than the underlying Holywell Chalk. New Pit Chalk is exposed in the valley bottom south of Urchfont Hill [SU 0470 5550], within the downlands of the Salisbury Plain Training Area. Here the valleys are incised, downcutting through the geological sequence. Field brash consists of hard smooth off-white chalk lacking in flints.

Farther eastwards, a small exposure in the side of a footpath, about 1.44 km at 170° from Manor Farm, Conock [SU 0704 5567], contained fragments of the bivalve Mytiloides ex gr. hercynicus/subhercynicus confirming a horizon within the basal New Pit Chalk Formation and the Turonian, T. lata Zone.

Another nearby locality at [SU 0690 5547] exposed the ‘Chalk Rock,’ near the base of the Lewes Nodular Chalk Formation. No fauna was collected.

The New Pit Chalk Formation is generally poorly exposed in Avon and Water Dean Bottom valleys, but much disturbed chalk is exposed in the military Casterley demolition area at [SU 10182 53105] and [SU 10192 5313] about 2.5 km at 250° from Widdington Farm. Here, a wide area of ground is covered in numerous craters, and small pits that expose lumps of blocky, smooth, white chalk, with rare small flints. The fauna from the two localities collected hereabouts comprises the inoceramid bivalve Mytiloides ex gr. hercynicus/subhercynicus, indicative of the lower T. lata Zone, and by inference, the lower part of the New Pit Chalk Formation. Nearby, a tank track rut [SU 1036 5311] exposes smooth, white blocky chalk overlain by hard nodular Lewes Chalk.

Where Water Dean Bottom joins the Avon near Compton, approximately 1 m of blocky, smooth white Chalk is exposed in a small pit in a private garden [SU 1315 5212]. The top of the New Pit Chalk Formation is commonly marked by a well-developed positive break of slope and is exposed in a track near Widdington Farm [SU 1246 5335]. Here, four closely spaced well-developed marl seams separated by smooth blocky white chalk can be identified. Hard glauconitic nodular chalk hardgrounds occur a short distance above.

Immediately to the west of the Devizes district the New Pit Chalk Formation is exposed in three sections, two on the Southern Transit Route around [ST 935 447] and one on the road between the Southern Transit Route and Knook Camp at Bevin’s Barn [ST 938 431]. The section on top of the ridge at [ST 9358 4462] about 450 m at 022° from East Hill Farm cuts through about 3 m of soft white, blocky, flaky, weathered chalk. Specimens of Inoceramus cuvieri were collected from about 2.5 m of firm to hard chalk with marl horizons. I.cuvieri is typical of the upper T. lata Zone, occurring in the higher part of the New Pit Chalk and the lower part of the Lewes Nodular Chalk Formation. Although the chalk exposed at this locality is much harder than typical New Pit Chalk, there is no evidence of the conspicuous nodularity that marks the base of the Lewes Nodular Chalk at Westbury. The section is therefore presumed to represent the higher part of the New Pit Chalk Formation. Hard nodular chalk is known to crop out a short distance to the north. Just to the west, a large cutting on the Southern Transit Route at [ST 9360 4496] to [ST 9354 4474] about 650 m at 013° from East Hill Farm, exposes nearly the entire thickness of the New Pit Chalk. The basal contact with the Holywell Nodular Chalk can be seen low down in the cutting at approximately 130 m OD, while the base of the Lewes Nodular Chalk Formation occurs on top of the ridge, just above the cutting at 145 m OD. The New Pit Chalk is about 15 to 20 m thick here. A fauna was collected by M Woods from a 9 m section, comprising hard, nodular, shell-rich chalk in the lower part, overlain by smoother textured, non-nodular chalk with a medium to large nodular flint about 1.5 m below the top of the section. The junction of these contrasting lithologies is marked by a marl seam. The fauna from the lower, nodular interval, comprises shell fragments of Mytiloides, including M. mytiloides. The higher part of the section contains Mytiloides ex gr. hercynicus/subhercynicus, a large terebratulid brachiopod, and the ammonite Collignoniceras?. The fauna shows that the marl seam that separates the different chalk lithologies in the section is the likely correlative of the Gun Gardens Main Marl/Lulworth Marl, which marks the junction of the Holywell Nodular Chalk and New Pit Chalk formations. The Knook Camp section [ST 938 431] is a shallow cutting no more than 2 m deep, which exposes approximately 9.5 m of New Pit Chalk. The lower 3.7 m consists of massive slabby, blocky, smooth white chalk, generally unfossiliferous with a few small nodular and rounded flints. Three major marl seams occur: one at 3.7 m, is 8 cm cm thick, a second, is 5 cm thick at 4.3 m, and a third 10 cm thick, 6 m above the base of the section. Micropalaeontological samples [ARF 1530; ARF 1531] suggest that the two lower marl seams occur in the basal New Pit Chalk. The upper marl seam is in the upper New Pit Chalk (ARF 1532, micropalaeontological zone BGS11iii). At 6.5 m up from the base of the section is a zone 0.6 m thick of hard, blocky and weakly nodular chalk, with an acme of bivalves. Above this is about 1.5 m of massive generally blocky smooth white chalk that is locally fossiliferous. A micropalaeontological sample at 8.2 m [ARF 1533] indicates the upper New Pit Chalk (Zone BGS11i). Above is good hard nodular chalk and very hard smooth, blocky, locally nodular porcellanous chalkstone, which marks the base of the Lewes Nodular Chalk Formation. Macropalaeontological specimen numbers: ARF 1436–1452, collected 6.6 m above the base of the section include samples of nodular chalk. The fauna comprises specimens of the inoceramid bivalve Inoceramus cuvieri. ARF 1438–1446, upper New Pit Chalk and ?basal Lewes Nodular Chalk Formation is of the Turonian, upper T. lata Zone.

Lewes Nodular Chalk Formation (LeCk)

The Lewes Nodular Chalk Formation comprises interbedded hard to very hard nodular chalks and hardgrounds with soft to medium-hard grainy chalks and marls. 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. The flints are typically black or bluish black with a thick white cortex. Sheet flints are common. The formation is between 15 and 40 m thick within the district. It includes the ‘Chalk Rock’ of traditional usage (Bromley and Gale, 1982) at its base.

In this district, the Lewes Nodular Chalk Formation forms the highest steep slopes at the top of the primary Chalk scarp (and dip slopes beyond onto the major interfluves).

In the standard Sussex succession the Lewes Nodular Chalk Formation includes the strata from the Glynde Marl 1 to the base of the Shoreham Marl 2 (Mortimore, 1986) but this was subsequently modified in Mortimore and Pomerol (1996, fig. 2) where the Lewes Nodular Chalk Formation commences above the marl at the inception of nodularity. In exposed sections the Lewes Nodular Chalk Formation can be divided informally into two units. The lower is mainly medium- to high-density chalks and conspicuously iron-stained hard nodular chalks whilst the upper is mainly low- to medium-density chalks with regular thin nodular beds. Between the two informal units the boundary is marked by the Lewes Marl and an extensive system of black cylindrical burrow-form flints called the Lewes Flints. 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 Nodular Chalk Formation. The Lewes Nodular Chalk includes the top of the Terebratulina lata Zone, and all of the Plesiocorys (Sternotaxis) plana and Micraster cortestudinarium zones. The higher beds of the Micraster cortestudinarium Zone contain carious, ‘rinded’ flints.

In this 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. Thus, 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 in this district. The top of the formation is placed at the highest recognisable bed of nodular chalk below the incoming of smooth white chalks with Platyceramus. This part of the succession is also marked in this district by a belt of large nodular and carious black and grey flints that span the boundary between the Lewes Nodular Chalk and Seaford Chalk formations.

Details

The Lewes Nodular Chalk Formation crops out over much of the northern downs area. The hardgrounds at the base of the Lewes Nodular Chalk (Chalk Rock) can be identified in the sides of the valleys either side of the primary escarpment. 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 acmes of Cremnoceramus sp. The contact between this and the basal part of the 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.

In the extreme west and south-west of the district, the Lewes Nodular Chalk Formation forms the wide expanse of the ridge that carries the Northern Transit Route of the Salisbury Plain Training Area and also the narrowing spurs that trend towards the south-west away from that crest and bound the Berril valley and its tributaries. Throughout this area the soil carries a heavy brash of hard to intensely hard nodular and grainy chalks together with a copious flint cover. Towards the centre of the crest, particularly around Stokehill Farm the soils become thicker but still carry a chalk and flint brash in an orange-brown stiff silty clay soil. This probably represents a remnant clay-with-flints or older head but it is too thin to map as a separate entity. Its thickness is variable, as demonstrated by the quantity of the included chalk brash. There are no extensive exposures in the area but numerous localities where the formation can be seen in worn tracks. Hard off-white chalk with large nodular flint seams are visible in the banks of the worn track between [ST 9563 5112] and [ST 9572 5119]. Three closely spaced glauconitic, intensely hard, nodular chalk beds were seen in a track at [ST 9667 5008] and similar grainy and nodular chalks can be seen on the opposite side of this minor valley to the south-east at [ST 9681 5004] (Plate 12).

In the Berril valley (essentially the dry course of the Chitterne Brook above its perennial spring) the formation is limited to the lower and steeper valley sides adjacent to the head and gravelly head fill. Only the highest 15 to 20 m of the formation are seen and its contact with the overlying Seaford Chalk Formation is identified at the change from intensely hard nodular and grainy chalk with flints up into white flaggy smooth chalk with flint. In places this formation contact is marked by a moderately strong positive feature. The formation is exposed along a great number of worn tank tracks that radiate out from the major crossing point at [ST 990 473] for example at [ST 9911 4731], [ST 9914 4727], [ST 9889 4699], [ST 9882 4723], [ST 9868 4748] amongst many other localities (Plate 13).

A degraded pit [ST 9841 4742] with no large exposure contains numerous animal burrows where fragments of hard grainy and nodular chalks have been excavated. Intensely hard nodular grainy chalks are exposed in a deeply cut dry ford at [ST 98581 47497].

The Lewes Nodular Chalk Formation has a broad outcrop around the Knook valley in the extreme south-west of this district, a narrow outcrop in a valley on the western side [ST 950 444] and a broad intricate outcrop on the valley flanks within the Chitterne valley and its tributaries to the east. Throughout the area arable fields carry a copious hard grainy and nodular chalk brash together with many nodular and shattered flints. The base of the formation includes an intensely hard porcellaneous splintery chalk that forms the basal strong positive feature. The glauconitic nodular and intensely hard chalks equated with the Chalk Rock Member are seen (rarely) in field brash about 5 m above the base of the formation.

The Chalk Rock Member is seen in brash at [ST 9544 4345], [ST 9562 4318], [ST 9763 4269] but not seen in section. The Lewes Nodular Chalk Formation was formerly seen in a pit on Codford Down [ST 9775 4275] but this area is now completely degraded with no exposure. Again, in a degraded pit at [ST 9897 4362], behind a barn [ST 9904 4434] at Manor Farm Chitterne, a small cut exposes about 1.5 m intensely hard to hard flaky grainy chalks with nodular flints that must be high in the succession.

The best exposure in the higher part of the formation is at Valley Farm [ST 98208 43819] to the west of Chitterne and just to the north of the Salisbury sheet boundary in the south of the district. Here a bench has been cut into the valley side to accommodate a new barn and an exposure of 3.5 m can be seen (Figure 19), (Plate 14) and (Plate 15).

In the west of the district, a track cutting at Chapperton Down [ST 9828 4803] exposes approximately 4 m of very weathered flaky, glauconitic, hard nodular chalk with two glauconitic hardgrounds. Glauconitic hard nodular chalk is also exposed in the track at the base of the tributary 100 m to the south-west. The Lewes Nodular Chalk Formation forms the very top of the escarpment to the south of Erlestoke. Here, a strong positive feature is created by the Chalk Rock Member close to the base of the formation. Most exposures are small where material has been brought to the surface by animal activity revealing a hard, off- white, grainy chalk with a hackly fracture and usually associated with flints.

A series of small quarries occur around Chirton Down [SU 066 546] where the middle and lower part of the Lewes Nodular Chalk Formation has been quarried, presumably for roadstone. The quarries are now nearly totally overgrown, although a few small exposures of about 0.5 m of hard blocky nodular flinty chalk occurs at [SU 0662 5469]. A sponge-rich hardground occurs near the top of the quarry. The specimens comprise the bivalve Cremnoceramus ex gr. waltersdorfensis and the echinoids Sternotaxis? and Micraster sp. The Micraster possesses ‘inflated’ ambulacra, which combined with the record of C. ex gr. waltersdorfensis, suggests assignment to the highest part of the P. (S.) plana Zone or lower M. cortestudinarium Zone (below the level of the Hope Gap Hardground). The middle or lower part of the upper Lewes Nodular Chalk Formation might be inferred. Very hard porcellaneous nodular chalk with some glauconitic and phosphatic hard-grounds is exposed in a tiny outcrop in an old quarry 330 m to the north-east at [SU 0632 5472].

The scarp south of The Lavingtons and eastwards, south of Urchfont, is capped by the Lewes Nodular Chalk Formation, and extends for some distance southwards along long dip slopes. Brash consists of hard nodular grainy chalks with some sponges and abundant nodular flints. Lewes Nodular Chalk Formation is exposed on the top of the scarp near Urchfont Hill and southwards towards Gibbet Knoll. A series of cuttings in the tank training area [SU 0375 5530] clearly exposes small sections of the highly glauconitic Chalk Rock Member.

South of Urchfont Hill, in the Westdown Artillery Range area, the Lewes Nodular Chalk Formation also crops out extensively within the deep valleys. The Chalk Rock Member has been identified at a number of localities [SU 0396 5016], [SU 0436 5034], [SU 0468 5064], [SU 0402 5249], [SU 0475 5113] and this feature-forming bed is approximately 5 m to as much as 10 m above the base of the formation that is itself marked by another positive feature and an intensely hard white porcellanous splintery chalk. The highest glauconitic horizons some 10 m above the base of the formation may be equated with the Hitch Wood Hardground.

Elsewhere in this impact area the formation is exposed in a great number of craters particularly in the vicinity of the static targets placed over the eastern half of the district. Examples are too numerous to detail, see (Plate 16) and (Plate 17) but nodular, grainy and spongiferous chalks, generally hard to intensely hard are seen widely.

The memoir (Jukes-Browne, 1905) describes ‘another good section of the Chalk Rock’ just a little to the north-east of Candown Farm (estimated at [SU 0432 5042]). Neither farm nor the quarry exist today, but the locality must be on the northern side of the valley hereabouts and (an estimate of [SU 0435 5058] at an area of steeper ground adjacent to a deeply incised tank track). The original description is given below with thicknesses converted into metres.

Continuing eastwards within the Salisbury Plain Training Area, the Lewes Nodular Chalk Formation up to 25 m in thickness, has an outcrop within the valley of Charlton Down and in the headwaters of Water Dean Bottom. For the most part the area is under dense ungrazed grassland within this impact area but the top of the formation is placed between a pair of strong positive features marking the change from the steep gradient of the valley sides to the less steep spur top. Hard to intensely hard nodular chalks and grainy chalks with a hackly fracture are noted at a number of localities within these valleys and within numerous craters across the area. Flint nodules are ubiquitous and the formation gives rise to a thin, very flinty silty soil packed with small hard chalk fragments. The change up into white smooth chalks and weakly grainy flaky chalks occurs at a strong double positive feature. There are no large scale exposures although a degraded pit (or perhaps large entrenchment) at [SU 0785 5200] shows a great amount of moderately hard flaky hackly fractured grainy chalk above a positive feature and intensely hard grainy chalk below.

The lower part of the Lewes Nodular Chalk Formation is exposed on a track near Widdington Farm, West Chisenbury [SU 1246 5335], just above the New Pit Chalk Formation. Here, several well-developed, glauconite-stained hard-grounds can be seen in the track. A short distance above, near the top of a steep section of track, 270 m south of Widdington Farm and 1.25 km south-east of Vedette Point 13, Casterley, [SU 1239 5336] an acme of echinoids comprises fragmentary specimens of the echinoids Micraster and Echinocorys. One of the Echinocorys specimens has a very rounded shape, and the periplastron morphology of the Micraster specimens suggests assignment to the P.(S.) plana Zone of the lower to middle Lewes Nodular Chalk Formation. Above this [SU 1234 5333], another hardground with very hard porcellaneous and nodular chalk occurs.

Hard, glauconite-stained hardgrounds near the base of the Lewes Nodular Chalk Formation can also be seen in tank track ruts and small exposures [SU 1155 5292], [SU 1039 5310], [SU 1173 5166], [SU 1208 5170], [SU 1356 5186] and in a small pit spoil heap at [SU 1269 5378]. Hard, grainy, nodular chalk is seen in many places as field brash, especially around Widdington Farm.

In the central district, around the Avon valley, the uppermost 10 m of the Lewes Nodular Chalk Formation forms the steep sides to the valley that runs east to west through Enford Farm [SU 1222 5079] and the minor tributary dry valleys running south-westward from it. The upper boundary of the formation is placed at a significant positive feature that marks the change from hard grainy chalks up into soft to firm white flaky smooth chalks. There are no exposures.

To the south-west of the Devizes district, around Heytesbury the Lewes Nodular Chalk Formation caps the hills. The lower part of the Lewes Nodular Chalk Formation has been quarried in the past at several areas on Cotley Hill [ST 916 436], [ST 916 438], [ST 918 437], [ST 920 430], West Hill [ST 938 448], and above Knook Camp around Bevin’s Barn [ST 938 432] and [ST 942 429]. The quarries are generally very shallow, around 2 to 3 m deep and are now disused and overgrown. A few small exposures generally less than 1.5 m high of hard blocky nodular chalk with glauconitic hardgrounds exist. The best section is a small quarry on Cotley Hill at [ST 9193 4334] where approximately 1.2 m of hard, nodular and porcellanous chalkstone is exposed. Three green-stained glauconitic hardground surfaces occur (Plate 18). This section has been recorded by Bristow (1995). A small quarry on West Hill [ST 938 448] formerly exposed 2.1 m of white rubbly chalk with hard, nodular cream-coloured chalk with layers of green-coated nodules. A small, discontinuous and overgrown 1.5 m section of hard glauconitic nodular chalk occurs just east of Bevin’s Barn at [ST 9395 4327]. The nodular and porcellanous chalk at the base of the Lewes Nodular Chalk Formation can be picked up at many places in the brash.

A small degraded cutting on the Southern Transit Route, about 1 km at 339° from Willis’s Field Barn, north-north-east of Knook Camp at [ST 9436 4450], shows a few metres of weathered rubbly chalk. Fossil material was collected from brash of hard, nodular chalk covering the degraded section. The fauna comprises the gastropod Bathrotomaria perspectiva, the inoceramid bivalve Mytiloides labiatoidiformis? and fragments of the echinoid Echinocorys. The fauna strongly suggests the middle part of the P. (S.) plana Zone, in the lower part of the Lewes Nodular Chalk Formation. Bathrotomaria perspectiva is typical of the fauna associated with the Hitch Wood Hardground, at the top of the ‘Chalk Rock’, although Mortimore (1986) shows that this gastropod is generally distributed through most of the plana Zone and the topmost lata Zone. M. labiatoidiformis does not range below the middle of the plana Zone (Mortimore, 1986).

Farther north, around Bratton, the Lewes Nodular Chalk Formation is identified by the presence of two strong positive features at the top of the primary scarp and by the copious hard nodular and dirty chalk and flint brash. The formation forms the extensive ridge from Winkland’s Down (extends to west onto (ST85SE) up to the margins of Westbury (Beggar’s Knoll) Quarry), through Bratton Down, in the vicinity of White Horse Farm [ST 9009 5144], and Warden’s Down in the west and extends to the south and east to Four Hundreds Down and south-east of the Northern Transit Route. The base of the formation is represented by the intensely hard, glauconitic, porcellanous Chalk Rock Member at [ST 9062 5144], [ST 9076 5142], [ST 9121 5116], [ST 9152 5102], [ST 9223 5099], [ST 9206 5075], [ST 9051 5058], [ST 9054 5034]. Elsewhere arable fields to the east of White Horse Farm carry a copious brash of hard to intensely hard, nodular to hackly fractured and grainy chalks, with many large nodular flints. There is a similar extensive brash in a series of borrow pits (? Neolithic) at and around [ST 905 514] east of Bratton Castle and the same brash is seen in ploughed ground created within the Salisbury Plain Training Area and south of the Northern Transit Route at [ST 912 508]. Generally to the south of the Northern Transit Route, the area is in grassland or, within the Warminster Ranges of Warden’s Down and Four Hundred Down, in unimproved rough grass and gorse but the hard chalk brash is still recognisable in thin humic soils.

Above these lower beds, the formation is generally represented in brash by nodular grainy chalk varying in hardness from hard to intensely hard, and with a distinctive dirty hackly fractured appearance. Around [ST 918 482], and between 5 and 10 m below the Seaford Chalk contact, the brash yields a number of examples of an inoceramid bivalve with coarse growth lines that is identified as a species of Cremnoceramus.

Over the interfluve of the Warminster Ranges to the west of this area the formation is identified from a subrounded grainy chalk brash within a reddish brown silty very flinty clay soil that supports a dense gorse shrub and grass cover. The rounded nature of the chalk brash suggests a strongly acidic soil.

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 district. It underlies much of the Chalk dip slope and broad interfluves between the primary escarpment and the negative break of slope below the secondary Chalk escarpment. Topographically, the Seaford Chalk Formation forms characteristic smooth convex slopes of the major ridges between the dry valleys over 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, with thin harder nodular chalk near the base. The flints in the lower part of the unit are generally 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), form almost continuous seams and in places create local topographical features. Examples can be seen in the valleys around Shrewton. Thin planar sheet flints are common in parts of the succession.

Typically in brash, the lower part of the Seaford Chalk Formation is very similar to the upper part of the Lewes Nodular Chalk Formation, and even in exposures they can be hard to distinguish. The lower Seaford Chalk Formation contains an abundance of fragments of the bivalves Volviceramus and Platyceramus, whilst the upper part contains Cladoceramus and Platyceramus (Mortimore, 1986). In the absence of these bivalves the flaggy nature and pure whiteness of the soft chalk serves to distinguish it from the Lewes Nodular Chalk Formation below. About 15 to 20 m up from the base of the Seaford Chalk succession a very large semitabular flint (commonly up to 30 cm thick) with characteristic brown staining is common 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 particularly characteristic semitabular flint occurs near the top of the Seaford Chalk in this 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 Whitaker’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 this flint band is a thin (1 to 2 m) horizon of intensely hard porcellanous indurated chalk (the ‘Winchester Hardground’ of Farrant, 1999), now formally called the Stockbridge Rock Member; it lies about 5 m below the Newhaven Chalk and is shown on the 1:10 000 maps as a limestone bed. 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 of about 30 cm containing an acme occurrence of Conulus sp. with other echinoids, occurs just above Barrois’ Sponge Bed (Robinson, 1986) and is known to occur 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 this 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 short intervals of white chalk.

Biostratigraphically, the Seaford Chalk Formation is co-extensive with the Micraster coranguinum Zone and spans the Coniacian–Santonian boundary that is placed at the Michel Dean Flint (Mortimore, 1986). This boundary is also marked by the incoming inception of Cladoceramus.

Details

In the south-west of the district, the Seaford Chalk Formation covers most of the area except in the valleys around Tilshead and Imber, which are cut down into the underlying Lewes Nodular Chalk Formation. An acme of Platyceramus and Volviceramus, occurs about 10 to 15 m above the base of the Seaford Chalk Formation; it forms a good stratigraphical marker horizon, and is probably at approximately the same stratigraphical level as the Seven Sisters Flint. This fossil horizon contains numerous large, very thick fragments of Volviceramus that can be readily traced across country. It occurs either side of the Westdown camp ridge, and can be followed along strike south of Tilshead and westwards towards Kill Barrow [SU 0001 4788] to Chapperton Down [ST 992 482]. Numerous specimens of these fossils were collected from many places including [ST 9941 4800], [ST 9958 4790], [SU 0111 4776], [SU 0192 4772], [SU 0231 4758], [SU 0371 4783], [SU 0416 4846], [SU 0463 4902], [SU 0456 4760].

Along the central southern border of the district around Larkhill, Durrington and Shrewton and northwards over the Larkhill Artillary Range the entire area is underlain by Seaford Chalk, which forms an extensive dip slope. Larkhill Barracks is sited on the upper Seaford Chalk, just below the Newhaven Chalk boundary, but there is no evidence for the Stockbridge Rock Member in this area. The rest of the area is developed on the middle Seaford Chalk. There were no good exposures in this vicinity at the time of this survey.

All of the dry valleys draining north-east from the Yarnbury Castle–Chitterne ridge are approximately coincident with the dip and follow the same stratigraphical horizon. The floor of these valleys is at or close to an acme of Volviceramus involutus, probably at or close to the level of the Seven Sisters Flint at the top of the Belle Tout Beds. A small, but pronounced positive feature, marks this about 2 to 5 m above the valley floor. Good specimens of Volviceramus were collected at several localities on this feature just to the south in the Salisbury district. This acme gradually dips below the valley floor to the north in the Devizes district, just south of the Chitterne–Shrewton road. Platyceramus and Volviceramus have been identified at [SU 0283 4310] and [SU 0290 4359]. The area north of the road is developed in the middle and upper Seaford Chalk. There are few significant exposures but to the north on the Salisbury Plain Training Area there are frequent small exposures of Seaford Chalk with large pink-rinded flints in tank tracks where these climb steep valley sides. A very large tabular flint band set in soft to firm, smooth white chalk is seen in a deep tank track rut at [SU 0161 4463]. A small 4 m-deep excavation at [SU 0434 4291] exposes approximately 2 m of soft white slabby chalk with occasional scattered very large tabular flints up to 60 cm across.

The Seaford Chalk Formation caps the highest ground to the north-west of Boreham Down Combe. Seaford Chalk can be found on the hills to the south-east of Imber Clump [ST 915 480] and onwards towards Middleton Down [ST 924 470]. The boundary is recognised and placed above the highest nodular, hard or grainy chalk and below the inception of weakly grainy or smooth, firm, white chalk. Both the Lewes Nodular and Seaford Chalk formations have significant large nodular flint seams at this level that are also carious in nature. The lithology change and belt of carious flints are a good proxy for the boundary but it is also confirmed by judiciously placed micropalaeontological samples.

There are few natural exposures and the formation is mapped on the distribution of a copious firm to soft, white, smooth chalk brash and very flinty thin silty clay soil over the ridge crest. To the east of Imber Clump there are a number of tank firing positions excavated into the ridge and these show a blocky to flaggy, white, smooth chalk beneath a thin soil layer. Three notable fresh excavations amongst a number in this vicinity are on Summer Down [ST 9156 4827], [ST 9162 4811], [ST 91655 48199]; this latter shown below in (Plate 19).

In the extreme south-west of the district the Seaford Chalk Formation outcrops on either side of the Berril valley. It forms the spur crests and the minor valleys and underlies the Berril valley in its middle course, around [ST 998 462]. There are very many small exposures in worn tank tracks throughout this area and these show predominantly firm, white, smooth, blocky and flaggy chalk with regularly spaced flint seams. This area was surveyed immediately after a major exercise and numerous foxholes still remained on the valley flanks that expose the formation, see (Plate 20). Towards the base of the unit seams of moderately hard and weakly grainy chalks are encountered. Above these, and within 10 to 15 m of the base, the chalk contains a significant amount of shell fragments of the bivalve Platyceramus.

The Seaford Chalk Formation forms rounded, convex ridges in the area. Only the basal 5 to 10 m are present, which comprise soft white chalk with flint. Two exposures showing around 0.5 m of soft white chalk and flints were seen in excavations near Bowl’s Barrow [ST 9400 4670].

Notable exposures are seen along the Southern Transit Route of the Salisbury Plain Training Area that runs from the south-western corner east-north-eastward towards the crossing over the Berril valley. A small section at [ST 98828 45108] shows 0.55 m of chalky flinty soil over 0.7 m of white moderately hard smooth flaggy chalk becoming blocky with depth and containing two marly partings. Similar chalks but this time containing flints and a copious fossil debris of Platyceramus and Volviceramus are exposed in a shallow cutting between [ST 9975 4558] and [ST 9987 4567]. This succession is some 15 m above the base of the Seaford Chalk Formation and is correlated with the horizon about the level of the Seven Sisters Flint where an acme of Volviceramus involutus occurs. A deeply cut tank track climbing over a small spur between [ST 9766 4675] and [ST 9760 4695] exposes many seams of nodular flints within a succession of about 15 m of firm to moderately hard, blocky, white, smooth chalk.

The Seaford Chalk Formation forms the broad rounded spurs around Chitterne and typically carries a thin soil with a copious firm, white, flaggy to flaky, smooth chalk and large nodular flint brash. Volviceramus associated with large flints (Seven Sisters Flint) were seen in a tank crossing of the Chitterne to Tilshead road at [SU 0026 4563].

To the South of Urchfont and The Lavingtons, lies the Westdown Artillary Range. Here, the lower part of the Seaford Chalk succession caps the crests of the gently rounded downs and forms the long dip slopes to the south within the Salisbury Plain Training Area. Brash consists of soft white smooth pellety chalk with occasional scattered large flints, Platyceramus fragments and flint bands near the base. The base of the formation is placed on a major positive feature upon which the highest hard grainy chalk of the Lewes Nodular Chalk is commonly recognised. In this area the Seaford Chalk is 10 to 15 m in thickness. The top of the formation is not present. In general exposure is poor over this area and the dense grassland is relatively undisturbed by cratering. The soils are thin and very flinty but carry a limited brash of soft to firm white smooth chalk that contains fragments of Platyceramus in places. The formation is visible in worn tracks [SU 0278 5016], [SU 0260 5052] and around the buildings at [SU 0260 5124] and chalk with Volviceramus was seen in an excavation [SU 0253 5116]. The Seaford Chalk Formation also has a small outcrop on the high ground of Wilsford Hill.

In the Charlton Down area, north of the Larkhill Artillary Range, the Seaford Chalk Formation forms the uppermost valley slopes, the extensive spur tops above the valleys and the strong ridge that trends east-south-east across the centre of this area. The area is heavily cratered and there are many small exposures of blocky to flaky soft to firm white smooth chalk with flints throughout the area. Chalks containing abundant Platyceramus are found along a small positive feature between 15 and 20 m from the base of the formation and Volviceramus has been identified [SU 0910 5110] and [SU 0917 5088] about 25 m from the base. Very thick fragments of Platyceramus were found along the ridge at [SU 0803 5133] some 5 to 10 m higher stratigraphically than the Volviceramus.

The Seaford Chalk crops out over much of the Netheravon area and forms two distinct ridges trending roughly east to west (Plate 21). The most southerly of these ridges, around Larkhill Racecourse, is capped by Newhaven Chalk. The formation forms broad rounded slopes and spurs, and carries a thin soil with many flints. Large exposures are absent but there are many exposures of soft to firm white smooth chalk in small excavations made by both the military and animals. In the east on the western side of the Avon valley where arable fields are found, the thin soil carries a copious brash of flaky and flaggy, firm to moderately hard smooth powdery chalk with many nodular flints. The ground is well featured presumably picking out the nodular flint horizons. Platyceramus is found infrequently within the lower third and upper few metres of the deposit.

A sponge bed near the top of the Seaford Chalk was noted in a track 300 m north of Knighton Barrow [SU 1278 4535]. Many of the valleys have classic well-developed convex slopes characteristic of the Seaford Chalk. The stratigraphical marker horizon of Platyceramus and Volviceramus, was located about 10 to 15 m above the base of the Seaford Chalk. In this area it occurs on the north side of the ridge west of Enford Farm and can be traced east down the south side of the valley to the south of Enford Farm to the Avon valley at Fifield. Abundant fragments were collected at [SU 1187 5029], [SU 1214 5034], [SU 1156 5042], [SU 1186 5104], [SU 1129 5092], [SU 1081 5079].

Newhaven Chalk Formation (NCk)

Lithologically similar to the Seaford Chalk, the Newhaven Chalk Formation is composed of soft to medium-hard, blocky smooth white chalks with numerous marl seams and flint bands and is between 55 and 70 m thick in this district. Typically, the marls vary between 20 and 70 mm thick but they are generally much thinner, often little more than a few millimetres, in this area, as they die out over syn-sedimentary positive features (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 elsewhere in the succession and were noted in the boreholes drilled for the A303 dual carriageway south-east of Stonehenge in the Salisbury district.

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. But although the secondary escarpment is well developed in this 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 10 m 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.

Biostratigraphically, the Newhaven Chalk covers the whole of the Uintacrinus socialis, Marsupites testudinarius and the Uintacrinus anglicus zones and most if not all of the Offaster pilula Zone. It crosses the Santonian–Campanian stage boundary that is placed at the Friars Bay Marl in Sussex (Mortimore, 1986). 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 further farther up-succession than described in Mortimore (1986)

Details

Three isolated outliers of Newhaven Chalk occur within this district. Firstly, a small outlier 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 encompassing Sidbury Hill [SU 21600 50600], Tidworth, and following the 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 within the basal Newhaven Chalk.

Culver Chalk Formation (CCk)

The Culver Chalk Formation is up to 25 m thick in this district and forms 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) that is just above the level of the Arundel Sponge Bed (Mortimore, 1986). Following Bristow et al. (1997), the boundary is placed in the upper Offaster pilula Zone perhaps as low as the Telscombe–Meeching Marls of the standard Sussex succession (Mortimore, 1986). 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 in the succession as seen in the sections at West Harnham, East Grimstead and at Pepperbox Quarry itself (see Salisbury Sheet description, Hopson et al., 2008) compared to the Sussex standard. In parts of Dorset and Sussex, the Culver Chalk Formation can be divided into a lower Tarrant Chalk and an upper Spetisbury Chalk (formations in Bristow et al., 1997) but current practice treats these as members of the Culver Chalk (Rawson et al., 2001) (Table 11).

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. In places, a negative feature occurs a few metres below this level, and where present this has been taken as the base of the Culver Chalk. Where the secondary Chalk escarpment is not so well developed, 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.

Biostratigraphically, the Culver Chalk mostly or entirely lies within the Gonioteuthis quadrata Zone, with the base possibly extending downwards into the Offaster pilula Zone in some areas outside this district. It is entirely within the Campanian Stage (Mortimore, 1986; Bristow et al., 1997).

Micropalaeontological determinations in the Salisbury district to the south show that the Culver Chalk has been significantly attenuated, with the lower Tarrant Member probably reduced to less than 8 m in thickness in places, and possibly being entirely absent in the extreme south, whilst the overlying Spetisbury Member may well be thicker where this occurs. This attenuation might be a consequence of relatively localised erosion during the Campanian as a consequence of channel development on the contemporary sea floor (Evans and Hopson, 2000; Evans et al., 2003). It is also possible that part of the Newhaven Chalk was removed by the same erosional process, consequently leaving it thinner, in places. Alternatively, the reduced thickness of the Culver Chalk may represent a regional northwards thinning of Campanian strata, or an overstep of the lower part of the Culver Chalk by the upper part towards the London Platform. In mapping terms the distribution of the members within the Culver Chalk could only be determinable by close micropalaeontological sampling over wide areas since exposure is limited and the two members do not show separate geomorphological featuring in this area. Such an intensive study is beyond the remit of this survey and the Culver Chalk is in consequence shown undivided.

Details

The presence of the Culver Chalk Formation in this district is very limited. Three small outliers occur on the hilltops at Sidbury Hill [SU 2160 5060], Beacon Hill [SU 2093 4434] and south-east of Bulford [SU 1949 4275] on the southern margin of the district.

Chapter 7 Palaeogene

The Reading Formation, part of the Lambeth Group, is the only Palaeogene deposit present in the district.

Reading Formation (RB)

The deposits of the Reading Formation typically consist of a basal bed of well-rounded chatter-marked flint pebbles in a reddish- brown sandy clay matrix. Above this basal bed, the Reading Formation is lithologically highly variable and comprises mottled red to yellowish or lilac brown silts and clays with occasional some fine-medium and coarse-grained red cross-bedded ferruginous sands with clay intraclasts and small well-rounded patinated flint pebbles. Concentrations of large oysters are known to be present in the basal part of the formation. In the Salisbury district towards the south-east, the formation comprises principally fine- to medium-grained sand with only minor amounts of clay, and represent a major fluvial channel within the otherwise mottled clay, floodplain, over-bank deposits that generally make up the unit.

Details

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. It 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. In this district there are no exposed sections.

Chapter 8 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 in (Figure 20).

Head

Head is a heterogeneous group of superficial deposits that have accumulated by solifluction, hillwash and hillcreep. Essentially it is very gravelly silty, sandy clay to clayey sandy gravel, with variable proportions of coarser granular material, and with 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’. Head is, in part, shown on the older geological maps of the area as ‘river and valley gravel’ or ‘valley gravel’. While these terms are accurate, they are imprecise and are not used in the current BGS scheme of nomenclature for superficial deposits.

In general head comprises pale yellow-brown, silty, sandy clays. The pebble content varies depending on the local bedrock source and is noticeably pebblier where material is derived from areas with a former Palaeogene cover. Similarly, head derived from Seaford Chalk, contains far more fresh large nodular or broken angular flints. Most of the dry valleys on the Chalk have a head deposit covering the valley floor. This is usually thickest and most prevalent in the lower reaches of the dry valley network, where the gradient lessens markedly, but can be absent where the valley is narrow or steep. In many cases, the lower limit of the head deposits occurs at the highest springhead, where it becomes reworked and merges into the alluvial deposits and the coarser less clayey gravelly head.

Head is very rarely exposed, and due to the considerable flint content cannot be regularly penetrated with a soil auger. The thickness of most head deposits in the area is therefore unknown. Borehole records suggest that the head is mostly less than 2 m in thickness, but could 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, possibly of varying age.

Details

Head is ubiquitous in the district within the minor dry valleys and it is inappropriate to include sheet-by-sheet descriptions, except where a small exposure can be used as an example. There are many small exposures throughout the district in ditches, along tracks and in man-made cuttings; few show the full succession locally but demonstrate the local derivation of the material. For example, the head deposits crossing the Upper Greensand carry much chert and sand debris, whilst those over the Chalk tend to have an upper flinty clay layer above a clayey and silty, weathered chalk gravel. For the most part, the deposit is identified by the change in soil and the flattened profile of transects across the valley bottom. Head is limited down-valley where there is a permanent stream.

Within the Devizes district, head can be classified on a lithological basis into two types.

The first is essentially a very gravelly silty, sandy clay to clayey sandy gravel, with variable proportions of coarser granular material closely associated with the Chalk Downs. 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 down the Chalk dip slopes. The term includes the chalky, flinty materials which were formerly mapped as ‘dry valley deposits’ or coombe deposits. The outcrop of this head type is extensive across the southern half of the district.

The second type is generally pale yellow-brown, sandy, silty clay derived principally from the Upper Greensand, Gault Clay, Lower Greensand and Kimmeridge Clay. These crop out as extensive dip-slope spreads and more limited valley bottom accumulations. The pebble content varies depending on the local bedrock source. In the area adjacent to and south-eastwards away from the Chalk scarp (north of Rowde) a spread of thin, orange to grey-brown, sandy silty clays with only infrequent scattered pebbles of flint and hard chalk is closely associated with a pale grey to off-white highly calcareous silty clay (‘marl’) that appears to be a distal outwash from the deep coombes that punctuate the scarp at regular intervals. This apron or dissected fan of material is sparse on the Lower Greensand but is commonly encountered in auger holes on the Gault.

In the far south-west of the district narrow outcrops of head are commonly found within the Berril valley and its tributaries. The tributary valleys generally carry a thin silty clay soil with chalk and flint clasts but in the main valley it is likely that some reworking of this similar material has taken place. Here the deposits are likely to be thicker and have an alluvial element at the top of the succession. An example of the head within the steeper gradient tributary valleys is given in (Plate 22).

Thin head is also mapped in each of the tributary valleys that join the main Berril–Chitterne Brook valley on both flanks. It is of limited thickness and generally has a narrow outcrop. There are few sections visible in the deposit and it gives rise to a yellow-brown to dark yellow-brown silty clay soil packed with angular flint brash, see (Plate 23).

Head forms the outcrop in the minor dry valleys draining into the Berril valley and the River Till. There are few exposures other than in worn tracks or excavations. The deposit is mapped on the basis of its yellow-brown, very flinty, silty clay soil and its association with narrow flats at the base of valleys. A characteristic profile of head material is shown in (Plate 24).

Within the area of the Westdown Artillery Range, head deposits form the sinuous flat base to the major north–south valley and its tributaries. It is rarely exposed but gives rise to a pale greyish brown silty clay soil packed with chalk and flint debris. In places the valley floor is very hummocky and these areas e.g. [SU 0450 5075] may be sites where groundwater rises during periods of heavy rainfall.

West of the Avon valley, south-west of Netheravon, head forms the floor of the dry valleys that dissect Netheravon Down (Plate 25). There are few exposures, but see (Plate 26) and the deposit is mapped on the basis of soil and flat valley form, generally below a strong negative feature.

In the far west around Bratton, just outside this district, head forms the valley bottom deposits creating narrow sinuous flinty chalky silty clay deposits. These deposits have been mapped in the base of Combe Bottom, the valley from Bratton southward into the scarp including Longcombe Bottom and within the valley to the south of the district. There are no exposures. See (Plate 27) for a general view of valleys to south of this area.

Older head

This older head consists of a series of deposits ranging from flinty gravels to orange-brown or reddish brown clays and sandy clays 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-with-flints or the plateau from which the clay-with-flints has been removed, and again is commonly a transitional boundary.

Small outcrops of older head occur in this district, many capping interfluves, and on upper valley slopes. Its distribution bears little relationship to the aspect of the slope. Most are no more than a few metres thick. The deposits merge laterally into areas with only a thin flinty veneer, and in these cases the presence of chalk in the soil is used to delimit the mapped edge of the head.

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.

Details

The broad interfluve of the Warminster Ranges to the west of this sheet district and extending south-eastward through Imber Clump carries a generally thin very flinty reddish brown silty clay soil over the Lewes Nodular Chalk and Seaford Chalk formations. Clasts comprise angular flint shards and broken nodular flints, commonly bleached white, together with a variable quantity of subrounded intensely hard chalk fragments. The deposit, where visible in tank tracks, is little more than 0.5 to 0.8 m thick and is not mapped separately. Its colour and flint content suggest that it is not in situ clay-with-flints but is derived from that deposit. (Plate 28) gives an example from a tank firing position on Seaford Chalk to the east of Imber Clump. Although thin, the complex soil profile suggests that it may well have developed over a number of phases.

A deposit of older head has been mapped capping the primary scarp south of Chirton forming a long narrow covering of red-brown clayey soils with angular flints.

To the north-west of the district, around Devizes, older head consists of a series of deposits ranging from flinty gravels to orange-brown or reddish brown clays and sandy clays containing abundant flint nodules and pebbles which have been derived from solifluction and mass movement of the clay-with-flints. The flints in older head are generally much more shattered than those in the clay-with-flints due to solifluction processes. The downhill boundary is taken where it thins and chalk becomes apparent in the soil, or at the negative break of slope, below which are the relatively flat flat-lying and finer grained valley head deposits. Where a break of slope is not marked, older head commonly grades imperceptibly into valley head. The upper boundary is taken at a positive break of slope at the edge of the clay-with-flint plateau, and again is usually a transitional boundary.

Gravelly head

The gravelly head is essentially alluvial, made up of 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 this district, this deposit occurs in the floor of the River Till near Winterbourne Stoke and in the Bourne River upstream of from 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, commonly dry, stream channel. Downstream of the perennial springs, the gravel is usually overlain by over-bank alluvial deposits of silt, sand and peat.

The near-surface part of the deposit has been decalcified, presumably by running water. While most of the flint clasts resemble those found in head, and are predominantly angular, some are subangular or subrounded and have evidently undergone transport by water. Jukes-Browne (1908, p. 56) states ‘sometimes the gravel is sharp and shingly without admixture of sand or loam, but in places it is sandy, and sometimes the stones are embedded in a chalk sand or chalky paste’.

This valley-bottom gravel is interpreted as the product of both solifluction and fluvial transport in a periglacial environment. Note that solifluction can occur on slopes of as little as 1º (Ballantyne and Harris, 1994) and that lobes of soliflucted material could have travelled along a valley floor for distances of hundreds of metres, if not kilometres. It is envisaged that material transported into the valley bottom by mass-movement was progressively reworked by seasonal stream flow. Under permafrost conditions no infiltration of surface waters would occur and during a spring thaw the valleys might carry a considerable flow for short periods. This would tend to rework the mass-movement deposit, removing fine-grained material and abrading the clasts. Some chalk clasts would survive short periods of fluvial transport, especially if they remained frozen. However, such reworking would be confined to the active layer (above the permafrost), which is likely to have been less than 1 m thick, and the alluvial gravels are themselves likely to have been subsequently reworked by cryoturbation and to have been buried by later mass-movement deposits.

Within the district the formation of these valley-floor gravels is thought to have been dominated by mass-movement processes, and so they have been named as a form of head. This gravelly head is in part shown on the older geological maps as ‘alluvium’. It differs from the deposits here recognised as alluvium in being predominantly very coarse and very weakly bedded. Other parts are shown on the older geological maps as ‘river and valley gravel’ or ‘valley gravel’. While these terms are accurate, they are imprecise and are not used in the modern BGS scheme of nomenclature for superficial deposits. Moreover, some areas previously shown as ‘valley gravel’ are now shown partly as gravelly head and partly as head. The presence of a central channel is taken as a simple criterion to separate head from gravelly head where these occur in the bottom of otherwise similar valleys.

Details

The Bourne valley, in the extreme south-east of the district, has a continuous floor of gravelly head from Southgrove [SU 220 590], southwards through Aughton (just east of the Devizes district) and beyond Tidworth [SU 230 480]. These deposits consist of flint gravel and fine chalk debris.

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, commonly 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.

In some places the deposit forms a distinct sloping bench several metres above the present valley floor but in others any such terrace feature that might have existed has been degraded. Although their topographical position suggests that these deposits belong to an older generation of head deposits, some reworking of the older deposits will have occurred in most places. The deposit is thus 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. Solifluction has transported the material down slope so that it now interdigitates with the deposits flooring the present valleys. The type, size and shape of pebbles and cobbles in these deposits resembles that in the gravelly head more than that in other forms of head, and they are therefore thought to have formed in a similar fashion.

It is most commonly 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 (i.e. it is effectively immature terrace).

Details

In the south-west of the district, head gravel is mapped in the middle course of the Berril valley/Chitterne Brook above the perennial spring and below the most significant area where the stream rises during excessive rainfall periods.

The deposit is effectively a head from which much of the fines- material has been removed or redistributed, by a flushing process, as the groundwater table rises through the deposit during the winter period. The valley bottom tends to be very gravely and hummocky (perhaps reflecting areas where the groundwater breaks the surface).

A tank trap excavated in the valley bottom at [ST 99141 46871] gives a fair representation of the profile of this deposit, see (Plate 29) and (Plate 30). (Plate 31) gives a view of the solution/collapse/spring issue hummocky terrain seem seen in this section of the valley.

Clay-with-flints

The clay-with-flints is primarily a remanié deposit created by the twin agencies of the modification of the original Palaeogene cover and solution 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. In places, notably on the high ground around Salisbury to the south, and the Marlborough Downs to the north, very gritty hard coarse sandstone pebbles and small rounded sarsens (pebbles of very hard fine-grained sandstone) occur in situ.

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, and small deposits near Everleigh Ashes [SU 195 563] and on Sidbury Hill [SU 216 506]. There were no good exposures of this deposit within the district and it was mapped on the basis of its characteristic reddish brown sticky clayey soil with nodular, commonly stained (orange), flints. In general it forms the flat top to hills and long dip slope spurs. The base is taken at the strong positive feature around the margins of these crests. For the most part it represents the eroded remnants of solution pipe fills and the Palaeogene cover. Its distribution reflects the remnants of the relict pre-Cenozoic 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 fine- to medium-grained multicoloured sand and stone-free clay that are commonly thought to be Palaeogene in origin. These pipes are generally detached from the Palaeogene outcrop and closely associated with a clay-with-flints cover. They are mapped herein with the clay-with-flints. The deposit is closely associated with older head which is a solifluction deposit derived directly from the clay-with-flints.

The margin of the clay-with-flints is sharply defined on scarp edges but the boundary becomes diffuse on the Chalk dip slopes where it commonly merges with head deposits. The occurrence of surface suffusion solution hollows (dolines) is a characteristic feature of the thinner clay-with-flints outcrops and along their margins, and the distribution of these features can be a useful indicator of the margin of the deposit. In places their distribution takes on a rectilinear pattern that may reflect the underlying jointing pattern within the chalk where joint intercept loci form a preferred site for surface water recharge of the Chalk aquifer and doline development.

Evidence from outside this district suggests that the clay-with-flints is developed in a number of phases and is closely associated with periglacial climates.

Details

A limited outcrop of the clay-with-flints occurs in the south-east corner of this district between Chitterne and Shrewton. The clay-with-flints is mapped over the interfluve ridge between the Berril valley in the west and the headwater valleys of the River Till draining east. Heavy rutting on either side of the Southern Transit Route over Breach Hill [SU 008 465] adjacent to Vedette V4 [SU 0104 4653] shows up to a metre of orange-brown stiff sticky and waxy clay with many nodular and broken flints. The deposit extends, as a ridge-capping, from Breach Hill southwards towards Copehill Down and then eastward through the training village towards Copehill Plantation [SU 031 463]. There are no boreholes to suggest thickness but the soil colour and flint content is so characteristic that the deposit may be as little as 0.5 m thick and assuming a relatively level base would have a maximum thickness in the order of 5 m.

On the western sides of the Avon valley, dark red-brown clayey stiff soils can be seen in deep ditches and as soil brash adjacent to the Southern Transit Route near [SU 1475 4610], north of Durrington. In the absence of any contained chalk debris the area is mapped as in situ clay-with-flints. Similar red flinty soils but with some chalk debris are noted around Round Covert (actually octagonal) [SU 1283 4715] and on the spur top to the south-south-east, and along the ridge centred around [SU 130 496] south of Enford Farm.

Peat

Peat is the term used for deposits of an organic nature that are generally fibrous and contain discernible organic material, but the term can also cover richly organic fine-grained 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 flood plain marginal woodland or reed/sedge beds in slow-flowing backwater situations. They often may 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.

Details

Within the district there are a number of small areas of peat delimited by mapping. In the north-west of the district, the upper reaches of the stream north and east of Coate are founded on the Upper Greensand at the level of the groundwater interface and consequently sandy peat areas are found. Reed beds and wet grassland with willow and sedge help delimit the area of peat. Augering proves very wet black to brown fibrous peat with varying proportions of silt, clay and sand. In the more clayey beds freshwater molluscs are found. 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.

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 over-bank deposits along the margins of the alluvial tract in the broader streams.

The sandy gravel, generally found at depth below fine-grained over-bank deposits is of variable thickness. It is composed of fine- and coarse-grained clasts of subangular to rounded flints with subordinate amounts of chalk, quartz, quartzite and sandstone, and rare exotic rock types. The fine- to medium-grained sand matrix also has a variable silt and clay content. This sandy gravel unit represents the bed-load of the stream. It was 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 exact relationship of this unit to the adjacent terrace and gravelly head deposits is not clearly demonstrated in boreholes and exposures, and they may well be a single unit on which the over-bank materials have accumulated. As there is generally a small bluff, which marks the lateral extent of the floodplain, the higher gravels are conventionally mapped as terrace.

The peat material is intimately associated with the fine-grained over-bank deposits laid down within a mature stream environment. It occurs in beds and as disseminated fragments a dark brown or black organic material with varying admixtures of silt and clay, and is usually fibrous and spongy. Its presence indicates areas of slow-flowing waters and significant plant growth and is most commonly, in this district, associated with sedge and reed-bed development within the larger alluvial tracts but may also be the result of organic accumulations in flood plain marginal situations where there is a significant shrub and wet woodland (commonly called alder–carr woodland after the main tree species encountered). There are currently limited areas of peat accumulation within the present floodplain. These may be remnants of naturally wet slow-flow backwaters or initiated by man (since the Middle Ages) in cut-off channels created during the creation of navigable streams. The detailed survey of surface distribution of peat within the floodplain has not been attempted during this survey because the relationship between the over-bank deposits and the organic beds is complex, the deposits are commonly thin and many of the accumulations are associated with man-made features. Thin peat units can be expected enclosed within the other deposits of the alluvium as mapped in this district.

The principal unit of the alluvial tract is the fine-grained sandy muds (and muddy sands) representing 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 muds (an admixture of silt- and clay-grade material) with varying proportions of very fine- and fine-grained sand and may contain shell material as individuals or in thin shell beds. It contains peat and disseminated organic material (see above) and commonly includes very thin fine-grained gravel beds of flint and chalk. These gravel lag deposits may be often no more than a string of pebbles, and represent the initial deposits of flood events. The more peaty soils tend to be in the backwater marshy areas away from the main flow. However, most of the rivers within the district have been subjected to extensive modification by man to create navigable streams, watermeadows and mill leats, hence, the original valley floor morphology is often no longer preserved.

Details

One small area of alluvium is mapped in the south-west of the district within 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. 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 in the deep channel particularly south of the village of Chitterne itself.

Alluvium is mapped in the headwaters of the Bristol Avon to the west of Devizes and in the headwaters of the Hampshire Avon in the Vale of Pewsey to the east. The alluvium in both river systems is a complex interdigitation of three distinct lithologies: gravel, peat and silty, fine sands to fine-grained sandy silty clays. The gravel is composed of generally small loose worn and rounded flints in the east but to the west ironstone and chert with some rare limestone/septaria fragments dominate. Peat occurs as a dark brown or black organic deposit with varying admixtures of marl and loam, and is commonly fibrous and spongy. The silty sandy clays are usually light or silvery grey and commonly contain fragments of flint/chert and chalk or ironstone and freshwater molluscs. The more peaty soils tend to be in the backwater marshy areas away from the main flow. There are few exposures and these are generally bankside exposures showing only partial sections usually in the overbank silty sandy clay facies.

River terrace deposits

(1 to 4 and undifferentiated)

River terrace deposits are associated with the River Avon in the district and are designated as fourth to conform with those in the Salisbury district. The Hampshire Avon has a history of terrace development. Outside the district to the south a full suite of terraces has been designated. The highest is topographically associated with the development of the proto-Solent and forms 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 outside the district but are not geomorphologically very distinct. Terraces in the Avon headwaters have undergone weathering and degradation by solifluction and grade both upslope and downslope into spreads of gravelly head deposits of various types. These terraces have been labelled as undifferentiated.

The term 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.

Details

The terrace deposits are widespread throughout the Avon valley but have not been widely exploited as a source of aggregate. Consequently there are no major exposures and few small-scale pits. Notable occurrences are given above and detailed descriptions on a sheet-by-sheet basis are inappropriate.

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 Devizes Sheet 282. Within urban areas the amount of artificial ground is often difficult to determine and its limits commonly 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 experienced a large amount of landscaping and the degree of ‘cut and fill’ is commonly impossible to determine. The artificial ground shown on the maps within those areas is probably an underestimate.

Worked ground delimits areas where natural resources have been extracted. In this area Chalk is the most commonly extracted material for the manufacture of cement, as a filler and whitening agent and as an agricultural dressing. To the south-west, the Portland and Purbeck groups are the source for architectural stone and extraction has occurred since Roman times. However, the Portland outcrops on in the Devizes Sheet district have not been exploited. There are few sand and gravel workings, perhaps reflecting the generally poorer quality of the aggregate locally. In the past most villages had a small brick and tile quarry to supply local needs. In this area, the Gault Formation and a number of the Quaternary deposits have been utilised in the past.

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). 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.

Infilled ground is used to delimit areas where former sites of extraction have been utilised for landfill. The type of fill is commonly difficult to determine but sites are known to have been used for both household waste and inert fill.

Chapter 9 Hydrogeology

The principal aquifer within the district is within the Chalk Group. Public water supplies also come 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. Although 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. Stream sinks are known at the margin of the Palaeogene strata and at the boundary of the Gault Formation.

Chalk hydrogeology

The Chalk is a microporous limestone and water flow is generally along fissures and joints that can become enlarged due to dissolution. 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 high porosity, but low transmissivity rates and is therefore slow to release its resources. Indeed unbroken chalk does not normally drain under gravity. 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 commonly regarded therefore as a dual porosity aquifer.

Permeability is generally only developed towards the top of the Chalk through the unsaturated and into the top of the saturated Chalk where fracturing and circulating groundwater is prevalent. With depth fracturing declines due to increased overburden, change in lithology and a general reduction in circulating groundwater. As circulating groundwater has a significant role in the enlargement of inherent fractures the base level to which the aquifer drains locally becomes important in enhancing and maintaining flow.

Springs issuing from the Chalk fall into two categories. Those related to overflow from the main water table and those resulting from major lithological changes in the rock mass intercepting water migrating through the aquifer. Springs resulting from lithological changes are found for example at the base of the Chalk where it overlies the Gault Formation or impermeable Upper Greensand (permeable Upper Greensand is in hydraulic continuity with the Chalk and the two aquifers act as a single resource); at the contact in the Grey Chalk Subgroup where the chalk becomes significantly more argillaceous (i.e. at the boundary between the West Melbury Marly Chalk and Zig Zag Chalk formations); at the top of the Plenus Marls Member below the fissured Melbourn Rock Member; and at other stratigraphical levels where marl seams, rock bands or continuous flint seams become locally important in the succession. Springs that occur on the dip slope of the Chalk are usually at valley bottom sites where the water table intersects the surface. During periods of low rainfall when the water table falls these springs successively dry up down the valley. They are reactivated again as water tables rise during wet periods hence their name as ‘winterbournes’ or simply ‘bournes’.

The hydrogeology of numerous of streams within the district (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 will have 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 commonly form where flow is concentrated along these horizons. The strength of the chalk is also important. Fractures in very soft chalk are commonly sealed by remoulded chalk putty, and thus form aquitards or even aquicludes. Joints in harder, nodular chalks often can remain open and thus solution cavities can develop more readily.

The Grey Chalk Subgroup (West Melbury Marly Chalk and Zig Zag Chalk formations) comprises alternating layers of clay-rich marls and thin limestone bands. The limestone bands have more open vertical joint sets, which feed water to the interface with the underlying marl. These marl layers can give rise to perched water tables. The Cast Bed (at the base of the Zig Zag Chalk Formation) is also known to form a perched water table and associated spring lines in the Dover area (Mortimore, 1993). In this district this horizon outcrops along the southern flank of the Vale of Wardour and in the headwaters of the River Wylye. In general, the Grey Chalk Subgroup is far less permeable than the White Chalk Subgroup.

Similar perching of the water table and horizontal flow occurs at the base of the Holywell Nodular Chalk Formation at the base of the White Chalk Subgroup. Here, the hard nodular Melbourn Rock Member and the overlying shelly nodular unit is extensively fractured by steeply inclined conjugate joints. The underlying clay-rich Plenus Marls Member, which impedes vertical water movement, is not jointed but dissipates stress sub-horizontally, so opening fractures in the Melbourn Rock. Spring lines occur at this level throughout southern Britain and in this district. They have been exploited for water supply, for example at Holywell near Eastbourne (Mortimore, 1993), the type site of the Holywell Nodular Chalk Formation. The New Pit Chalk and the Newhaven Chalk also display well-developed conjugate joint sets, which commonly dissipate along marl seams. These marl seams can be the loci for dissolution as water flows down the joints until it meets a marl seam and is forced to flow horizontally. This is not so apparent where the marls are thin or absent over syn-sedimentary highs.

Like the Holywell Chalk, the Lewes Nodular Chalk Formation Chalk is hard and nodular, but also has extensive large nodular and sheet flint seams. Joint sets tend to be relatively open steeply inclined conjugate sets. These joints are commonly solutionally enlarged; as they are more pervasive and are more likely to remain open than in the softer chalks. Small solution cavities are known to have developed along the sheet flints, for example in the upper Lewes Nodular Chalk Formation at a quarry at Upper Woodford [SU 1235 3700].

Both the Seaford Chalk and the Culver Chalk formations are massively bedded soft to medium hard pure chalks with regular orthogonal joints. They are characterised by numerous large tabular and sheet flint horizons. Large conduits are known in the Seaford Chalk at Beachy Head (Waltham et al., 1997; Reeve, 1979) where flow is concentrated along a sheet flint and at Shoreham Cement works (Mortimore, 1993) along faults and master joints. The Seven Sisters Flint is also known to exhibit solutional cavities on the Sussex coast and possibly does so in this district.

Details of yields and flows are held at BGS Wallingford and within the physical properties document (Allen et al. 1997). Below are details of significant springs identified during the survey. Rivers and streams elsewhere across the district are maintained by run-off and base recharge where the groundwater surface intersects the valley-floor deposits.

Details

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 along 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. This situation is repeated across the district with a series of springs that issue from the base of the Upper Greensand at or near the Gault contact. These springs occur all along its outcrop from Coulston to Urchfont and to the north, by 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 rock-head 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.

The Bourne River, in the far east of the district, and its tributary, the Nine Mile River are also examples of this. The Bourne River rarely rises at Southgrove Copse near Everley, sometimes flowing only for a short distance. Numerous springs break out at various points downstream.

Minor aquifers

Jurassic strata

The Portland Group and to a lesser extent the Purbeck Group are significant minor aquifers outside the district and indeed the Portland Limestone is a major aquifer in Dorset.

In this district the Purbeck Group (Lulworth and Durlston formations) is a poor aquifer because of its generally more argillaceous nature with interbedded shales, limestones, sandstones and evaporitic beds. The limestones are fissured and their water resources are limited, not least because of their small outcrop area.

The principal water-bearing lithology in the Portland Group is limestone with minor calcareous sands in the Wardour Formation at the base. The limestones tend to be cemented and intergranular permeabilities are low. Water movement is through fractures that have been enlarged by dissolution. High yields can be obtained where these openings are closely interconnected. In this district their limited surface outcrop and highly fissured and fractured nature (compartmentalising the aquifer) make the aquifer vulnerable to fluctuations in water table and contamination. Because of their fissure flow characteristics the limestones tend to have high transmissivities (particularly where the aquifer is karstic) but low storage coefficients and excessive abstraction can affect surface water flows. The yields from springs issuing from the group are highly variable. Water quality information is sparse for this aquifer throughout southern England but is generally hard to very hard with high concentrations of CaCO3.

Lower Greensand Group

Lower Greensand is a poor aquifer providing poor quality water and can be regarded here as a low-yield concealed aquifer. There are numerous small springs and sinks associated with the Gault–Lower Greensand and Lower Greensand–Portland contacts around the Vale of Pewsey.

Upper Greensand

The Upper Greensand Formation is an important minor aquifer in southern England. In this district the formation comprises three members all sandy in nature, or is undivided. It is commonly 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 the formation is at outcrop, such as in the Vale of Pewsey, the formation is an aquifer in its own right. The Gault Clay Formation below acts as an aquiclude.

The formation is highly permeable (intergranular flow predominates), consisting of alternating sands and sandstone with a little chert usually concentrated in this district in the Boyne Hollow Chert Member. Where the degree of cementation is high, fracture flow can become important. Towards the south-west in the Shaftesbury district the formation is the principal aquifer. One of the seven major public water supply wells is in Berwick St John [ST 9415 2182] just to the south-west of the district. Springs are commonly used for abstraction and issue from various horizons within the formation, depending on local topography and hydrogeology, and at the junction with the underlying Gault Clay Formation. Yields of up to 85 m3 per day are common for small springs and wells and they characteristically continue to flow even during extended dry periods. This is perhaps the result of tapping into the larger resources within the overlying Chalk.

Quaternary

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. Densities of over 100 per km2 have been reported in parts of Dorset (Sperling et al., 1977), but more typically, densities of between 10 and 50 per km2 occur across Hampshire and Wiltshire. Their location is unpredictable, but by assessing the geology and geomorphic 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 are common on the Chalk. The term sinkhole is interchangeable with the term doline, and can also be applied to surface features where a stream wholly or partially disappears underground. Buried sinkholes (as defined by Culshaw and Waltham, 1987) are typified by ‘pipe’ or cone-like cavities within the chalk, infilled by the overlying deposits that have subsided into the cavity as a result of dissolution. Most are circular or oval in plan and can be many metres deep, commonly bifurcating into several smaller ‘pipes’ at depth. They often have no surface expression and are commonly infilled with flinty gravelly clay derived from the superficial cover, usually clay-with-flints.

Subsidence sinkholes are closed surface depressions, usually either bowl, pipe or cone-like in shape. They can occur as isolated examples or as groups, commonly coalescing into large composite dolines. They can form rapidly as a dropout failure following the washing out of pre-existing infilled pipes. Most occur in covers of unconsolidated sediment between 1 and 10 m thick, such as the clay-with-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 anthropomorphic 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.

However, the main control on near-surface solution features is the geomorphic setting and the presence or absence of an impermeable cover. An area of impermeable strata either adjacent to or overlying the Chalk serves to concentrate recharge and hence dissolution at the contact between the two rock types. The highest density of sinkholes occurs around the margin of the overlying Palaeogene strata or around the clay-with-flints outcrop. Topography and drainage patterns affect the distribution of solution features. Dissolution is enhanced where underground drainage routes are concentrated such as along valley floors and at spring lines. Typically the chalk is far more weathered under valley floors than under interfluves. Topography also influences whether drainage from the Palaeogene outcrop flows onto or away from the chalk and thus influences the location of water recharge via stream sinks.

An understanding of the geomorphic evolution of an area is vital to identify potential areas of karst development that have little no surface expression today. This is especially the case for karst features formed under differing climatic conditions or relict karst formed prior to present topography. Where the present land surface is close to the sub-Palaeogene peneplane, solution features inherited from the former Palaeogene cover may still exist. For example, solution pipes may still exist below ground level in areas where a former clay-with flint or Palaeogene cover has now been eroded. Elsewhere, erosion and dissection has removed these relict solution features.

Distribution of solution features

The distribution of observed solution features is shown on the 1:10 000 scale geological maps. 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.

Chapter 10 Review of soil types

This review is based on the 1:250 000 scale Soils of England and Wales Sheet 5 (South West England, 1983) and Sheet 6 (South East England) published by the Soil Survey of England and Wales; and the compendium volumes, Bulletin No. 14 ‘Soils and their use in South West England’, (Findley et al., 1984) and Bulletin No. 15 ‘Soils and their use in South East England’ (Jarvis et al., 1984).

Within the district there are 22 soil associations whose major constituents and geological derivation are described below. Each soil association is closely associated with a number of ancillary subgroups and soil series. Full descriptions and representative soil profiles, which define each of the soil series, are available in numerous publications of the Soil Survey. The reference is given in brackets after each entry.

341 Icknield Association

(Cope, 1976, p. 61)

Description: Shallow, mostly humose, well-drained, calcareous soils over chalk on steep slopes and hill tops. Deeper flinty, calcareous, silty soils exist within small coombes and valleys.

Location: Broad spur tops and dip slopes underlain by the White Chalk Subgroup within the central Salisbury Plain area and on the Marlborough Downs to the north of the Vale of Pewsey.

Association: Andover, Upton, Coombe and Millington soils.

342a Upton 1 Association

(Cope, 1976, p. 76)

Description: Shallow, well-drained, calcareous, silty soils over chalk.

Location: Mainly on moderately steep, sometimes very steep land. Deeper fine silty calcareous soils exist within coombes and dry valleys. Most commonly found on the northern scarp face and deep coombes of Salisbury Plain and valley sides within the major streams, Avon and Bourne. Predominantly on the lower part of the White Chalk Subgroup but also found on the limestones of the Portland Group.

Association: Andover, Icknield, Panholes and Coombe soils.

342b Upton 2 Association

(Cope, 1976, p. 76)

Description: Shallow well well-drained calcareous, silty soils over argillaceous chalk. In places the association comprises deeper well-drained, calcareous, clayey soils. Location: Confined to the lower slopes of the Salisbury Plain scarp and similar slopes on the southern flank of the Marlborough Downs to the north of the Vale of Pewsey founded on the Grey Chalk Subgroup.

Association: Wantage and Blewbury soils.

343h Andover 1 Association

(Cope, 1976, p. 68)

Description: Shallow, well-drained, calcareous, silty soils over chalk, on slopes and crests. Deep calcareous and non-calcareous, fine, silty soils also occur in valley bottoms. Rainfall readily absorbed with little run-off and can suffer from drought particularly on the harder Holywell Nodular Chalk and Lewes Nodular Chalk formations. Striped soils patterns locally.

Location: Thin soils, generally developed on bedded chalks with flints; covers much of the dip slopes of the White Chalk Subgroup within Salisbury Plain.

Association: Panhole, Coombe, Upton and Charity soils.

343i Andover 2 Association

(Cope, 1976, p. 68)

Description: Shallow, well-drained, calcareous, silty soils over chalk. Associated with deeper non-calcareous, variably flinty, well-drained, fine silty, over clayey soils. Rainfall readily absorbed. Generally associated with the White Chalk Subgroup and particularly the flinty Lewes Nodular Chalk and Seaford Chalk formations.

Location: Common soil within broad valleys on Salisbury Plain. Commonly contains little chalk debris and very flinty on the Seaford Chalk.

Association: Garston, Carstens and Charity soils.

411a Evesham 1 Association

(Findley et al., 1984; Palmer, 1982 p. 138)

Description: Variably thick and wet depending on the nature of the underlying Jurassic clays and shales with limestone seams, calcareous clay with minor limestone clasts. Soils slowly permeable and seasonally waterlogged.

Location: Generally associated with the Lower Cretaceous and Purbeck Group strata in the most western part of the Vale of Pewsey.

Association: Haselor, Moreton, Sherbourne and Wickham soils.

511d Blewbury Association

(Findley et al., 1984, p. 382)

Description: Shallow, well-drained, calcareous, clayey, and fine silty over clayey soils, over argillaceous chalk. Some fine silty over clayey soils with slowly permeable subsoils and slight seasonal waterlogging.

Location: Shallow soils closely associated with the West Melbury Marly Chalk and the more argillaceous basal part of the Zig Zag Chalk formations.

Association: Winterbourne, Yatesbury and Wantage soils.

511f Coombe 1 Association

(Findley et al., 1984, p. 385)

Description: Shallow to deep, well-drained, calcareous, fine silty soils, deep in valley bottoms. The soil is shallow on chalk on valley sides in places. Slight risk of water erosion on steeper slopes. Can suffer occasional damaging floods and prolonged waterlogging by groundwater.

Location: Developed on gravelly head and head within the upper reaches of the Bourne and Nine Mile river valleys, and flanking the alluvium on terrace and gravel-rich head in the Till, Lower Bourne and Avon river valleys.

Association: Panhole, Millington, Andover and Charity soils.

512d Grove Association

(Jarvis, 1973)

Description: Shallow to deep gleyic brown calcareous slightly stony clay loam over gravel substrates.

Location: Generally associated with valley terrace-like fills within the upper Avon and upper Bourne River valleys where they form wide flats over the basal Grey Chalk Subgroup and Upper Greensand Formation.

Association: Evesham, St Lawrence and Kelmscot soils.

512e Block Association

(Jarvis et al., 1984)

Description: Shallow to deep permeable gleyic brown calcareous fine loamy and chalky soil that may be seasonally affected by groundwater.

Location: Generally associated with the Selborne Group and Kimmeridge Clay Formation at the base of the Salisbury Plain scarp to the west of The Lavingtons.

Association: Ford End and Burwell soils.

541B Bearsted 2 Association

(Fordham and Green, 1980, p. 69)

Description: Deep, well-drained, coarse loamy and sandy soils over sand or sandstone, in places ferruginous. Some permeable coarse and fine loamy soils affected by groundwater. There is a distinct risk of water erosion and gullying during heavy rainfall.

Location: Deep soils closely associated with the lower part of the Upper Greensand Formation within the Vale of Pewsey.

Association: Lupitt and Shirrell Heath soils.

544 Banbury Association

(Jarvis et al., 1984)

Description: Deep well-drained stony fine and coarse loamy ferric brown earths, resting on shattered ironstone.

Location: Very closely associated with the Lower Greensand Group Seend Formation to the west and south-west of Devizes.

Association: Tadmaton and Irondown soils.

571h Ardington Association

(Findley et al., 1984, p. 380)

Description: Deep, well-drained, fine and coarse loamy, glauconitic soils. Generally permeable and naturally well drained. Some valley bottom soils can be affected by groundwater. Locally perennially wet.

Location: Closely associated with the Vale of Pewsey where it is underlain by the Upper Greensand Formation.

Association: Urchfont and Coate soils.

572h Oxpasture Association

(Palmer, 1982, p. 145)

Description: Dark brown slightly mottled and stony clay loam and silty clay loam soils over stone less yellow clay. The soils have a slow permeability and can become seasonally waterlogged.

Location: In this district found on the shallow slopes underlain by the Gault Formation and Kimmeridge Clay Formation west of Devizes.

Association: Burlesdon, Wickham and Holdenby soils.

572j Bursledon Association

(Jarvis et al., 1984, p. 374)

Description: Soils generally dark greyish brown, very slightly stony clay loam or sandy loam soils. They are slowly permeable and liable to seasonal waterlogging.

Location: In this district found on the shallow slopes underlain by the Gault Formation and Kimmeridge Clay Formation west of Devizes.

Association: Curdridge, Kings Newton and Oxpasture soils.

581d Carstens Association

(Jarvis et al., 1979, p. 215)

Description: Deep, well-drained, fine silty over, clayey and fine silty soils, commonly very flinty. Generally with reddish clayey subsoils and with good vertical drainage into the underlying chalk. Relatively deep soils closely associated with the clay-with-flints, and the older head and gravelly head derived from it.

Location: On the highest hill tops and surrounding areas close to the sub-Palaeogene erosion surface and closely allied to the basal Reading Formation. In these circumstances the soils contain significant well-rounded, chatter marked ‘Tertiary’ flints.

Association: Givendale, Winchester, Porton, Garston and Wallop soils.

582 a Batcombe Association

(Jarvis et al., 1979)

Description: Variably flinty red fine silty and fine loamy stagnogleyic palaeo-argillic brownearths.

Location: Closely associated with the clay-with-flints and derived older head capping the highest ground over the Chalk outcrop east of Tidworth.

Association: Hornbeam and Carstens soils.

711f Wickham 2 Association

(Jarvis et al., 1984, p. 297)

Description: Heavy fine loamy soils over clayey subsoils and are typically stagnogleys.

Location: These soils have a close association with the Gault and Kimmeridge Clay formations in the Vale of Pewsey. Here they are heavy fine loamy soils over clayey subsoils and are typically stagnogleys. The shallow slopes typical of these deposits and the slowly permeable subsoils give rise to seasonal waterlogging.

Association: Denchworth, Oxpasture and Evesham soils.

711g Wickham 3 Association

(Jarvis et al., 1984)

Description: Typically stagnogley soils developed in fine loamy to fine silty drift over clay substrates.

Location: Commonly found on the Gault Formation and Kimmeridge Clay Formation to the west of Devizes. Association: Burlesdon, Curdridge and Denchworth soils.

712b Denchworth Association

(Palmer, 1982)

Description: Wet clayey pelostagnogley soils

Location: Found over Cretaceous and Jurassic clay substrates as in such as the Gault Formation and Kimmeridge Clay Formation to the south and west of Devizes.

Association: Wickham, Dale, Lawford and Oxpasture soils.

813b Fladbury 1 Association

(Jarvis et al., 1984, p 157)

Description: Dark greyish brown, stoneless clay soils over prominently mottled pelo-alluvial gley subsoils; slowly permeable and there is seasonal flooding from both groundwater and run-off sources; clayey alluvial soils commonly associated with rivers that drain Jurassic areas.

Location: These soils are found within the valleys draining the Jurassic strata to the west and south-west of Devizes.

Association: Thame and Wyre soils.

814a Thames Association

(Hazelden, 1986, Soils in Oxfordshire II)

Description: Mainly grey calcareous clayey soils in close association with river alluvium (overbank deposits) affected by groundwater and occasional flooding. They are pelo-argilic alluvial gley soils.

Location: Head and alluvium of the upper Avon within the Vale of Pewsey and in the valley cutting south through the Salisbury Plain from Upavon to Durrington.

Association: Fladbury and Uffington soils.

Chapter 11 Economic geology

Building stone

Jurassic strata

Many buildings use stone imported from the Upper Jurassic succession in the Vale of Wardour, south-west of Devizes. 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, is the major source of freestone that comprise generally variably glauconitic and calcareous sandstones and sandy limestones. In this district, there has been limited extraction of building stone mainly due to the lack of suitable material.

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 rarely 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.

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

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 within 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 on a local scale to support farms. Their grade and potential as a source of aggregate has not been tested.

Brick clays

The Gault Clay 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.

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, 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 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. Either phenomenon can create difficulties during or following construction. Stream sinks and dolines may be locally present.

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.

Planning for future construction should allow for the possible existence of small areas of made, infilled landscaped ground. Such areas might 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. Commonly, the nature of the fill is unknown. In the case of landfill sites the presence of gas derived from the breakdown of the buried wastes may form a problem.

Areas of ground subject to landslides are relatively common in the north-west of the Devizes 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 itself. In most cases the area of slip is obvious from the disruption of the surface sediments.

Chapter 12 Sources of information

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

Other information sources include borehole records, fossils, rock samples, thin sections, hydrological data and photographs. Searches of indexes to some of the collections can be made on the Geoscience Index system in BGS libraries. The indexes are:

• Boreholes
• GeoIndex
• Outlines of BGS maps at 1:50 000 and 1:10 000 scale and 1:10 560 scale County Series
• Chronostratigraphical boundaries and areas from British Geological Survey 1:250 000 maps
• Geochemical sample locations on land
• Aeromagnetic and gravity data recording stations
• Land survey records

Books

British Regional Geology

• The Hampshire Basin and adjoining areas, 4th edition, 1982.

Memoirs

• Geology of the country around Winchester and Stockbridge (Sheet 299), 1912
• Geology of the country around Andover (Sheet 283), 1908
• Geology of the country around Alresford (Sheet 300), 1910
• Geology of the country around Southampton (Sheet 315), 1987
• Geology of the country around Salisbury (Sheet 298), 1903
• Geology of the country around Basingstoke (Sheet 284), 1909
• Geology of the country around Devizes (Sheet 282), 1905
• Geology of the country around Fareham (Sheet 316), 1913
• Geology of the country around Ringwood (Sheet 314), 1902

Sheet explanations

• Geology of the Winchester District (sheet 299), 2000
• Geology of the Fareham and Portsmouth District (sheet 316 and part sheet 331), 2000
• Geology of the Alresford district (Sheet 300), 2001

Sheet descriptions

• Geology of the Salisbury District (Sheet 298), 2008
• Geology of the Winchester District (Sheet 299), 2008

Technical reports

• Technical reports relevant to the district are arranged below by topic. Most are not widely available but may be purchased from BGS or consulted at BGS and other libraries.
• WA/00/11 - Aldiss, 2000
• WA/00/18 - Booth, 2000
• WA/00/23 - Hopson, 2000
• WA/00/24 - Farrant, 2000
• IR/01/157 - Farrant, Hopson, Booth, Aldiss, 2001

Biostratigraphy reports

• There are seven biostratigraphical reports by the authors A Woods and I Wilkinson, covering the Devizes district.
• IR/04/146 - Wilkinson, 2004a
• IR/04/148 - Wilkinson, 2004b
• IR/04/156 - Wilkinson, 2004c
• IR/06/003 - Wilkinson, 2006
• WH/99/142R - Woods, 1999
• IR/04/149 - Woods, 2004
• IR/05/107 - Woods, 2005

Maps

Geological maps

• 1:1 500 000
• Tectonic map of Britain, Ireland and adjacent areas, 1996
• 1:1 000 000
• Pre-Permian geology of the United Kingdom, 1985
• Geology of the United Kingdom, Ireland and the adjacent continental shelf (south sheet), 1991
• 1:625 000
• Bedrock Geological map of the United Kingdom, South sheet, 2007
• Quaternary map of the United Kingdom, South sheet, 1977
• 1:250 000
• Sheet 51N 02W Chilterns, Solid Geology, 1991
• 1:50 000
• Solid and Drift
• Sheet 299 Winchester (2003)
• Sheet 314 Ringwood (2002)
• Sheet 300 Alresford (1999)
• Sheet 316 Fareham (1997)
• Sheet 315 Southampton (1987)
• Sheet 282 Devizes (2008)
• Sheet 284 Basingstoke (1981)
• Sheet 298 Salisbury (2006)
• Sheet 283 Andover (1975)
• 1:10000
• For the most recent revision of the 1:50 000 series Sheet 282 Devizes, the component 1:10 000 National Grid maps are listed below, together with the initials of the surveyors and dates of survey.

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

Geochemistry maps

1:625 000

Methane, carbon dioxide and oil susceptibility, Great Britain (South Sheet) 1995

Radon potential based on solid geology, Great Britain (South Sheet) 1995

Distribution of areas with above the national average background concentrations of potentially harmful elements (As, Cd, Cu, Pb and Zn), Great Britain (South Sheet) 1995

Hydrogeological maps

• 1:625 000
• Sheet 1 (England and Wales), 1977
• 1:100 000
• Groundwater Vulnerability Map, North-west Hampshire (Sheet 44); produced by the Environment agency
• Minerals maps
• 1:1 000000
• Industrial minerals resources map of Britain, 1996

Hydrogeology

• BGS hydrogeology enquiry service; wells and springs and water borehole records are held at the British Geological Survey, Hydrogeology Group, Maclean Building, Crowmarsh Gifford, Wallingford, Oxfordshire OX0 8BB. Telephone 01491 838800. Fax 01491 692345.
• Whitaker, W. 1910. The Water Supply of Hampshire. Memoirs of the Geological Survey, England and Wales.
• Harvey, B I, Langston, M J, Hughes, M D A, Whalley, H A. 1975. Records of wells in the area around Frome and Devizes: inventory for one-inch geological sheets 281 and 282, new series. Metric Well Inventory. Institute of Geological Sciences, London.

Documentary collections

Boreholes

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

Geophysics

Gravity and aeromagnetic data are held digitally in the National Gravity Databank and the National Gravity Aeromagnetic Databank at BGS Keyworth.

BGS Lexicon of named rock unit definitions

Definitions of the named rock units shown on the 1:50 000 series Sheet 282 Devizes are held in the Lexicon database. This is available on the website http://www.bgs.ac.uk Further information on the database can be obtained from the Lexicon Manager at BGS, Keyworth.

Material collections

Palaeontology collections

Macrofossils and micropalaeontological samples collected from the district are held at BGS Keyworth. Enquiries concerning all the fossil material should be directed to the Chief Curator, Biostratigraphy Collections, BGS, Keyworth. See also http://www.bgs.ac.uk/collections/pal.html

Petrology collections

Hand specimens and thin sections are held in England and Wales Sliced rocks collection at BGS Keyworth. Specimens are available for study and a loan system is available to accredited scientists. General enquiries should be made to the Chief Curator, Petrology collections, BGS, Keyworth. See also http://www.bgs.ac.uk/collections/pet.html

Borehole core collections

The National Geosciences Records Centre, BGS Keyworth, holds samples and entire core from a small number of boreholes in the Devizes district. Contact the Chief Curator, BGS, Keyworth for further information.

BGS photographs

Copies of the photographs in this report are deposited for reference in the BGS Archive, Keyworth. Prints and transparencies can be supplied at a fixed tariff. BGS also has a National Archive of geological photographs (also GeoScenic) available to search on the web at http://www.geoscenic.bgs.ac.uk

Other relevant collections

Groundwater licensed abstractions, catchment management plans and landfill sites.

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

References

Other reports and publications are listed under References. These can be consulted at the BGS Library, Keyworth.

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

Aldiss, D T A. 2000. Geology of the Cholderton–Grateley area, Hampshire and Wiltshire. British Geological Survey Technical Report, WH/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, No. 8.

Ballantyne, C K and Harris, C. 1994. The Periglaciation of Great Britain. (Cambridge: Cambridge University Press.)

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

Booth, K A. 2002. Geology of the Winchester district. Sheet Explanation of the British Geological Survey. Sheet 299 (England and Wales).

Bristow, C R. 1995. Geology of the Heytesbury district (Wiltshire). British Geological Survey Technical Report, WA/95/86.

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.

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 and Lott, G K. 1996. The building stone potential of the Portland Stone in Chicksgrove Quarry, Wiltshire. British Geological Survey Technical Report, WA/96/19R.

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, C R, 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).

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Harvey, B I, Langston, M J, Hughes, M D A, Whalley, H A. 1975. Records of wells in the area around Frome and Devizes: Inventory for one-inch geological sheets 281 and 282, new series. Well Inventory Series, Institute of Geological Sciences.

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Hopson, P M, Aldiss, D T, Smith, A, Wood, C J, Wilkinson, I P, Woods, M A, Allsop, J M, and Cheney, C S. 1996. Geology of the country around Hitchin. Memoir of the British Geological Survey, Sheet 221 (England and Wales).

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. Sheet Explanation of the British Geological Survey, Sheet 298 (England and Wales).

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. 2008. Geology of the Salisbury district. Sheet Description of the British Geological, Sheet 298 (England and Wales).

Howard, A S, Warrington, G, Ambrose, K, and Rees, J G. 2008. A formational framework for the Mercia Mudstone Group (Triassic) of England and Wales. British Geological Survey Research Report, RR/08/04.

Hull, E. 1869. The Triassic and Permian rocks of the Midland Counties of England. Memoir of the Geological Survey of Great Britain.

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Jarvis, M G, Allen, R H, Fordham, S J, Hazelden, J, Moffat, A J, and Sturdy, R G. 1984. Soils and their use in South East England. Soil Survey of England and Wales, Bulletin No. 15.

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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, 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, No. 68.

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Figures, plates and tables

Figures

(Figure 1) Geology of the Devizes district.

(Figure 2) Structure of the Wessex Basin showing the major faults and other named structures.

(Figure 3) Location of the five deep boreholes within or close-by the Devizes district.

(Figure 4) Section through the Portland Stone Formation in a borehole (SU05NW/40) drilled at Stroud Hill Farm, Potterne [SU 0150 5833].

(Figure 5) Lithological log showing about 0.6 m of Upper Greensand Formation succession at an old quarry, south-west of Potterne church [ST 99475 58308].

(Figure 6) Lithological log showing the Upper Greensand Formation in the roadside section running south-south-east of Coulston [ST 95355 53947] to [ST 95376 53796].

(Figure 7) Upper Green­sand Formation in a cutting adjacent to a footpath approximately 650 m north-north-west of Devizes Castle, [ST 99860 61897] to [ST 99830 61940]. The lithology and biozonation suggest a level below the Potterne Rock Bed.

(Figure 8) An exposed section of Upper Greensand Formation with large, flattened ‘doggers’ north-west of Devizes Castle, [ST 99949 61536]. Fauna collected suggests the C.auitus Subzone of the M. (M.) inflatum zone a level below the Potterne Rock Bed.

(Figure 9) Composite section showing the Upper Greensand Formation including the Potterne Rock on the western side of the A360 south of Devizes [SU 00360 60400] to [SU 00240 60060].

(Figure 10) A 15 m exposure of Upper Greensand Formation in the bank of Coxhill Lane, west-south-west of Stroud Hill Farm, near Potterne [SU 0012 5840] to [SU 0062 5842]. The 2.4 m section of glauconitic sands with scattered phosphatic clasts are analogous to the Potterne Rock.

(Figure 11) Lateral variation in the development of the Potterne Rock in Coxhill Lane, west-south-west of Stroud Hill Farm, near Potterne. Zone A is analogous to the Potterne Rock. The hard beds perhaps represent a locally expanded development of the Potterne Rock.

(Figure 12) Lithological log of an old pit section on the eastern side of a lane leading up to Urchfont Hill, approximately 730 m at 185° from the church at Urchfont [SU 03982 56592]. Bed C represents the base of the Chalk Group.

(Figure 13) Lithological log showing the Upper Greensand Formation north-north-east of Easterton Sands [SU 0195 5683]. The section can be divided into three broad intervals (1–3). The lithology of the laterally persistent hard bed (4) is closely comparable with the Potterne Rock interval seen elsewhere in the district.

(Figure 14) Lithological log showing the Upper Greensand Formation with inferred Potterne Rock horizon, analogous to the Devizes section (Figure 9), Friar’s Lane, Urchfont [SU 0426 5717].

(Figure 15) Lithological log showing the Upper Greensand Formation with the Potterne Rock. The exposure is adjacent to a footpath about 400 m south-east of Wick Farm near Littleton Panell [SU 0025 5430].

(Figure 16) Lithological log showing the upper part of the Upper Greensand Formation in a cutting in Peppercombe Lane, north-west of the church at Urchfont [SU 0395 5738]. The distinctly micaceous character of the sediments at the top of the section suggests a level below the Potterne Rock.

(Figure 17) Lithological log showing the uppermost part of the Upper Greensand at Heath Knapp Farm [SU 05818 60407]. Lithology and field data suggest a horizon between the Pottern Rock and the base of the Chalk Group.

(Figure 18) Correlation of key sections in the Upper Greensand Formation of the Devizes and Frome districts.

(Figure 19) Lithological log showing the upper part of the Lewes Nodular Chalk Formation near Valley Farm, Chitterne [ST 98208 43819].

(Figure 20) General relationship between the superficial deposits of the district.

Plates

(Front cover) Front cover Caen Hill Locks on the Kennet and Avon Canal, Wiltshire. View looking east up the Upper Greensand scarp from the lower basin. (P698535).

(Plate 1) Upper Greensand succession in the Devizes Castle cliff section [499949, 161536] P598890.

(Plate 2) Upper Greensand in Peppercombe Lane near Urchfont [SU 0395 5738] (P598846).

(Plate 3) Zig Zag Chalk Formation exposed near Great Cheverell Hill [ST 9672 5197] (P584895).

(Plate 4) Holywell Nodular Chalk, Melbourn Rock, Plenus Marls and Zig Zag Chalk Formation (P584944).

(Plate 5) Zig Zag Formation, Etchilhampton Hill (P696968).

(Plate 6) Middle part of the Zig Zag Chalk Formation (P696887).

(Plate 7) The contact between Holywell Nodular Chalk Formation and Zigzag Chalk Formation, below, on the MOD Southern Transit Road [ST 9312 4493] (P584330).

(Plate 8) Longcombe Bottom [ST 92299 51601], a typical buttressed outcrop of the Zig Zag Chalk Formation within the steep scarp valley along the southern scarp to the Vale of Pewsey. Small scar to left of picture is the location [ST 9226 5150] described above (P584571).

(Plate 9) Melbourn Rock Member on the Plenus Marls Member, Holywell Nodular Chalk Formation (P584454).

(Plate 10) Very hard nodular porcellanous and grainy chalks of the Melbourn Rock Member rest on dark greenish grey soft marls (Bed 8 of Jefferies) of the Plenus Marls Member (at the hammer head [30 cms]) [ST 95459 50280] (P584453).

(Plate 11) Coombe Bottom, Bratton. The West Melbury Marly Chalk Formation underlies the shallow ramp, bottom left, below the strong negative feature at the base of the scarp. It is overlain by the Zig Zag Chalk Formation that forms the steep buttressed slope. The Melbourn Rock Member forms the prominent positive feature (marked by the tree in the foreground). The strong scarp-top feature is developed in the Holywell Nodular Chalk Formation. Bratton village lies at the base of the scarp, within the trees; Combe Bottom is the open ground to the left of the photograph. Vale of Pewsey and Marlborough Downs form the distant view to the skyline. View looking east from [ST 91043 51535] towards Picquet Hill and Edington Hill (centre middle distance) and Luccombe Bottom (partially shaded deep coombe) (P584578).

(Plate 12) Typical valley in this northern part of the Salisbury Plain Training Area. View looking south-east towards the Imber (Berril) valley. The major positive feature, where the ruts divide, is the Chalk Rock near to the base of the Lewes Nodular Chalk Formation. The bright white chalks in the track at the base of the slope are in the underlying New Pit Chalk Formation [ST 96727 50083] (P584442).

(Plate 13) Typical rough nodular grainy Lewes Nodular Chalk exposed in foxhole spoil [ST 98992 47377] on north flank of Berril valley (P584432).

(Plate 14) New cut for barn at Valley Farm Chitterne exposing the higher part of the Lewes Nodular Chalk Formation. Abundant micraster and inoceramids collected (P584400).

(Plate 15) New cut for barn at Valley Farm, Chitterne [ST 9821 4381] exposing the higher part of the Lewes Nodular Chalk Formation. Abundant Micraster and inoceramids collected. Hammer (30 cm) just below sponge bed and associated flint shown on section above (P584398).

(Plate 16) Uppermost Lewes Nodular Chalk Formation exposed in heavily cratered ground (P584511).

(Plate 17) Lewes Nodular Chalk Formation exposed in a recent crater (P584515).

(Plate 18) Lower part of the Lewes Nodular Chalk Formation (Chalk Rock), Cotley Hill (P584324).

(Plate 19) Seaford Chalk Formation exposed within a tank firing-trench (P584584).

(Plate 20) Seaford Chalk Formation exposed in a foxhole (P584417).

(Plate 21) Seaford Chalk, typical topography (P584363).

(Plate 22) Head exposed in a deep tank track (P584452).

(Plate 23) Dry valley cut in the lower Seaford Chalk (P584437).

(Plate 24) Cryoturbated head on Seaford Chalk Formation (P584378).

(Plate 25) Dry valley underlain by head (P584374).

(Plate 26) Clast-supported flint gravel (P584372).

(Plate 27) Typical asymmetric geomorphology below the Lewes Nodular Chalk Formation (P584492).

(Plate 28) Soil and thin head overlying the Seaford Chalk, Imber Clump (P584583).

(Plate 29) Tank trap exposure in the head gravel, Berril valley (P584428).

(Plate 30) Sarsen exposed in head gravel (P584429).

(Plate 31) Sinkhole within the head gravel, Berril valley (P584420).

(Back cover)

Tables

(Table 1) Lithostratigraphy of the Chalk Group.

(Table 2) Geological succession and bed thicknesses of the Devizes district.

(Table 3) Statal thicknesses in the five deep boreholes in and near the district.

(Table 4) Thickness variation of the Permian and Triassic succession proved in the five deep boreholes in and near the district.

(Table 5) Correlation of the Lias Group after Cox et al., 1999.

(Table 6) Lias Group in the Devizes No. 1 Borehole.

(Table 7) Comparative thicknesses for the named units within the Great Oolite Group.

(Table 8) Comparison of nomenclatural schemes developed for the Portland Group.

(Table 9) Traditional subdivision of the Upper Greensand Group.

(Table 10) Correlation of the old and new schemes as determined by Bristow (1995) for the Upper Greensand Formation of the Tisbury area.

(Table 11) Chalk Group nomenclature for the Southern Chalk Province.

(Table 12) Zonal schemes for the Chalk Group: traditional and current classification.

(Table 13) Foraminiferal and macrofaunal schemes for the Chalk Group of Southern England.

Tables

(Table 2) Geological succession and bed thicknesses of the Devizes district

(Table 3) Statal thicknesses in the five deep boreholes in and near the district

(Table 4) Thickness variation of the Permian and Triassic succession proved in the five deep boreholes in and near the district

(Table 5) Correlation of the Lias Group after Cox et al., 1999

(Table 6) Lias Group in the Devizes No. 1 Borehole

(Table 7) Comparative thicknesses for the named units within the Great Oolite Group

(Table 8) Comparison of nomenclatural schemes developed for the Portland Group

(Table 9) Traditional subdivision of the Upper Greensand Group

(Table 10) Correlation of the old and new schemes as determined by Bristow (1995) for the Upper Greensand Formation of the Tisbury area.

(Table 12) Zonal schemes for the Chalk Group: traditional and current classification