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Geology of the Bath district — a brief explanation of the geological map sheet 265 Bath

A J M Barron, T H Sheppard, R W Gallois, P R N Hobbs, and N J P Smith

Bibliographic reference: Barron, A J M, Sheppard, T H, Gallois, R W, Hobbs, P R N, And Smith, N J P. 2011. Geology of the Bath district — a brief explanation of the geological map. Sheet Explanation of the British Geological Survey. 1:50 000 Sheet 265 Bath (England and Wales).

Keyworth, Nottingham: British Geological Survey.

Printed in the UK for the British Geological Survey by B&B Press Ltd, Rotherham.

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(Front cover) The Roman Great Bath and Bath Abbey. (Photographer P J Witney; P756090).

(Rear cover)

(Geological succession) Geology of the Bath district. Summary of the geological succession in the district.

Acknowledgements

R J Wyatt advised on the stratigraphy of the Middle Jurassic, A R Farrant contributed to the Applied Geology chapter and the description of radon hazard is based on a contribution by C Scheib. Regional geophysical maps were prepared by C P Royles and other figures drawn by Henry Holbrook. Editing by M A Woods. The British Geological Survey gratefully acknowledges the co-operation of landowners in the district during the geological survey.

Notes

The word 'district' refers to the area of the geological 1:50 000 Series Sheet 265 Bath. National grid references (NGR) are given in square brackets. The district lies entirely within 100 km square ST and the letter prefix is omitted. Boreholes are identified by the BGS registration number in the form (ST87SW/1), where the prefix indicates the 1:10 000 scale National Grid quarter sheet. Symbols in parentheses after lithostratigraphical names are those used on the geological map.

The grid, where it is used on figures, is the National Grid taken from Ordnance Survey mapping. © Crown copyright reserved Ordnance Survey licence number GD272191/2011

Geology of the Bath district (summary from rear cover)

(Rear cover)

An explanation of sheet 265 (England and Wales) 1:50 000 series map.

This Sheet Explanation provides a short description of the geology of the Bath district, which embraces the city of Bath and parts of Bristol as well as the towns of Melksham and Chippenham in the east. The district includes the scenic countryside of the southern Cotswold Hills, as well as the picturesque Avon valley.

The bedrock of the Bath district includes rocks of Upper Palaeozoic and Mesozoic age, spanning approximately 300 million years of Earth's history. The Palaeozoic rocks represent the sedimentary deposits of rivers, coal swamps and coastal plains, and were uplifted, eroded, faulted and deformed into a series of regional-scale folds before deposition of the Mesozoic sediments. The Mesozoic rocks represent the deposits of deserts, lakes, shallow marine shelves and deeper seas, and are affected only by more minor, later tectonism. Also included is a description of the Quaternary (superficial) deposits of the district; mainly materials deposited by rivers and streams, as well as the mass-movement (landslide) deposits in the area around the city of Bath.

Information on the applied geological aspects of the district includes an account of water resources, building stones and the former coalfields in the west of the district, as well as descriptions of the famous hot springs and 'Bath stone'. There is also a synopsis of engineering ground conditions, potential geological hazards, and issues of geological conservation within the district, all of which are significant considerations for planning and development.

Chapter 1 Introduction

This Sheet Explanation provides a summary of the geology of the area covered by geological 1:50 000 Series Sheet 265 Bath. The main population centres are the eastern suburbs of Bristol together with Chippenham, Melksham, Corsham and the city of Bath itself, which is England's only World Heritage City. The majority of the district is however rural, and it lies at the southern end of the Cotswold Hills Area of Outstanding Natural Beauty (AONB). The Cotswold escarpment is the most significant principal geomorphological feature of the district, forming a prominent ridge which runs north from Upton Chew to Old Sodbury. This separates the low-lying undulating ground in the west from the rolling, upland country of the Cotswold Hills, which then fall gently eastwards towards the Avon valley. In the east, the ground rises towards the Chalk downlands of Salisbury Plain.The principal river of the district is the River Avon which enters the north of the district, following a circuitous route through Chippenham, Melksham and Bath to the western edge of the district at Keynsham. The Cam Brook and By Brook are significant tributaries, both entering the river at Bath.

The bedrock of the Bath district includes rocks of Palaeozoic and Mesozoic age, spanning at least 300 million years of Earth history. Strata of latest Silurian age are present at depth. The oldest exposed rocks are of Devonian age, and represent the sedimentary deposits of rivers and coastal plains. These are succeeded by Carboniferous strata formed in tropical seas, brackish-water deltas and freshwater swamps, and much affected by earth movements at the close of the Carboniferous and intermittently thereafter through the Mesozoic. Overlying Triassic rocks were deposited in arid, desert environments, but marine conditions returned in the Jurassic and Cretaceous and these successions were laid down in subtropical seas.

All these units are mantled by Quaternary (superficial) deposits on hillslopes and in the major river valleys. Slope failure (mass movement) is a significant feature in the Bath district, and a number of large landslides, also of Quaternary age, are preserved in the area around Bath and along the Cotswold and Corallian escarpments.

The geology of the district has been exploited by man for many centuries. Extensive underground workings and quarries supplied the famous 'Bath Stone' for building, whilst the Carboniferous rocks in the west of the district have been mined for coal. Today, the Middle Jurassic succession forms an important aquifer in the district, and the hot springs at Bath, used for medicinal and recreational purposes since Roman times, draw many visitors to the city.

Survey history

The Bath district has a special place in the history of geology. The first known British geological map, drawn by William Smith in 1799, was of the area around the city (Plate 1). Smith is sometimes known as 'Strata' Smith or 'the Father of English Geology'. He was the first to appreciate the significance of fossils in identifying beds of rock, thereby enabling him to place them in a succession, or 'Order of strata' (Torrens, 2000), and to deduce the probability of the presence of minerals, including coal.

The entire district was first surveyed at the scale of one inch to one mile and published as hand-coloured and engraved 1:63 360-scale 'Old Series' Geological Survey sheets 19 (1845), 14 (1857), 34 (1857), and 35 (1866). These surveys were largely the work of the Geological Survey's founder and first Director, Sir Henry de la Beche, who also published an account of parts of the geology (De la Beche, 1846). The district was resurveyed at the six-inch to the mile scale (1:10 560) in 1944–48 and 1957–59 by D R A Ponsford, G A Kellaway, R Cave, F B A Welch, W Bullerwell and G W Green, being published as 1:63 360 Geological Survey Sheet 265 in 1965. No memoir was produced to accompany this map, which was reconstituted without revision on the 1:50 000 scale in 1990. However, the memoirs describing the Bristol 1:63 360 Special Sheet (Donovan and Kellaway, 1984; Kellaway and Welch, 1993) include a strip 7 km wide covering the west of the Bath district. The present survey incorporates the addition of artificial deposits and amendments to the bedrock, superficial and mass movement geology at the 1:10 000 scale by A J M Barron, R A Edwards, R A Ellison, A R Farrant, P R N Hobbs, K R Royse, T H Sheppard, P J Strange, R K Westhead and R J Wyatt in 1985 to 2007. This work was published as a combined bedrock and superficial deposits map at the 1:50 000 scale in 2011.

Chapter 2 Geological description

Pre-Carboniferous rocks

Little is known of the pre-Devonian succession. Tremadoc strata are known to the north and it is probable that Silurian volcanic rocks are present at depth, connecting the outcrops in the Mendips and at Tortworth. The Hamswell Borehole (ST77SW/1) [ST 7348 7088]) penetrated around 300 m of red-brown mudstone, dipping 15 to 30° and belonging to the Raglan Mudstone Formation (Silurian, Pridoli age). The formation lies in the lower part of the Lower Old Red Sandstone Group, Old Red Sandstone Supergroup, and younger Devonian-age Old Red Sandstone strata are inferred to be present at depth more widely across the district (Figure 1). During the Devonian, the Bath district lay within a broad, low-lying coastal plain on the southern margin of a continent known as Laurussia, bordered to the north by uplands stretching from North Wales to the Pennines and East Midlands, and to the south by an ocean basin. Major (Acadian) uplift in the Mid Devonian created an unconformity between the Lower and Upper Old Red Sandstone groups. On the northern edge of the district, the Upper Old Red Sandstone is exposed at the eastern end of the Chipping Sodbury railway cutting [ST 733 816] (Green, 1992, fig. 21). Here, about 10 m of Tintern Sandstone Formation (TS), of latest Devonian (Famennian) age, comprises purplish brown sandstone with subordinate mudstone beds and scattered pebbles. The succession records the gradual change at the end of the Devonian from fluvial sedimentation to marine deposition that was fully established in the early Carboniferous. All these occurrences of Old Red Sandstone are inferred to form the culmination of the 'Bath Axis', a westerly-dipping monocline at depth, possibly overlying a Variscan displacement, and forming a basement high of dense rocks that is very apparent as a high gravity anomaly (Figure 2). To the west this axis is bounded by the north–south trending Coalpit Heath and Pensford–Radstock synclines and the intervening east–west Kingswood Anticline (Figure 1).

Carboniferous

The Carboniferous rocks of the district formed when the British Isles occupied a broadly equatorial setting. The Lower Carboniferous (Mississippian) succession is traditionally referred to as the 'Carboniferous Limestone' (now a formal supergroup), and in southern Britain comprises a thick sequence of limestone, dolomite and subordinate siliciclastic strata. They were deposited following major sea-level rise in the early Tournaisian and the drowning of the coastal floodplains of southern Laurussia. A shallow, southward-dipping carbonate ramp became established, and active tectonism led variously to periods of emergence and submergence. By the late Visean the ramp had been largely drowned and open shelf conditions prevailed.

During the Namurian, climate change, coupled with uplift of the Wales– Brabant High to the north (Besly, 1987), led to southward progradation of deltas which occluded the marine environments of the Mississippian. At the end of the Carboniferous, the closure of the Rheic Ocean to the south saw the uplift of a fold belt in the region of northern France, and the onset of the Variscan Orogeny in southern Britain. During the early part of the Westphalian, fluviatile and lacustrine depositional environments with coal mires became established on an open coastal plain subject to sporadic marine incursions from the east. By mid Westphalian times, however, marine incursions had ceased and continental red beds were deposited in the Bath district.

Mississippian (Tournaisian to Visean)

At outcrop in the Bath district, occurrences of the Tournaisian to Visean Carboniferous Limestone Supergroup (CL) are confined to the Chipping Sodbury railway cutting and a series of north to south-trending inliers within the Mesozoic outcrop at the foot of the Cotswold escarpment. In the subsurface, Carboniferous Limestone is also found at relatively shallow depths beneath the city of Bath (see Applied geology). These occurrences lie on the flanks of the Bath Axis; Carboniferous strata are absent over its culmination (Figure 1), and in the subcrop they are inferred to stretch east and north from Bath to the Lucknam Borehole (ST87SW/1) [ST 8338 7071]). This borehole proved 90.5 m of probable Carboniferous Limestone underlying the Penarth Group, but beyond this its eastern extent and structure are uncertain. The axis appears to be sinistrally offset by the Bitton–Tadwick Fault and other east–west faults in the Wick area (Figure 2), but this displacement maybe the result of complex movements including thrust and normal faulting.

The Carboniferous Limestone Supergroup is divided into the Avon Group (Av) and the overlying Pembroke Limestone Group. The Avon Group is poorly known in the district, being exposed only at the eastern end [ST 732 816] of the Chipping Sodbury railway cutting, where it is approximately 25 m thick and rests conformably upon the Tintern Sandstone. The basal part of the group comprises coarse bioclastic and ooidal limestone with subordinate mudstone beds, which is probably equivalent to the Shirehampton Formation recognised in the adjacent Bristol district (Barton et al, 2002). These strata are disconformably overlain by greenish grey mudstone with subordinate black crinoidal limestone. The lowermost 58 m of strata in the Lucknam Borehole include many beds described as shale and probably also represent the Avon Group.

The lowest division of the succeeding Pembroke Limestone Group is the Black Rock Limestone Subgroup (BRL), present in the Chipping Sodbury cutting and in a small inlier [ST 724 781], near Codrington. The uppermost 32.5 m of Carboniferous Limestone in the Lucknam Borehole may also represent this subgroup. The Black Rock Limestone comprises a unit of dark grey or black, well-bedded crinoidal limestones with a rich fauna of corals and brachiopods. It attains a thickness of 180 m in the district, and the uppermost 40 m are widely dolomitised.

In the outcrops at Chipping Sodbury and Codrington, the Black Rock Limestone is disconformably overlain by the Gully Oolite Formation (GuO), comprising white-weathering, thick-bedded, pale grey oolite, approximately 30 m thick and generally unfossiliferous. The formation represents the development of migratory ooid shoals in a high-energy, shallow-water environment following sea-level rise in the early Visean. The sharp, erosional contact between the Gully Oolite and the overlying Clifton Down Mudstone Formation (CDM) is exposed in the extensively quarried inlier [ST 7105 7315] at Wick (Kellaway and Welch, 1993). The basal part of the Clifton Down Mudstone Formation here comprises up to 3.5 m of brecciated and conglomeratic limestone with clasts of Gully Oolite and greenish grey mudstone, overlain by some 13 m of interbedded calcareous and non-calcareous mudstone that is typical of the Clifton Down Mudstone. At Wick, this facies is interrupted by a prominent 8.5 m-thick succession of hard, grey crinoidal and ooidal limestone, representing the Goblin Combe Oolite Formation (GCO). Above this, a further 18 m of lime mudstone and dolomitic limestone are regarded as the upper leaf of the Clifton Down Mudstone Formation. Stromatolitic algae are common in the Clifton Down Mudstone, but otherwise these rocks are poorly fossiliferous, and probably represent subtidal to peritidal deposits developed in a lagoon. In contrast, the Goblin Combe Oolite has yielded the gastropod Bellerophon at Wick, and a rich brachiopod fauna at localities in the Bristol district (Kellaway and Welch, 1993). It represents a higher-energy shoreface environment, and may be indicative of mid Visean transgression.

The Clifton Down Mudstone is overlain by the Lower Cromhall Sandstone (LCS) in the Chipping Sodbury railway cutting (where it is 4 m thick) and in quarries centred at [ST 7252 8263] immediately north of the district. It is the lowermost of three tongues of sandstone (Kellaway and Welch, 1993, fig. 9) constituting the Cromhall Sandstone Formation, which spread southward and interrupted carbonate deposition in the late Visean (Cave, 1977). However, no evidence of arenaceous strata has been found at this level at Wick, where the 240 m-thick Clifton Down Limestone Formation (CDL) occurs above the Clifton Down Mudstone. The basal 60 m comprises splintery limestone with algal beds and mudstone partings, overlain by bedded lime mudstone which passes upwards into ooidal limestone. The Clifton Down Limestone represents the transition from the Tournaisian– early Visean carbonate ramp to a more open marine shelf setting in the late Visean.

At both the Chipping Sodbury cutting and Wick, the Clifton Down Limestone is succeeded by the Middle Cromhall Sandstone (MCS). This interval is relatively thick where seen in a quarry [ST 7232 8420] north of the district, with some 27 m of sandstone, dolomitised limestone and mudstone (Murray and Wright, 1971). However, at Wick, only around 5.5 m of rippled sandstone, overlying 2 m of nodular mudstone, is exposed. Up to 20 m of strata may be present in the district, but southwards the beds have pinched out completely at Grandmother's Rock [ST 7090 7118], north-east of Beach.

Above the Middle Cromhall Sandstone at Wick is the Oxwich Head Limestone Formation (OHL; formerly known as the Hotwells Limestone Formation), where it is approximately 75 m thick. It also forms most of the Grandmother's Rock inlier where it may be over 100 m thick. The formation comprises massive, grey, crinoidal and ooidal limestone, with the most varied Visean fauna of the district, notably corals including Syringopora, and brachiopods including gigantoproductids, athyrids, chonetids and spiriferids. This rich fauna indicates that deposition took place in fully open sea, shelf conditions.

The uppermost tongue of Visean sandstone, the Upper Cromhall Sandstone (UCS), overlies the Oxwich Head Limestone Formation at outcrop, and is more than 210 m thick at Wick. It largely comprises interbedded sandstone and mudstone of fluviodeltaic origin, with several beds of crinoidal and ooidal limestone. The lowermost of these is the 20 m-thick Castle Wood Limestone (CWL), which may correlate with the Rownham Hill Coral Bed of Bristol (Kellaway and Welch, 1993). A thin bed of fossiliferous limestone, named the Mollusca Bed (Mo), lies about 15 m below the top of the formation.

Mississippian to Pennsylvanian (Namurian)

Namurian rocks are represented by the Marros Group (Mar), laid down in an isolated basin with relatively little marine influence. They crop out only in the Wick inlier, although they are present at depth throughout the west of the district (Figure 1). Near the base is a sequence of distinctive chert and cherty mudstone beds, up to 15 m thick, which in the Bristol district (Barton et al., 2002) has yielded an early Namurian (Pendleian) age fauna (Kellaway and Welch, 1993., p.63) correlated with the Aberkenfig Formation of South Wales (Waters et al., 2009). Above these, the Quartzitic Sandstone Formation (QS) comprises fluviodeltaic sandstone and mudstone with seatearth beds and thin coal seams, between 70 and 185 m thick.

Pennsylvanian (Westphalian)

Westphalian rocks crop out widely in the western part of the district, as part of the Bristol–Somerset Coalfield. The coalfield is divisible into three structural areas (Figure 1), two of which are at surface: the Kingswood Anticline running east to west from Wick towards Kingswood, and the Coalpit Heath Syncline, running north to south from Iron Acton towards Mangotsfield. Coal-bearing strata also occur in the Pensford–Radstock Syncline in the south-west, generally subcropping beneath Mesozoic rocks.

The lowermost division is the South Wales Coal Measures Group, which largely comprises rhythmic alternations of mudstone, silty mudstone, sandstone and coal. It was laid down on a coastal plain which stretched from the Bath district west to South Wales, and occupied an area to the south of the Wales–Brabant landmass. The intercalation of marine mudrocks ('marine bands') indicates infrequent marine incursions, probably related to eustatic transgressions and valuable for regional correlation. In the area west of Wick, marine mudstone with the gastropod Donaldina ashtonensis (Bolton) overlies Namurian strata (Kellaway and Welch, 1993), and is the local representative of the Subcrenatum (Ashton Vale) Marine Band that marks the base of the South Wales Coal Measures Group. Together with the strata up to the base of the Vanderbeckei (Harry Stoke) Marine Band, these comprise the South Wales Lower Coal Measures Formation (SWLCM) of Langsettian (Westphalian A) age. The formation, which is thought to be around 200 m thick, does not crop out in the district, and is poorly known; it has not been worked for coal and it is uncertain whether representatives of the main seams of the Lower Coal Measures in the Bristol district (the Ashton coals) are present.

Higher in the succession, the Aegiranum (Croft's End) Marine Band divides the Duckmantian (Westphalian B) and Bolsovian (Westphalian C) successions, and the youngest recorded marine horizon in the European Westphalian is represented by the Cambriense (Winterbourne) Marine Band. The rocks between the base of the Vanderbeckei Marine Band and top of the Cambriense Marine Band comprise the South Wales Middle Coal Measures Formation (SWMCM), which is around 685 m thick in the district. It crops out in the core of the Kingswood Anticline and its subcrop extends to the north and south (Figure 1). The formation contains a number of economically important coal seams, the Kingswood Great Coal formally being the main productive seam of the Bristol Coalfield and generally about 1 m thick.

In the Bristol Coalfield, the South Wales Middle Coal Measures Formation is conformably overlain by the Pennant Sandstone Formation — the lower part of the Warwickshire Group and here the lateral equivalent of the South Wales Upper Coal Measures Formation. The Cambriense Marine Band marks the base of the group south of the Kingswood Anticline, but north of this axis interpretation is complicated by the absence of a recognisable marine interval at this horizon. The Pennant Sandstone is fully developed in the Coalpit Heath Syncline and in the subsurface in the Pensford–Radstock Syncline, and it is inferred at depth in the east (Figure 1). It is Bolsovian in age, possibly ranging up to Asturian (Westphalian D), and up to 1100 m thick. During the Bolsovian, the Variscan Orogeny began to exert an influence on the depositional environment and patterns of sedimentation in southern Britain. The Pennant Sandstone is characterised by the development of massive sandstone bodies thought to represent increased sediment supply from orogenic highlands to the south. The lower division of the Pennant Sandstone is the Downend Member (Dn), which comprises thick sandstone and mudstone with coal seams in its lower part, and includes the Mangotsfield Coals (Mng) at the top. From over 650 m thick on the western edge of the district at Downend, it thins north-east to under 150 m at Yate [ST 706 820] (Kellaway and Welch, 1993, p.96). It also comes to crop in an inlier at Corston [ST 696 656]. The overlying Mangotsfield Member (Mg) similarly comprises massive sandstone with mudstone and rare thin coal seams, and is around 450 m thick.

The Pennant Sandstone is succeeded by the Grovesend Formation, which here is Asturian in age (Waters et al., 2009), and occupies the core of the Coalpit Heath Syncline. The High Coal marks the base of the formation, the constituent Farrington and Barren Red members (FaBR) not being differentiated on the map. The strata comprise grey mudstone with sandstone beds and coal seams, passing up into red mudstone and sandstone lacking coal, and may attain 500 m in total thickness.

Younger Westphalian strata are absent from the Coalpit Heath Syncline, and the succeeding Radstock Member (Rad) is found at outcrop only in the far south-west of the district. It comprises up to 100 m of grey mudstone with sandstone lenses and numerous thin, muddy coal seams.

At the close of the Carboniferous, the convergence of the Laurasia and Gondwana continents, closing the Rheic Ocean to form the Pangaean supercontinent, resulted in north-directed thrusting. This is especially evident on the Farmborough and Southern Overthrust faults, collectively causing the uplift of the Bath district, and more widely resulting in the establishment of an arid terrestrial environment. The district lay on the northern margin of the Variscan mountains, and northwards abutted against the Worcester Uplift (Smith and Rushton, 1993) that forms part of the Midlands Microcraton. In early Permian times deep erosion stripped the Pennsylvanian cover from the central part of the Bath district, exposing the Tournaisian to Visean and older rocks which lie along the axis of the Kingswood Anticline and along the Worcester Uplift.

Triassic

By the Early Triassic, the northern part of Pangaea, which included the British Isles, lay a little north of the equator. The wider region was still largely high ground, and although alluvial sediments (Sherwood Sandstone Group) were probably being deposited to the east, it was not until Late Triassic (Carnian to Norian) times that deposition recommenced in the Bath district.

Carnian to Norian

Rocks of Carnian to Norian age in the Bath district are assigned to the Mercia Mudstone Group (MMG), which crops out around Corston in the south, and in a continuous outcrop from Bitton northwards to Chipping Sodbury. The group below the Blue Anchor Formation largely comprises red dolomitic mudstone with greenish laminae and subordinate evaporite beds, totalling up to 80 m in thickness at outcrop. A number of pale grey sandstone and siltstone beds, known as 'skerries', occur towards the top. A celestite-bearing mudstone, about 1 m thick, is also recognised in the upper part in the area between Oldland Common [ST 68 71] and Siston [ST 67 75]. It is thought to represent the Yate Evaporite Bed (YE) of Kellaway and Welch (1993, p.135). Although absent over the Bath Axis, the group reappears at depth in the south and east, thickening to almost 300 m. The Mercia Mudstone Group represents the deposits of euhaline playa lakes and sabkha-type mudflats. In contrast, the Mercia Mudstone Marginal Facies (MMMF; formerly known as the 'Dolomitic Conglomerate') comprising breccia, conglomerate and sandstone with dolomitic cement, represents screes, alluvial fans and wadi deposits developed along the margins of the playa lakes and sabkha flats. The facies is markedly diachronous, persisting throughout the Mercia Mudstone Group wherever it onlaps the Palaeozoic rocks, but crops out only around Wick in this district, reaching perhaps 30 m in thickness.

Rhaetian

The red mudstone of the undivided Mercia Mudstone Group grades rapidly up into the Blue Anchor Formation (BAn), an up to 5 m-thick succession of grey, greenish grey and green partly dolomitic mudstone, its green colouration indicating a reducing environment and reflecting a change to a wetter climate. At this time the landscape of the district comprised extensive coastal flats, interrupted by denuded outcrops of Palaeozoic rocks which formed low-relief hills.

The Mercia Mudstone Group is disconformably overlain by the Penarth Group (PnG), formed by a continuation of the marine transgression which began with the deposition of the Blue Anchor Formation, and persisted throughout Rhaetian times and into the Early Jurassic. Consequently the group oversteps the older Triassic rocks to lie unconformably upon the Palaeozoic rocks in the north and east of the district, as seen in the inliers at Wick, Codrington, and in the Chipping Sodbury railway cutting. The group thins north from 10 m to less than 7 m, but its outcrop is too narrow to be subdivided on the map. It may also thicken south-eastwards at depth. The basal division, the Westbury Mudstone Formation, comprises 3 to 5 m of very dark grey to black pyritic mudstone, with thin muddy limestone beds and locally a basal bed containing pebbles and abundant fish teeth and vertebrate debris ('Rhaetic Bone Bed'). The formation yields a varied fauna including fish, gastropods and bivalves. Overall, the Westbury Mudstone represents deposition in a series of fringing lagoons. A transition from lagoonal to more open marine environments is represented by the overlying Cotham Formation, which comprises about 3 m of grey-green mudstone, calcareous mudstone and limestone, with a thin stromatolitic limestone (the Cotham Marble) at the top. The succeeding White Lias Formation is a 0.6 to 3 m-thick unit of pale, well-bedded limestone with thin mudstone interbeds (Plate 2) (Donovan and Kellaway, 1984). A shallow marine setting is envisaged for the deposition of this unit, with the sea bed suffering periods of emergence. An erosion surface that caps the highest limestone in the succession is marked by desiccation cracks and numerous Diplocraterion burrows.

Jurassic

Marine transgression continued into the Jurassic, and the uplands of the Triassic landscape became distinct landmasses: Cornubia to the south and west of the Bath district, the Anglo-Brabant landmass to the east and the Welsh Landmass to the north-west. Throughout the Jurassic, the Bath district occupied part of a shallow marine platform on the northern margin of the deeper waters of the Bristol Channel, Wessex and Weald basins. Rocks of Jurassic age crop out across most of the district and a relatively complete succession is present, with strata ranging from Hettangian to Oxfordian in age.

Lower Jurassic

Largely argillaceous rocks comprise the Lower Jurassic (Hettangian to Toarcian) Lias Group. This includes much limestone in the lower part with sand and sandy silt towards the top. The lowest unit of the group is the Blue Lias Formation (BLi) of latest Rhaetian to Sinemurian age, which is between 18 and 39 m thick, and is a rhythmic succession of interbedded limestone and mudstone (Plate 2). The formation is considered to represent hemipelagic shelf sedimentation in relatively quiescent, variably agitated waters. The relative proportions of limestone and mudstone vary through the succession, leading to the recognition of a tripartite subdivision in much of the district, with an upper and lower carbonate-rich unit, separated by a carbonate-poor silicate mudstone division. For the English Midlands, Ambrose (2001) defined these divisions as the lower Wilmcote Limestone Member (Wct), middle Saltford Shale Member (SaSh), and upper Rugby Limestone Member (RLs). They cannot be recognised south of the Newton Fault, where the formation is undivided. Further expansion of the Early Jurassic marine transgression led to the deposition of the grey, variably fossiliferous mudstone, known as the Charmouth Mudstone Formation (ChM). The formation thickens from typically 100 m at outcrop (although locally as little as 35 m) to about 160 m at depth in the north-east (Green, 1992).

In most of the district, the Charmouth Mudstone is overlain by the Dyrham Formation (DyS), a succession of poorly-indurated interbedded mudstone, siltstone and sandy siltstone up to 27 m thick. Above this, the Beacon Limestone Formation (BnL; formerly known as the 'Junction Bed') forms a discontinuous outcrop of up to 3 m of ferruginous and ooidal limestone. The sand and silt content of the Dyrham Formation is thought to indicate an increase in clastic sediment flux during a minor regression, whereas the thin and very condensed succession of the Beacon Limestone Formation may be the result of prolonged deposition in sediment-starved deeper water (Hesselbo, 2008).

In the south and east the Dyrham and Beacon Limestone formations are overstepped by younger Lias Group strata. Above the Beacon Limestone, the Bridport Sand Formation (BdS) is a succession of poorly indurated, bioturbated sand and silty sand, with a few carbonate-cemented masses of sandstone ('doggers'), which thickens north from 20 to 45 m and east to perhaps 60 m.

Middle Jurassic

Rocks of Middle Jurassic age extend from the Cotswold escarpment eastward forming the broad dip slope of the Cotswold Hills. Toarcian regression, accompanied by crustal upwarp and volcanism in the North Sea, led to radical palaeogeographical changes in southern Britain by the onset of the Aalenian. Much of the Midlands became emergent, and the Aalenian to Bathonian rocks of the Bath district represent relatively condensed carbonate deposition on a shallow marine shelf, interrupted at times by the influx of considerable volumes of mud.

The lowest division of the Middle Jurassic is the Inferior Oolite Group (InO), which forms a prominent cuesta along the entire Cotswold escarpment. A tripartite division of the succession was recognised in the wider Cotswold region by early workers (Buckman, 1893, 1895; Witchell, 1882), and allocated to three formations in the north Cotswolds by Barron et al. (1997). Uplift and erosion in early Bajocian times (Kellaway and Welch, 1993) was most pronounced in the Bath area and probably removed any strata representative of the lower parts of the group (Aalenian and lower Bajocian), forming a significant unconformity. Renewed transgression led to the deposition of the youngest of the three divisions, of late Bajocian to early Bathonian age, comprising a succession of bioclastic, ooidal grainstones 12 to 23 m thick, but these are too dissimilar to the coeval strata of the north Cotswolds to share the same formation name. The rocks represent the deposits of ooid shoals and barriers formed in a shallow, high-energy environment.

Later in the Bathonian there was a considerable influx of clastic sediment, and the clays and limestones deposited at this time form the Great Oolite Group, totalling about 100 m in thickness. The basal unit is the Fuller's Earth Formation (FE), which is approximately 40 m thick, and comprises a series of thick silicate and calcareous mudstone beds interleaved with beds of coarsely bioclastic limestone. Immediately south of the district, the upper part of the formation contains a commercially exploited bed of montmorillonite clay, which may extend into the southern part of the Bath district (see Mineral resources). The presence of abundant montmorillonite indicates contemporaneous volcanic activity. A sequence of shelly limestone 2 to 5 m thick forms a feature between Englishcombe in the south [ST 721 625] and near Dyrham [ST 745 750] in the north, and has been named the Fuller's Earth Rock Member (FER; (Figure 3)). North from Dyrham, it passes into the fine-grained Dodington Ash Rock Member (DA), up to 2 m thick. Above it, the Tresham Rock Formation (TR) forms a succession of fine-grained limestone with calcareous mudstone beds that is up to 5 m thick at outcrop, and is overlain by the Lansdown Clay Formation (LC) comprising up to 10 m of calcareous mudstone beds with lenses of limestone. In the outcrop in the extreme north [ST 770 814] and in the subsurface in the eastern part of the district, the Lansdown Clay Formation passes into and is replaced by the Athelstan Oolite Formation (AO). This is a succession of hard, pale grey, fine-grained ooidal limestone with subordinate shell debris, 5.2 m thick in the Lacock No. 2 Borehole (ST96NW/2); [ST 9205 6926]). These rocks represent the development of ooid shoals in higher-energy waters to the east and north of the district.

The Fuller's Earth Formation and its lateral equivalents are disconformably succeeded by the Chalfield Oolite Formation (ChO) of Wyatt and Cave (2002), formerly the 'Great Oolite' of Green and Donovan (1969) (Figure 3); (Plate 3). The formation is between 15 and 31 m thick, and is a succession of fine to coarse, largely matrix-free ooidal grainstones with, at certain levels, much bioclastic debris. At its greatest development in the south of the district, the formation is divisible into three formal units, in ascending order: the Combe Down Oolite Member, the Twinhoe Member and the Bath Oolite Member (Figure 3). North of Box, the Twinhoe Member is thought to pass into the lower part of the Bath Oolite, although the subdivision can still be recognised in the north around Castle Combe (Wyatt and Cave, 2002), and in the east, where the Lacock No. 2 Borehole records a 1 m-thick succession of ferruginous peloidal limestone and calcareous mudstone at the Twinhoe horizon. The formation thins northwards as a result of erosion beneath the Forest Marble Formation, resulting in the removal of probably the entire thickness of the Bath Oolite (Wyatt and Cave, 2002, fig. 1).

The Combe Down Oolite Member (CDO) is a 9 to 18 m-thick unit of creamy-grey to yellowish, medium- to coarse-grained, cross-bedded ooid-grainstone (Plate 3), interleaved in the lower part with prominent beds of calcareous mudstone. The Twinhoe Member (Tw) comprises a succession of up to 11 m of limestone with ferruginous pisoids and ooids, bioclasts and corals, whilst the succeeding Bath Oolite Member (BO) is a monotonous sequence of up to 15 m of fine- to medium-grained, cross-bedded ooid-grainstone, with very subordinate shell debris and without prominent mudstone interbeds. In the north of the district where the members are not distinguished on the map, a prominent succession of up to 5 m of coarse bioclastic limestone (ls) lies at the base of the Chalfield Oolite Formation, and has been informally named the 'Grickstone Beds' (Cave, 1977; Wyatt and Cave, 2002). The Chalfield Oolite Formation represents the distal deposits of an extensive carbonate ramp covered by a shallow, subtropical sea, where high-energy, tidally-dominated conditions led to the development of shifting ooid shoals and migratory sand waves. The deposition of such sediments in the Bath district is attributable to an abrupt southward shift of the ooid grainstone facies belts in the late Bathonian, possibly as a result of the reactivation of deep-seated faults along the northern margin of the Wessex Basin (Bradshaw et al., 1992).

In the south of the district, the uppermost cross-bedded ooid-limestone beds of the Bath Oolite Member are overlain by a succession of white or brown, variously ooidal, shell detrital limestone beds with in places one or more lenticular coralliferous beds (Plate 4), totalling up to 9 m thick. This unit was termed the 'Upper Rags' by Green and Donovan (1969), but not separated from the Bath Oolite on the maps published at that time. The Upper Rags have been subsequently included either in the Forest Marble Formation (Cave, 1977; Wyatt, 1996; Wyatt and Cave, 2002) or as the Corsham Member in the Chalfield Oolite Formation (Sumbler, 2003). In the course of the present survey, it has been concluded that the Upper Rags generally show insufficient similarity in lithofacies with either formation and are termed here the Corsham Limestone Formation (Cm), and where possible are distinguished above the Bath Oolite on the map. The coralline units probably represent patch reefs or their remains. The formation overlies the Chalfield Oolite non-sequentially, and the boundary with the overlying Forest Marble Formation is disconformable, and both contacts may be marked by a hardground.

The Forest Marble Formation (FMb) is 24 to 31 m thick, and on the map it is generally divisible into two parts. The basal unit is composed of lenticular, locally channel-form, flaggy or rubbly, coarse, bioclastic limestone and subordinate ooidal grainstone beds, generally not more than 10 m thick. The higher part is mudstone dominated with lenticular or impersistent limestone beds, and is up to 21 m thick. The limestone beds represent the deposits of a shallow, transitory, probably tidally dominated carbonate shelf or inner ramp. In the north between North Wraxall and Badminton (Malmesbury district) the basal limestone beds are represented by a more argillaceous variant, informally named 'Acton Turville Beds' by Cave (1977). The overlying clay-dominated part of the formation results from an increased fine clastic influx and the occlusion of carbonate production. At one or more levels near the base there is a distinctive brachiopod-dominated benthic assemblage (the 'Bradford Clay Fauna') which flourished at this time.

The Forest Marble Formation is conformably overlain by the Cornbrash Formation (Cb), which is 4 to 6 m thick (Cave, 1977, fig. 20) and consists of poorly bedded, non-ooidal argillaceous bioclastic limestone with partings of mudstone. The Cornbrash represents renewed carbonate deposition following the onset of a major transgression which began during the latest Bathonian. Previous workers have recognised a lower and upper division at outcrop (e.g. Cave, 1977; Douglas and Arkell, 1932), with a non-sequence between them approximating to the Bathonian–Callovian stage boundary and representing a minor marine regression. However, this distinction has not been made during the present survey.

The Kellaways Formation (Kys) represents relatively deeper water sedimentation following continued Callovian transgression. The district includes the type section (Tytherton No. 3 Borehole, (ST97SW/2) [ST 9440 7445]) and type area of the formation at Kellaways [ST 947 758], which is also the origin in latinised form of the stage name Callovian (Cox and Sumbler, 2002). The formation is 21 to 25 m thick, and north of Chippenham is divisible into two members, the Kellaways Clay and Kellaways Sand, but these cannot be distinguished in the south. The Kellaways Clay Member (KlC) is dark grey mudstone with thin and lenticular beds of fine sand or sandstone. The Kellaways Sand Member (KlS) forms a relatively thin development of sand and calcareous sandstone above the Kellaways Clay, perhaps related to a brief marine regression in the mid Callovian. It is well exposed in the banks of the River Avon at Kellaways [ST 9466 7577] (Plate 5), first recognised here by William Smith around 1800.

The top of the Middle Jurassic succession occurs within the mudstone-dominated Oxford Clay Formation (OxC). Its thickness is uncertain but from regional considerations it is estimated to be about 150 m. Where better exposed elsewhere in southern England, the formation is divided into three members, but they cannot be distinguished on the map in this district. The lower part (Peterborough Member) is olive-grey, thinly laminated organic-rich mudstone, which is highly fossiliferous at some levels. This unit represents sedimentation in association with poor bottom-current circulation, resulting in sea-floor anoxia and the widespread preservation of organic matter. Improved circulation occurred in the later Callovian and early Oxfordian, and the upper part of the formation (Stewartby and Weymouth members) is blocky grey mudstone with some shell debris-rich beds and calcareous siltstone horizons.

Upper Jurassic

Upper Jurassic rocks are found only in the extreme south-east of the district. The transgression which characterised the latest part of the Mid Jurassic continued during the Late Jurassic, but tectonic movements in the Celtic Sea area led to the emergence of an enlarged landmass to the west of the district. By the mid Oxfordian, calcareous and sandy sediments spread over extensive areas of southern Britain, forming the rocks of the Corallian Group, which pass laterally south and east into more argillaceous rocks in the Weald and Wessex basins.

The basal unit of the Corallian Group is the Hazelbury Bryan Formation (Hz), a 15 to 30 m-thick unit of yellow, brown and grey very fine- to medium-grained quartzose sandstone, with thin sandy siltstone and sandy mudstone beds in parts. The sediments represent deposition on a quiescent, low-energy clastic shelf. Above this, the Kingston Formation (Ki) is a 5 to 20 m-thick succession of calcareous mudstone and medium-grained quartzose sand, with carbonate-cemented beds and masses of calcareous sandstone or sandy limestone (ls). On the eastern edge of the district, it is overlain by shell detrital limestone, ooidal limestone and calcareous mudstone of the Stanford Formation (St), of which not more than 5 m is present. Sea level fall in the very latest Jurassic led to the removal from the district of all Jurassic strata younger than the mid Oxfordian.

Cretaceous

The Cretaceous is represented only by rocks of the Lower Greensand Group (LGS) Approximately 20 m of strata are preserved within the district, and lie with a marked unconformity on the Upper Jurassic rocks, capping a plateau around Bowden Park. They probably form a local representative of the Seend Ironstone Formation seen further south (Hopson et al., 2008), and consist of glauconitic shelly medium- to coarse-grained sand and sandstone, weathering to reddish brown.

Quaternary

The Quaternary deposits of the district comprise unlithified ('superficial') sedimentary materials laid down during the Pleistocene (2.6 to 0.01 Ma) and Holocene (to present). During the Pleistocene, the British Isles experienced repeated episodes of glaciation, but generally the Bath district is thought to have lain well beyond the maximum ice limits. Consequently, the Quaternary is represented almost entirely by deposits which accumulated through periglacial and fluvial processes.

However, isolated high-level deposits of gravelly sandy clay with flint, quartz, chert and other exotic pebbles occur around Bath, including at up to 175 m OD south-west of Claverton, and as fissure-fills in the Chalfield Oolite (Donovan, 1995). These are thought to be material deposited either by outwash or by fluvial reworking of remanié till, predating the incision of the Avon valley probably before 0.35 Ma (mid Mid Pleistocene) times (Self, 1995), and indicating that an early Mid Pleistocene ice-sheet may have encroached on the Bath district (Hunt, 1998). These are termed the Bathampton Down Member (BD) of the Kenn Formation (Campbell et al. in Bowen, 1999).

The modern River Avon is flanked along much of its course by river terrace deposits which represent abandoned floodplains. They are the Bristol Avon Valley Formation, of Mid Pleistocene age, and consist of clay, silt and sand overlying gravel, and three levels can be identified. However, the unified Avon scheme of Campbell et al. (in Bowen, 1999), with named Bathampton, Stidham and Ham Green members, is not adopted herein as it fails to allow adequately for the influence of the Hanham and Clifton gorges.

The active floodplains of the Avon and its tributaries are underlain by alluvium, which comprises clay, silt, sand and gravel, in places with beds or lenses of peat. On the east bank of the Avon at Warleigh Wood, narrow tributaries running over limestone bedrock have deposited small spreads of calcareous tufa. More widely across the district, slopes are commonly mantled with a discontinuous veneer of head deposits, which represent slopewash and colluvial materials, deposited both under periglacial conditions in the Pleistocene and under broadly modern climatic conditions in the Holocene. They consist of gravel, sand, silt and clay in variable proportions, reflecting the composition of the geological materials upslope.

Superficial structures and mass-movement deposits

Along the escarpment and in the valleys of the Cotswolds, slopes have been extensively affected by superficial disturbances dating from Pleistocene times to the present. Within the district there are many areas where strata capping slopes and hills have begun to tilt or move downslope as blocks, due to the deformation of underlying, less competent mudstone/clay beds that have become ductile (Hobbs and Jenkins, 2008). The process, known as cambering, particularly affects slopes in the Fuller's Earth Formation and Lias Group rocks, which are capped by the Chalfield Oolite Formation or Inferior Oolite Group respectively (Figure 4), (Figure 6). Cambered masses that have not become clearly detached from their parent bedrock outcrops (and thus included in landslides) typically have poorly defined lateral extents, and consequently are not distinguished on the geological map. The early stages of cambering lead to the development of large, joint-bounded blocks of limestone, with extensional downslope movement opening the joints and leading to the development of cavities (known as gulls) between blocks (Hobbs and Jenkins, 2008). Most of the gulls are subsequently partly or wholly filled with rubble, soil or other deposits. Gulls present a significant geological hazard (see Applied geology), and although their overall distribution is poorly known, numerous gulls have been recorded within the district. They may be up to ten metres deep, two metres wide, and several tens of metres long. Where slippage occurs along the interface between the thick beds of the Chalfield Oolite, permitting lower blocks to move, closely spaced open gulls may be present up to 20 metres below ground surface and some distance in from the valley side; gull caves may develop and extend for several hundred metres (Self, 1986; 1995; Self and Boycott, 2000).

Cambering is thought to take place largely under glacial or periglacial conditions (Forster et al., 1985). Cambered strata are overlain by river terrace deposits at Twerton (Chandler et al., 1976) indicating that at least some movements are ancient. It is not thought that cambering is active here under the present temperate climate, although landslides that are more recent commonly incorporate cambered (e.g. block-toppled) material.

Discrete areas of mass down-slope movement of rock and/or soil are depicted as landslide deposits, and comprise rock falls, mudflows, and either rotational or translational slides which may include detached cambers (Figure 4). Their formation results from several principal processes, possibly in combination: weathering-induced limestone fragmentation and conversion of mudstone to clay; undercutting or loading of slopes by natural or human actions, and changes in the groundwater regime. An increase in pore water pressure in overconsolidated clay or silt-dominated formations (e.g. Charmouth Mudstone, Bridport Sand, Dyrham, Fuller's Earth and Oxford Clay) and in overlying fine-grained head deposits, can result in reduced shear strength and relatively shallow failure of the hillslope. This commonly occurs by rotational sliding, or by translational sliding (Anson and Hawkins, 2002), particularly within the Fuller's Earth Formation and Lias Group (Figure 6). The majority of the landslides within the district are of these types, and probably formed during the wetter, periglacial climate of the latest Pleistocene. Many of the older landslides within the district, including those at Bailbrook [ST 773 673], Beacon Hill [ST 751 659], Beechen Cliff [ST 751 641], Twerton [ST 726 644] and North Stoke [ST 700 687], are large rotational failures on the lower slopes, thought to be initiated by the downcutting of the River Avon, perhaps during the later Pleistocene (Kellaway and Taylor, 1968). Minor movements, usually of mudflow or translational type associated with prolonged saturation, continue to the present day (Anson and Hawkins, 2002) and landslides continue to present a hazard within the district (see Applied geology).

An additional form of ground disturbance that may be prevalent within the district is valley bulging. This occurs when river downcutting results in significant unloading and the upward bulge of parts of the valley floor, typically under periglacial freeze-thaw conditions. It may have taken place in many of the valleys in the district during the Pleistocene, including the Avon valley around Bath, although much of the affected strata may have been removed by Late Pleistocene erosion. Bulging is not thought to be an ongoing process today, but deposits affected may pose a hazard to engineering.

Artificially modified ground

Worked ground is shown where the ground has been excavated, generally for minerals, and has not been backfilled. Only the larger areas are shown, most of which are quarries for building stone and aggregate, as at Brown's Folly [ST 796 662] and Wick [ST 709 737] respectively. Made ground is shown where substantial thicknesses of material have been deposited by human activity, including mine or quarry waste, spoil and refuse, together with areas of engineered or non-engineered embankments and extensive tracts of fill on the Avon floodplain through Bath. Where excavations have been backfilled, infilled ground is shown; this includes many surface workings in the area of Combe Down [ST 76 62].

Geological structure and regional geophysics

The rocks of the Bath district present a long record of earth movements, represented by faults and folds in both the Palaeozoic and Mesozoic strata. Structural interpretation of the district has been undertaken by study of seismic reflection profiles, and is aided by colour-shaded Bouguer gravity anomaly (Figure 2) and aeromagnetic anomaly (Figure 5) maps.

Folding and faulting

The major, regional-scale folds of the district are developed in the Palaeozoic rocks (Figure 1), and represent deformation associated with the Variscan Orogeny, at the close of the Carboniferous; a consequence of the collision of Laurasia with the southerly continent Gondwana. This compressional tectonic regime deformed the rocks of southern Britain into a series of open folds. Within the present district, the two most prominent folds seen at outcrop are the Coalpit Heath Syncline and the Kingswood Anticline (Figure 1). Both folds involve relatively young (Bolsovian–Asturian) Carboniferous strata, and it is possible that deformation was already taking place as these end-Carboniferous rocks were being deposited. Both folds are markedly asymmetrical; the eastern limb of the Coalpit Heath Syncline dips west at around 40°, whilst the western limb dips east at only 10°. Cave (1977) suggested that the syncline may have been developed as a result of buckling in association with the Variscan reactivation of older faults along the Malvern Fault Belt. The latter is a north–south trending fracture zone cored by Precambrian rocks and with a history of movement extending into the Proterozoic. Fault reactivation during the Variscan was widespread in southern Britain, with many older extensional structures being reactivated as reverse faults; however, the sense of movement on most major faults of the district, including the Coalpit Heath, Kidney Hill, Bitton– Tadwick, Newton and Pennyquick faults is normal. This is exemplified by Early Jurassic synsedimentary growth-faulting associated with extension, leading to differential thickening across these structures. Several thrust fault belts are recognised in the ground to the south and west of the district (Barton et al., 2002), and it is known from subsurface workings that a zone of intense deformation associated with the Avon Thrust (Kellaway and Hancock, 1983) extends into the district, occupying the core of the Kingswood Anticline. The Farmborough Thrust (or Fault Belt) seen in the Somerset Coalfield (Barton et al., 2002) can be traced from the south-west in the seismic data towards the Warleigh Fault, and the Southern Overthrust (Green, 1992) may continue east as the Trowbridge Fault (Figure 1). These thrusts are thought to have vertical throws of 300 m and horizontal displacements of over 1 km.

In comparison with the Palaeozoic rocks, the Mesozoic rocks of the district are little deformed, showing a gentle tilt to the south-east. In the south-east the dominant fault trend is north-east to south-west, seen in the Warleigh, Corsham and Monkton Farleigh faults and the Atworth–Lacock Fault Belt. Some of these define minor grabens in the Mesozoic cover, but are also expressions of significant displacements in the basement which may be manifest as changes in the gravity field (Figure 2). The Warleigh Fault downthrows south-east, and from seismic reflection profile evidence appears to influence thickening in Triassic rocks. A prominent anticline in both Carboniferous and Mesozoic rocks is formed in the hanging wall of the probable antithetic fault to the Warleigh Fault and these structures can be traced at depth into the fault belt around Lacock (Figure 1).

The Radstock Basin is well defined by a magnetic low in the west of the district (Figure 5), whilst to the east the progressively stronger magnetic response indicates the presence of Lower Palaeozoic volcanic rocks at shallow depths uplifted where the Variscan trend meets that of the Worcester Graben.

Chapter 3 Applied geology

Geological factors have a significant influence on the activities of man and as such are major considerations for land-use planning and development. Consideration of earth science issues early in the planning process can help ensure that site and development are compatible, that local resources are not damaged or contaminated, and that any appropriate mitigation measures are taken prior to development. Potential geological hazards may present a public health risk or require costly remediation. Engineering ground conditions and designated sites of geological conservation strongly influence the location and design of any new development.

Hydrogeology

Several major aquifers lie within the district. These are rocks which have a high permeability and/or a known or probable presence of significant fracturing. The Carboniferous Limestone is a major aquifer with water flowing largely in fractures and voids, whilst the conglomerate and breccia of the Mercia Mudstone Group Marginal Facies form the only significant aquifer of Triassic age. Younger aquifers include the sand and sandstone of the Bridport Sand Formation, the limestone beds of the Inferior Oolite Group and Forest Marble Formation, the sandstone of the Lower Greensand Group, and most importantly the ooidal limestones of the Chalfield Oolite Formation. These rocks may be highly productive, and able to support large abstractions for public water supply.

Minor aquifers are rocks which do not have a high primary permeability but may host water in a series of variably-connected fractures. These include the Cornbrash Formation, the Kellaways Sand Member, limestone units in the Fuller's Earth Formation, the Blue Lias Formation and the sandstone units of the South Wales Lower and Middle Coal Measures, Pennant Sandstone and Grovesend formations. Whilst these aquifers are unlikely to yield enough water to sustain public water supplies, they are important for local private abstraction and in supplying groundwater to rivers and streams. The mudstone formations of the district, including the Oxford Clay Formation, Kellaways Clay Member, Fuller's Earth Formation, Charmouth Mudstone Formation, Penarth Group and Mercia Mudstone Group are regarded as non-aquifers. Whilst water flow through such rocks is small, some of these rocks can yield sufficient water for domestic use.

Superficial deposits within the district do not form good aquifers. Where they overlie a non-aquifer bedrock they may yield small quantities of water, but they tend to drain rapidly and are unlikely to support continuous demand. In comparison, superficial deposits above bedrock aquifers are commonly dry. Alluvium and river terraces adjacent to streams and rivers may support limited abstraction, although this will be at the expense of surface flow.

Major public supply groundwater abstractions from the Jurassic aquifers are found in the Malmesbury area, and surface water abstraction from the River Avon takes place in the region of Bath. These provide water to a large part of the district including the city itself. Abstraction has raised concern about river levels in late summer, and as a consequence groundwater is used to support streams and rivers during low-flow periods. Details of groundwater and surface water abstraction and licences can be found in the Environment Agency CAMS (Catchment Abstraction Management Strategy) documentation for the Bristol Avon.

Hot springs

The famous hot springs of Bath are one of only five groups of thermal springs in the UK, and the only that qualify as 'hot', emerging at a temperature of about 45°C with a combined flow of about 15 litres per second (Stanton, fig. 8.3 in Kellaway, 1991). The King's, Cross Bath (Figure 7) and Hetling springs, all of which are found within a small distance of each other in the centre of the city, would in their natural state have risen to the surface through the Charmouth Mudstone bedrock and the terrace gravels of the River Avon. They were discovered by the native Britons, perhaps as long ago as 863 BC, when it is reputed that their healing powers cured a Celtic prince of leprosy. A shrine, dedicated to the local water goddess Sulis, was in existence at Bath at the time of the Roman conquest of AD 43; from this the Roman town derived the name Aquae Sulis. The Romans identified Sulis with their goddess Minerva, responsible among other things for medicine, and they first began to develop the bath-house complex soon after the conquest of Britain, possibly during the reign of the Emperor Claudius (AD 41–54). The baths fell into disrepair following the collapse of the Roman Empire, but were rebuilt in association with renewed interest in bathing waters during the 18th and 19th centuries (Front cover). In 1810, the springs diverted and failed, and it was none other than William Smith who restored the water to its original course. Today, the springs and bath houses form a major British tourist attraction.

Like all the other British thermal springs, including Hotwells in the adjacent Bristol district, the waters at Bath are sourced in the Carboniferous Limestone Supergroup, which lies beneath the city at depths as shallow as 50 m below ground level (Figure 7). It is widely accepted that rainwater falling on the Carboniferous Limestone outcrop in the Mendip Hills, south of the district, descends to great depths in the Radstock Basin where it becomes geothermally heated, before rising beneath Bath and breaking through the aquiclude formed by the Mesozoic rocks. The nature of the conduit to the surface is more controversial: Andrews et al. (1982) favoured a fracture zone over a Variscan thrust fault, and Kellaway (1996) suggested that the hot water escapes to the surface via fractures located over a deep-seated crustal lineament (the Avon– Solent Fracture Zone). However, seismic reflection surveys (McCann et al., 2002) failed to find evidence of the fracture zone beneath the city. Several boreholes have penetrated the Carboniferous Limestone in the area of the springs, of which two were sited adjacent to the hot springs (Kellaway, 1991). In all cases, the upper part of the Carboniferous Limestone was found to be heavily karstified and affected by dissolution and the formation of large voids. This surface formed an exposed part of the post-Carboniferous (Permo-Triassic) land surface, which was subsequently overlain by mudstone-dominated Mesozoic strata (see Geological description). A number of faults (including possibly the eastward extension of the Pennyquick Fault) and fractures were propagated through the cover by post-Early Jurassic structural reactivation (Figure 7), and were exploited by the downcutting River Avon during the Pleistocene (Gallois, 2007). Eventually this erosion brought the Carboniferous Limestone sufficiently close to the surface for the thermally-heated waters within it to escape. Once escape-pathways had become established the rate of flow would have increased, flushing fine-grained material from the voids in the karst and further increasing discharge, until a series of stable hot springs became established. Although not discounting the presence of fractures and minor faults, Gallois (2007) suggested that the conduits are conical debris-filled pipes, which at least at the King's Spring, formed by collapse over the karstic cavities in a shallow-buried knoll of Carboniferous Limestone (Figure 7). The debris includes blocks of Triassic and Jurassic rock, river gravel and even fragments of Roman building materials (Gallois, 2006).

Mineral resources

Coal

A detailed account of the history of coal mining in the region is given by Cornwell (2003), from which much of the following is taken. Colliery names in bold are shown on the published 1:50 000 geological map. The earliest workings for coal are in the Coalpit Heath Syncline, where mining was underway by the 1680s, and steam-powered pumps were in use by the mid 18th Century. Mines such as the Half Moon [ST 680 811], Oxbridge [ST 681 815] and Upper Whimsey [ST 678 807] pits were working seams in the Grovesend Formation, principally the High Coal. Larger pits at Ram Hill [ST 679 803] and Serridge [ST 675 796], located further south, were operating by 1790. The last colliery in the Coalpit Heath syncline was the Coalpit Heath Colliery, worked from the Frog Lane Pit [ST 687 816], which was sunk in the early 1850s. The Hard, Top and High coals were all worked, but the colliery was abandoned in 1949. In the Mangotsfield area, the Brandy Bottom (Parkfield South) Colliery [ST 682 772] had two shafts, one nearly 200 m deep. To the north-east, the Parkfield Colliery [ST 689 778] had nine shafts, and was one of the most successful in the area. Both Parkfield and Brandy Bottom worked coals in the Grovesend Formation, and both were closed in 1936.

Mining was very extensive in the area of the Kingswood Anticline on the crop of the more productive South Wales Middle Coal Measures Formation. The oldest workings are bell pits dating back at least to 1680. Major pits include the very early Soundwell colliery [ST 659 750], which had several shafts, and worked a number of coals in the Middle Coal Measures, including the Five Coals, the Kingswood Great Coal and the Soundwell Hard Venture Coal. The lowest seam known in the district was worked at Soundwell, but following accidents and flooding it was closed in 1853.

To the south, the Siston Common (or Syston Hill) Colliery [ST 669 739] opened around 1790, but it worked complex, faulted ground and was closed by 1889. The Crown Colliery [ST 672 735] was sunk around 1820, and faced similar difficulties. The Goldney Pit [ST 670 726] at Cadbury Heath was opened at about the same time, mining seams around the level of the Coking Coal and the New Smith's Coal. Further south, workings were largely undertaken in the Pennant Sandstone Formation. The California collieries [ST 665 716] exploited the Millgrit, Chick and Hen Coals from 1875, but an inrush of water in 1904 led to their closure. In the Bitton area, the New [ST 686 710] and Old [ST 690 708] Golden Valley collieries were the deepest workings within the district, extending to a depth of some 625 m to work the upper parts of the South Wales Middle Coal Measures Formation.

Away from the main coalfield, a limited number of coal workings have been undertaken in the district around Bath. These include a trial shaft at Batheaston [ST 7820 6750] and the Pennyquick (or Twerton No. 1) Colliery [ST 715 646], which worked the Lower Five Coal and others in the South Wales Middle Coal Measures Formation at depths between 109 and 260 m.

Metalliferous minerals

Metalliferous mineralisation is scarce in the Bath district, and largely confined to replacive bodies of hematite (iron ore) found along faults and fractures in the Carboniferous rocks. They have only been worked in the region of the Coalpit Heath Syncline, around Frampton Cotterill. Iron ore was also worked in the Parkfield area of Pucklechurch at the end of the 19th century.

Building stone and bulk minerals

The various limestone and sandstone formations of the district have all, at some time or other, been exploited for building stone on a local scale. However, the Bath district is best known for the building stone worked from the rocks of the Chalfield Oolite Formation, the so-called 'Bath Stone' or 'Great Oolite Freestone'; 'freestones' being limestones which can be sawn or trimmed in more than one direction, and which are thus suitable for complex carving or moulding. The best freestones in the Chalfield Oolite are found in the upper part of the Combe Down Oolite Member and within the Bath Oolite Member, where the rocks are composed of fine- to coarse-grained ooid-limestone with a sparry cement and little matrix.

The freestones of Bath have been worked since Roman times: villas at Box and Bathford used local stone, as did parts of the Great Baths. During medieval times, Bath Stone was used for the construction of several great buildings in the south-west, including Lacock Abbey and Longleat. During this period, the stone was won mostly from surface workings, but by the 18th century the rise in demand led to the development of underground stone mines, where the rocks are unweathered and the stone generally of better quality. Mining was constrained by various practical issues, including the need for a sound roof bed above the mined freestone horizon; consideration of the nature and thickness of the overburden with regard to the potential for subsidence, and the need to avoid areas of disturbance such as fault zones or areas of landslide and camber. The Chalfield Oolite is also a major aquifer (see above), and groundwater ingress presented a further problem. The principal quarries and mines were at Box, Corsham (Plate 6), Monkton Farleigh (Brown's Folly) and Combe Down. The mined freestones were used in the construction of many buildings, including Buckingham Palace in London and the Royal Pavilion in Brighton, and were widely exported, being employed in the Town Hall of Cape Town, South Africa, and Union Station in Washington DC. Demand began to decline during the economic depression in the 1930s. During the Second World War, many of the mines were used for ordnance factories and storage. Production in the later part of the 20th Century has been relatively small, and today only three sites remain in work: the Upper Lawn Quarry [ST 766 624] at Combe Down, and Elm Park Mine [ST 885 682] and Hartham Park Quarry [ST 855 701] at Corsham, the last two being underground mines.

The large quarries in Carboniferous Limestone at Wick continue to be worked for crushed rock aggregate and coated roadstone. Construction sand has formerly been extracted from sand beds in the Hazelbury Bryan Formation and in the Lower Greensand to the north-east of Melksham, but these workings are now disused.

Fuller's earth

The name 'fuller's earth' derives from the ability of certain clays to absorb oil and grease, and therefore to be used for cleansing or 'fulling' woollen cloth. These clays are composed mainly of the mineral montmorillonite, and today they are used in oil refining, as a suspension agent in drilling muds and agricultural sprays, and as a component of grouts in civil engineering applications. Until 1980 a bed of fuller's earth of relatively low grade was mined at Midford [ST 754 616] and the Combe Hay Works [ST 729 612], just south of the Bath district. Resources are still to be found south and east of the city, around Wellow, extending northwards as far as Bathampton Down [ST 77 65]. A full account is given by Forster et al. (1985).

Geological hazards

Knowledge of ground conditions is a prerequisite when identifying land suitable for development, and it underpins cost-effective design. Engineering ground conditions vary depending upon the physical and chemical properties of materials, the topography, behaviour of groundwater and surface water, and the nature of past and present human activity. The most significant problems likely to be encountered within the district are due to the weathering of the solid rocks and their disturbance on slopes. These can be effectively dealt with by obtaining adequate information, including properly designed site investigations, to confirm the characteristics of individual sites.

Limestones within the district, including those of the Carboniferous Limestone Group and the Cornbrash and Forest Marble formations, tend to have high load-bearing capacities associated with a great degree of cementation and/or recrystallisation. The ooidal limestone units of the district, including the Inferior Oolite and Chalfield Oolite, tend to be weaker as they lack strong cementation between individual grains. All limestones within the district are susceptible to the ingress of water via fractures, and the formation of voids at depth may significantly reduce the load-bearing properties of the rock. High load-bearing capacities are also shown by sandstone formations of late Carboniferous and Triassic age, except where mudstone interbeds are present.

In contrast, muddy and silty sandstones, including the Bridport Sand, Dyrham Formation and Kellaways Sand, tend to have moderate load-bearing capacities, and their geotechnical properties vary considerably with saturation and they may be susceptible to significant deformation or collapse where the water table is high. The increased susceptibility of mudstone units to deep weathering and chemical breakdown drastically reduces their load-bearing capacity in the near-surface zone. This particularly affects the Mercia Mudstone, Charmouth Mudstone, Oxford Clay, Kellaways Clay, Forest Marble and Fuller's Earth, and some mudstone-rich units may also be prone to shrink-swell behaviour.

River terrace deposit gravels typically have moderate to high load-bearing capacities, and alluvium may also be load-bearing, although lenses of sand may give rise to running conditions in excavations and beds of peat may produce compressible ground. Shear surfaces in deposits of head can become unstable if loaded and artificial ground is highly variable and may include contaminated land requiring remediation.

Special consideration must be given to engineering ground conditions in areas where landslide deposits (Table 1) have been identified because these present a significant geological hazard. They may be very variable in composition, with intermixed materials of differing load-bearing capacities and strengths. They are invariably underlain by one or more shear planes and movement on these may be reactivated by excavation or loading, or by changes in the groundwater regime. If engineering works are planned for landslide areas it is imperative that adequate site investigations are undertaken to define their nature, extent and stability.

Gulls produced by cambering (see Superficial structures and mass movement deposits) present a further hazard (Hobbs and Jenkins, 2008) and may be either air-filled or contain an admixture of sand, clay, limestone and superficial debris. Rubble-filled gulls may sometimes express themselves at surface as a linear depression. Air-filled gulls are commonly bridged by intact limestone beds, making them difficult to distinguish from rubble-filled ones, and in many cases there is no surface expression of either type of fissure. Foundations which cross debris-filled gulls may suffer differential settlement, and those which cross air-filled gulls may lack sufficient support. In areas where gulls are suspected, development should always be preceded by adequate site investigations.

The distribution of alluvial deposits as shown on the geological map provides an indication of those areas that are prone to river flooding. However, low terraces may also suffer flooding, and other areas may be inundated during anomalously high rainfall, by groundwater flooding, or as a result of drains and culverts becoming blocked. Areas regarded as at risk of flooding are shown on Environment Agency maps. Periods of enhanced catchment discharge may increase the pollution potential of ground and surface waters. At these times, leachate, soluble contaminant or contaminative sediments from areas of artificial ground, including poorly lined landfill sites, agricultural waste-disposal sites and active or former industrial sites (such as sewage works, pits and quarries) may enter surface water, groundwater or alluvial sediments.

Within the district, gas emissions represent a hazard in areas associated with the accumulation of methane or radon. Methane may accumulate in coal mine voids and shafts, and may also be generated by decomposition of material in landfill sites and organic deposits such as peat. It is toxic, an asphyxiant, and explosive in high concentrations. Methane is less dense than air, is capable of migrating through permeable strata, and accumulating in poorly ventilated spaces such as basements, foundations or excavations. Although methane emissions are unlikely to pose a significant hazard in the district, the risk can be further mitigated through correct design of landfills and developments in the vicinity of 'at risk' sites. Radon is a natural radioactive gas produced by the radioactive decay of radium and uranium, and is found in small quantities in all rocks and soils, although the amount varies from place to place. Geology is the most important factor controlling the source and distribution of radon (Miles and Appleton, 2005), and approximately a third of the Bath district is classified as a radon affected area (Miles et al., 2007). The government recommends that houses in radon affected areas should be tested for radon. Radon protective measures will be required in new buildings, extensions, conversions and refurbishments in parts of the district (Scivyer, 2007). A study of geological radon potential indicates that the radon affected areas include the crop of the following units: Carboniferous Limestone Group; Quartzitic Sandstone Formation; sandstones in the Coal Measures formations; Mercia Mudstone Marginal Facies; Blue Lias Formation; Dyrham and Bridport Sand formations; Inferior Oolite Group, and Chalfield Oolite Formation. Advice about radon and its associated health risks can be obtained from the Health Protection Agency–Radiological Protection Division, Chilton, Didcot, Oxfordshire, OX11 0RQ.

Former coal mining in the west of the district may induce a risk of subsidence. Mining activities include the construction of shafts, adits and galleries and, in the case of ancient and commonly undocumented shallow mining, the extraction of coal by means of bell-pits and pillar and stall workings. Any of these activities can give rise to voids at shallow or intermediate depths. Settlement into such voids has the potential to cause fracturing, general settlement or the formation of crown-holes in the ground above, as well as more significant collapses. For further information regarding underground and opencast coal mining, the location of mine entries (shafts and adits), and matters relating to subsidence or other ground movement induced by coal mining, please contact The Coal Authority. Subsidence is also associated with some of the freestone mines, notably those at Combe Down, which are currently undergoing remediation works. For information, please contact the Combe Down Stone Mines Project Team, Bath & North East Somerset Council, 10 Palace Yard Mews, Bath, BA1 2NH.

Geological conservation

Geological localities considered to be of national importance are protected as Sites of Special Scientific Interest (SSSI's). These are statutory designated conservation sites which have some protection under the Wildlife and Countryside Act 1981. Further information on the extent and designation of SSSIs and locally-designated Regionally Important Geodiversity Sites (RIGS) within the district can be obtained from Natural England, Northminster House, Peterborough, PE1 1UA. Special features of interest, being considered for notification as SSSIs, are described in the Geological Conservation Review (GCR) series, published by the Nature Conservancy Council.

Information sources

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

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

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

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

Maps

• Geological maps
• 1:250 000
• Sheet 51N-04W Bristol Channel, Solid Geology, 1987
• Sheet 51N-04W Bristol Channel, sea bed sediments and quaternary geology, 1986
• 1:63 360/1:50 000
• Sheet 250 (Chepstow), 1972
• Sheet 251 (Malmesbury), 1970
• Sheet 264 (Bristol), 2004
• Sheet 265 (Bath), 2011Sheet 266 (Marlborough), 1974
• Sheet 280 (Wells), 1984
• Sheet 281 (Frome), 2000
• Sheet 282 (Devizes), 2008
• Bristol District (Bristol Special Sheet), part of sheets 250, 251, 264, 265, 280 and 281, 1962.

1:10 000

The most recent revision of the component 1:10 000 scale National Grid maps of 1:50 000 Series sheet 265 Bath are listed below, along with the surveyors' names and dates of survey. The surveyors were A J M Barron, R A Edwards, R A Ellison, A R Farrant, K R Royse, T H Sheppard, P J Strange and R K Westhead. Amendments to some maps were made by R J Wyatt in 1977–1985. Maps which have not been resurveyed since the 1965 publication of Sheet 265 at 1:63 360 are marked with an x. These maps, together with those listed as being surveyed only in part, have been reconstituted at the 1:10 000 scale from older component maps at the scale of 1:10 560 scale of either the National Grid or County Map series.

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

Digital geological map data

In addition to the printed publications, many BGS geological maps are available in digital form. Details are given on the BGS website. National coverage of digital geological map data (DiGMapGB) is derived from geological maps at scales of 1:625 000, 1:250 000 and 1:50 000. Selected areas also have digital geological data derived from 1:10 000 scale geological maps. Digital geological data for offshore areas is derived from 1:250 000 scale geological maps.

Geophysical maps

1:250 000

51N 04W Bristol Channel, aeromagnetic anomaly, 1988

51N 04W Bristol Channel, Bouguer anomaly, 1988

Hydrogeological maps

1:100 000

Sheet 37 Groundwater Vulnerability of the Southern Cotswolds, 1990

Groundwater vulnerability maps are published by the Environment Agency from data commissioned from The Soil Survey and Land Research Centre and BGS, and are available from The Stationery Office (020 7873 0011).

Books and reports

Books and technical reports relevant to the district, including biostratigraphical reports, are listed in the References. Reports may be consulted at the BGS library or purchased from the BGS Sales Desk.

Documentary records collections

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

Borehole and trial pit records

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

Hydrogeological data

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

Geophysical data

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

BGS Lexicon of named rock units

Definitions of the stratigraphical units shown on BGS maps, including those named on Sheet 265 (Bath), are held in the BGS Stratigraphical Lexicon database, which can be consulted on the BGS website. Further information on this database can be obtained from the Lexicon Manager at BGS Keyworth.

BGS photographs

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

Materials collections

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

References

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

Ambrose, K. 2001. The lithostratigraphy of the Blue Lias Formation (late Rhaetian–early Sinemurian) in the southern part of the English Midlands. Proceedings of the Geologists' Association, Vol. 112, 97–110.

Andrews, J N, Burgess, W G, Edmunds, W M, Kay, R L F, and Lee, D J. 1982. The thermal springs of Bath. Nature, Vol. 298, 339–343.

Anson, R, and Hawkins, A B. 2002. Movement of the Soper's Wood landslide on the Jurassic Fuller's Earth, Bath, England. Bulletin of Engineering Geology and the Environment, Vol. 61, 325–345.

Barron, A J M, Sumbler, M G, and Morigi, A N. 1997. A revised lithostratigraphy for the Inferior Oolite Group (Middle Jurassic) of the Cotswolds, England. Proceedings of the Geologists' Association, Vol. 108, 269–285.

Barton, C M, Strange, P J, Royse, K R, and Farrant, A R. 2002. Geology of the Bristol District. Sheet Explanation of the British Geological Survey, Sheet 264 (England and Wales).

Besly, B M. 1987. Sedimentological evidence for Carboniferous and early Permian palaeoclimate of Europe. Annales de la Société Géologique du Nord, Vol. 106, 131–143.

Bowen, D Q (editor). 1999. A revised correlation of Quaternary deposits in the British Isles. Special Report of the Geological Society of London, No. 23.

Bradshaw, M J, Cope, J C W, Cripps, D W, Donovan, D T, Howarth, M K, Rawson, P F, West, I M, and Wimbledon, W A. 1992. Jurassic.107–129 in Atlas of Palaeogeography and Lithofacies. Cope, J C W, Ingham, J K, and Rawson, P F (editors). Memoir of the Geological Society of London, No. 13.

Buckman, S S. 1893. The Bajocian of the Sherbourne district. Quarterly Journal of the Geological Society of London, Vol. 49, 479–522.

Buckman, S S. 1895. The Bajocian of the mid-Cotteswolds. Quarterly Journal of the Geological Society of London, Vol. 51, 388–462.

Cave, R. 1977. Geology of the Malmesbury District. Memoir of the Geological Survey of Great Britain, Sheet 251 (England and Wales).

Chandler, R J, Kellaway, G A, Skempton, A W,and Wyatt, R J. 1976. Valley slope sections in some Jurassic strata near Bath, Somerset. Philosophical Transactions of the Royal Society of London Series A, Vol. 283, 527–556.

Cornwell, J. 2003. The Bristol Coalfield. (Ashbourne, Derbyshire: Landmark Publishing.)

Cox, B M, and Sumbler, M G. 2002. British Middle Jurassic Stratigraphy. Geological Conservation Review Series, No. 26. (Peterborough: Joint Nature Conservation Committee)

De la Beche, H T. 1846. On the Formation of the rocks of South Wales and South Western England. Memoir of the Geological Survey of Great Britain, Vol. 1.

Donovan, D T. 1995. High level drift deposits east of Bath. Proceedings of the University of Bristol Spelaeological Society, Vol. 20, 109–126.

Donovan, D T, and Kellaway, G A. 1984. Geology of the Bristol district: the Lower Jurassic rocks. Memoir of the British Geological Survey.

Douglas, J A, and Arkell, W J. 1932. The stratigraphical distribution of the Cornbrash: II. The north-eastern area. Quarterly Journal of the Geological Society of London, Vol. 88, 112–170.

Forster, A, Hobbs, P R N, Monkhouse, R A, and Wyatt, R J. 1985. An environmental geology study of parts of West Wiltshire and south-east Avon. British Geological Survey Technical Report, WA/85/25.

Gallois, R W. 2006. The geology of the hot springs at Bath Spa, Somerset. Geoscience in south-west England, Vol. 11, 168–173.

Gallois, R W. 2007. The formation of the hot springs at Bath Spa, U K. Geological Magazine, Vol. 144, 741–747.

Green, G W. 1992. British regional geology: Bristol and Gloucester region. Third edition. (London: HMSO for British Geological Survey.)

Green, G W, and Donovan, D T. 1969. The Great Oolite of the Bath area. Bulletin of the Geological Survey of Great Britain, Vol. 30, 1–63.

Hesselbo, S P. 2008. Sequence stratigraphy and inferred relative sea-level change from the Onshore British Jurassic. Proceedings of the Geologists' Association, Vol. 119, 19–34.

Hobbs, P R N, and Jenkins, G O. 2008. Bath's 'foundered strata' — a re-interpretation. British Geological Survey Open Report, O R/08/052.

Hopson, P M, Wilkinson, I P, and Woods, M A. 2008. A stratigraphical framework for the Lower Cretaceous of England. British Geological Survey Research Report, RR/08/03.

Hunt, C O. 1998. The Quaternary history of the Avon valley and Bristol district. Chapter 10 in Quaternary of south-west England. Campbell, S, Hunt, C O, Scourse, J D, Keen, D H, and Stephens, N (editors). Geological Conservation Review Series, No. 14. (London: Chapman and Hall.)

Kellaway, G A (editor). 1991. Hot springs of Bath — investigations of the thermal waters of the Avon valley. (Bath: Bath City Council.)

Kellaway, G A. 1996. Discovery of the Avon–Solent Fracture Zone and its relationship to Bath hot springs. Environmental Geology, Vol. 28, 34–39.

Kellaway, G A, and Hancock, P L. 1983. Structure of the Bristol district, the Forest of Dean and the Malvern Fault Zone. 88–107 in The Variscan Fold Belt in the British Isles. Hancock, P L (editor). (Bristol: Adam Hilger Ltd.)

Kellaway, G A, and Taylor, J H. 1968. The influence of landslipping on the development of the city of Bath, England. Proceedings of the 23rd International Geological Congress, Czechoslovakia, Vol. 12, 65–76.

Kellaway, G A, and Welch, F B A. 1993. Geology of the Bristol district. Memoir of the British Geological Survey.

McCann, C, McMann, A C, McCann, D and Kellaway, G A. 2002. Geophysical investigation of the thermal springs of Bath, England. 15–40 in Sustainable Groundwater Development. Hiscock, K M, Rivett, O, and Davison, R M (editors). Special Publication of the Geological Society of London, No. 193.

Miles, J C H, and Appleton, J D. 2005. Mapping variation in radon potential both between and within geological units. Journal of Radiological Protection, Vol. 25, 257–76.

Miles, J C H, Appleton, J D, Rees, D M, Green, B M R, Adlam, K A M, and Myers, A H. 2007. Indicative Atlas of Radon in England and Wales. (Didcot: Health Protection Agency and British Geological Survey.)

Murray, J W, and Wright, C A. 1971. The Carboniferous Limestone of Chipping Sodbury and Wick, Gloucestershire. Geological Journal, Vol. 7, 255–270.

Penn, I E, Merriman, R J, and Wyatt, R J. 1979. The Bathonian strata of the Bath–Frome area. Report of the Institute of Geological Sciences, No. 78/22.

Scivyer, C. 2007. Radon: guidance on protective measures for new buildings (including supplementary advice for extensions, conversions and refurbishments). Building Research Establishment Report, B R 211.

Self, C A. 1986 (for 1985). Two gull caves from the Wiltshire/Avon border. Proceedings of the University of Bristol Spelaeological Society, Vol. 17, 153–174.

Self, C A. 1995. The relationship between the gull cave Sally's Rift and the development of the river Avon east of Bath. Proceedings of the University of Bristol Spelaeological Society, Vol. 20, 91–102.

Self, C A, and Boycott, A. 2000 (for 1999). Landslip caves of the southern Cotswolds. Proceedings of the University of Bristol Spelaeological Society, Vol. 21, 197–214.

Smith, N J P, and Rushton, A W A. 1993. Cambrian and Ordovician stratigraphy related to structure and seismic profiles in the western part of the English Midlands. Geological Magazine, Vol. 130, 665–671.

Sumbler, M G. 2003. Comments on Wyatt and Cave's 'The Chalfield Oolite Formation (Bathonian, Middle Jurassic) and the Forest Marble overstep in the South Cotswolds and the stratigraphical position of the Fairford Coral Bed'. Proceedings of the Geologists' Association, Vol. 114, 181–184.

Torrens, H S. 2000. Timeless order: William Smith (1769–1839) and the search for raw materials. William Smith Lecture, Geological Society of London.

Waters, C N, Waters, R A, Barclay, W J, and Davies, J R. 2009. A lithostratigraphical framework for the Carboniferous successions of southern Great Britain (Onshore). British Geological Survey Research Report, RR/09/01.

Witchell, E. 1882. On the Pisolite and the basement beds of the Inferior Oolite of Gloucestershire. Proceedings of the Cotteswolds Naturalists' Field Club, Vol. 8, 35–49.

Wyatt, R J. 1996. A correlation of the Bathonian (Middle Jurassic) succession between Bath and Burford, and its relation to that near Oxford. Proceedings of the Geologists' Association, Vol. 107, 299–322.

Wyatt, R J, and Cave, R. 2002. The Chalfield Oolite Formation (Bathonian, Middle Jurassic) and the Forest Marble overstep in the South Cotswolds, and the stratigraphical position of the Fairford Coral Bed. Proceedings of the Geologists' Association, Vol. 113, 139–152.

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

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

(Index map)

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

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

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

Figures and plates

Figures

(Figure 1) Generalised pre-Permian subcrop map of the district. Sections 1 and 2 are lines of section shown on the published 1:50 000 geological map (Sheet 265, Bath).

(Figure 2) Bouguer gravity anomaly map of the Bath district and adjacent areas.

(Figure 3) Simplified cross-section showing lithostratigraphical relationships in the Bathonian strata of the south Cotswolds. Not to scale. Formations in capitals, members in upper and lower case, informal units in inverted commas. ls – Forest Marble Formation basal limestone beds; FC – Frome Clay Formation. Modified after Penn et al (1979), Wyatt (1996) and Hesselbo (2008).

(Figure 4) Stages of cambering and mass movement of slopes in Middle Jurassic strata in the district. For key to bedrock units, see Geological Description.

(Figure 5) Aeromagnetic anomaly map of the Bath district and adjacent areas.

(Figure 6) Summary of principal mass-movement features in the district.

(Figure 7) Cross-section showing the geological setting of the Bath hot springs. For key to bedrock units, see Geological Description.

Plates

(Plate 1) Geological map of Bath drawn by William Smith in 1799.

(Plate 2) The Penarth Group at Chipping Sodbury railway cutting. Exposed during excavation in 1901, mudstone beds of the Cotham Formation are overlain by the prominent limestone of the White Lias Formation, succeeded by interbedded mudstone and limestone of the basal part of the Blue Lias Formation. Approximate location [ST 7285 8160]. (Photographer S H Reynolds; P239116).

(Plate 3) The Chalfield Oolite Formation near Ford. Roadside cutting [ST 8510 7468] in the Combe Down Oolite Member. In the upper part, a prominent cross-bed set can be seen dipping to the left (west) (Photographer A J M Barron; P731874).

(Plate 4) Corsham railway cutting (as seen in 1967). The uppermost beds of the Bath Oolite Member, forming the ledge upon which the hammer rests, are overlain by the Corsham Limestone Formation, including its basal coralliferous limestone lens (the Corsham Coral Bed) succeeded by flaggy bioclastic limestone with mudstone beds [ST 8602 6947] (Photographer C A F Friend; P210749).

(Plate 5) The Kellaways Sand exposed in the bank of the River Avon at Kellaways [ST 9465 7577] (Photographer A J M Barron; P692881).

(Plate 6) The 'Portland Sheds' at the West Wells Masonry Works, Corsham. Seen in the mid 20th Century, a variety of worked and unworked freestone is visible (Photographer H O'Neil; P539467).

(Front cover) The Roman Great Bath and Bath Abbey. (Photographer P J Witney; P756090).

(Rear cover)

(Geological succession) Geology of the Bath district. Summary of the geological succession in the district.

Figures

(Geological succession) Geology of the Bath district. Summary of the geological succession in the district.

(Figure 6) Summary of principal mass-movement features in the district