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Geology of the Bristol district — brief explanation of the geological map Sheet 264 Bristol

C M Barton, P J Strange, K R Royse and A R Farrant

Bibliographic reference: Barton, C M, Strange, P J, Royse, K R and Farrant, A R. 2002. Geology of the Bristol district—a brief explanation of the geological map. Sheet Explanation of the British Geological Survey. 1:50 000 Sheet 264 Bristol (England and Wales).

© NERC 2003 All rights reserved. Copyright in materials derived from the British Geological Survey's work is owned by the Natural Environment Research Council (NERC) and/or the authority that commissioned the work. You may not copy or adapt this publication without first obtaining NERC permission. Contact the BGS Intellectual Property Rights Manager, British Geological Survey, Keyworth. You may quote extracts of a reasonable length without prior permission, provided a full acknowledgement is given of the source of the extract.

Keyworth, Nottingham: British Geological Survey, 2002.

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

(Front cover) Cover photograph. Neptune overlooking the docks. This lead statue is one of the last surviving relics of the Bristol Lead works that used Mendip lead. (Photographer H J Evans) (A11396).

(Rear cover)

Notes

The word 'district' refers to the area of Sheet 264 Bristol. National Grid references are given in square brackets; all lie within the 100 km square ST. Borehole records referred to in the text are prefixed by the code of the National Grid 25 km² area on which the site falls, for example ST57SE, followed by its registration number in the BGS National Geosciences Records Centre.

Acknowledgements

The authors acknowledge the assistance provided by members and staff of the City of Bristol Unitary Authority, Bristol Central Library, Taunton Library and Bristol Region Environmental Records Centre. James Fussell (James Fussell Associates) and John Cornwell (Bristol Coalmining Archives Limited) provided historical data for the principal coal seams incorporated into (Figure 3). Landowners, tenants and quarry companies are thanked for permitting access to their lands.

The grid, where it is used on figures, is the National Grid taken from Ordnance Survey mapping. © Crown copyright reserved Ordnance Survey licence no. GD272 191/2002.

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

(Rear cover)

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

The Bristol district is justly famous for its spectacular scenery and picturesque gorges, and displays a rich diversity of rock types with a varied geological history. The striking scenery results from the partial exhumation of an ancient rugged landscape of Palaeozoic age from beneath a blanket of soft limestones and mudstones. Recent erosion, superimposed on the relict topography, has sculpted the landscape and created the dramatic gorges we see today.

Devonian Old Red Sandstone and Carboniferous Limestone, transported northward on large thrust faults and folded into a series of large-scale anticlines, form the backbone of the region, and underlie much of the high ground of Broadfield Down as well as the conspicuous ridge between Bristol and Clevedon. The spectacular Avon Gorge, spanned by Brunel's famous suspension bridge, dissects the ridge and displays an important section through the Carboniferous Limestone. The overlying Coal Measures are also highly deformed and include numerous high-quality coal seams, formerly exploited in the Bristol and Nailsea coalfields. Although production from collieries ceased in 1963, the legacy of coal mining is still evident with abandoned shafts, acid mine drainage and local subsidence.

A regional unconformity is spectacularly exposed along the coast at Portishead and details of the buried landscape can be determined below the Triassic sandstones and mudstones that comprise the Mercia Mudstone Group. Today, these soft strata form the low ground upon which the city of Bristol was built. Lower Jurassic rocks, widely quarried for building stones, underlie much of the northern suburbs around Horfield, while Dundry Hill, its flanks masked by landslips, is an outlier of Middle Jurassic strata that dominates the city's southern skyline. Extensive Quaternary deposits of low-lying, flood prone, estuarine and fluviatile alluvium, river terraces and peat underlie much of the Somerset Levels bordering the Severn estuary and along the Avon and Gordano valleys.

The industrial heritage of Bristol is strongly linked to the extraction of coal, lead, and iron, as well as bulk minerals such as sand, limestone and a wide variety of building stones. The range of building stone has given Bristol an architectural diversity that does much to enhance the city.

Chapter 1 Introduction

This Sheet Explanation provides a summary of the district covered by the geological 1:50 000 Series Sheet 264 Bristol published in a solid and drift edition in 2001. A fuller description of the geology is provided by both the general Special Sheet Memoir (Kellaway and Welch, 1993) and the Special Sheet Memoir for the Lower Jurassic rocks (Donovan and Kellaway, 1984). The regional guide to the Bristol and Gloucester region (Green, 1992) provides a geological synthesis of the district and surrounding areas.

The district lies within the Unitary Authorities of the City of Bristol, South Gloucestershire and North Somerset. The main population centre is the conurbation of Bristol, Kingswood and Avonmouth; smaller towns include Nailsea, Portishead and Clevedon. The principal semi-rural areas are in the south of district around Broadfield Down and Dundry.

Physical setting

The scenic variety for which Bristol is famous is a direct consequence of the varied geology. Higher ground, 100 to 200 m above sea level, is typically developed on hard Carboniferous Limestone. The Long Ashton to Clevedon ridge and the area around Broadfield Down comprise limestone uplands of this type, comparable with the Mendip Hills to the south of the district. In contrast, the scenery of the Coal Measures to the east and south-east of Bristol comprises rather monotonous low ground with little differentiation of relief, apart from the incised valleys of the Avon and Frome. Although the Coal Measures include both soft mudstones and hard sandstones, these areas were eroded to a relatively uniform surface during a late Triassic denudation event. The Flax Bourton valley, and much of the central and west Bristol areas forms gently undulating, low-lying ground developed on mudstones and sandstones. The exception is Dundry Hill in the south of the district, capped by Jurassic limestones, and over 230 m above sea level.

The district is drained by rivers that all flow into the Severn Estuary. The Bristol Avon enters the district near Keynsham and flows west or north-west to Avonmouth, cutting gorges first at Hanham, in the Coal Measures south-east of Bristol and then in the Carboniferous Limestone to form the celebrated Avon Gorge at Clifton (Plate 1). The river is tidal as far as St Anns, Bristol. Much of the modern channel in central Bristol has been modified with the excavation of the 'New Cut', in 1809, and creation of the Floating Harbour. The Avon is joined by the River Chew, which flows east and north-east around the south of Dundry Hill, by the south-west flowing River Frome at Bristol, and by the River Trym at Sea Mills. The River Trym cuts its own gorge through the Carboniferous Limestone at Blaise Castle and the River Frome flows through gorges incised into the Pennant Sandstone at Oldbury Court and Frenchay. Rivers that flow directly to the Bristol Channel include the Land Yeo and Kenn rivers, draining the area around Nailsea, and the Congresbury Yeo, which flows north-west along the Mendip foothills, in the south of the district. The drainage of the district is widely considered to be superimposed, that is it developed on a former landscape of Triassic age, now partly removed by erosion. The River Avon formerly flowed toward the north-west across this landscape, at a time when the hard limestones now seen in the Avon Gorge were buried beneath soft Triassic red beds. Probably as a result of global lowering of sea level during the Pleistocene glaciations, the river cut down into these underlying strata to create the present-day gorge while still retaining its earlier course. The river does not take the easier route to the sea near Clevedon by following the outcrop of soft sandstones along the Flax Bourton valley for this reason. Steeply incised valleys along the Frome and Trym formed by glacial overflow in a similar way; all gorges in the district have provided good defensive sites, outcrops that are easily quarried, and define good communication corridors.

History of research

The district covered by Sheet 264 Bristol was originally surveyed on a scale of one inch to one mile and published as engraved and hand coloured 1:63 360 'Old Series' Geological Survey sheets 19 and 35 in 1845. These were largely the work of Sir Henry T De la Beche, the first Director of the Geological Survey, who also published an early written account of the geology (De la Beche, 1846). Other important geological maps of Bristol were produced in the 19th century, particularly those by William Sanders in 1862 at a scale of four inches to one mile. These maps were used by John Anstie, a mining engineer who produced the first systematic survey of the Bristol coalfield as part of the 1871 Coal Commission.

Many early advances in stratigraphy and palaeontology are based on work carried out in the Bristol district. William Smith discovered the principles of stratigraphy in 1792–1795 while working on the Bristol and Somerset coalfields (Smith, 1815). Soon after production of Smith's maps, Buckland and Conybeare (1824) published a classic paper giving subsurface data on the coalfields. Arthur Vaughan produced the first internationally accepted zonal classification of the Lower Carboniferous by documenting corals and brachiopods in the Avon Gorge sections (Vaughan, 1905, 1906). The work of S S Buckman on Jurassic ammonites in the district (Buckman, 1895) was also extremely influential.

The modern primary survey of the district, on a scale of six inches to one mile (1:10 560) began in 1939 by G A Kellaway and F B A Welch. Following interruption for the war, the work was completed by them, and after 1950 by G W Green, in 1943–1953, and published at 1:63 360 scale in 1963. It was thirty years before the accompanying memoir was published (Kellaway and Welch, 1993), although by then, an account of the Lower Jurassic strata (Donovan and Kellaway, 1984) was available.

The present map edition incorporates the addition of Artificial Deposits, and minor amendments at a scale of 1:10 000, carried out by K R Royse, P J Strange, A C Pople, R K Westhead, R A Edwards and A R Farrant in 1995–1999 and published at 1:50 000 scale in 2002.

Chapter 2 Geological description

Pre-Carboniferous rocks

Although Cambrian and Silurian rocks are present in the Tortworth area, just north of the district, the oldest strata represented on the Bristol Sheet comprise the Old Red Sandstone, of largely Devonian age (spanning late Silurian to early Carboniferous). The Old Red Sandstone comprises a thick continental red-bed sequence, principally sandstones, mudstones and conglomerates with intervals of calcareous concretions in the lower part. The red beds occur within anticlinal fold cores, both in the Westbury-on-Trym Anticline and along the Clevedon–Portishead ridge where there is a well-exposed coastal section.

Red-bed deposition was not continuous and there is a non-angular unconformity within the median part of the sequence that allows a subdivision into Lower and Upper Old Red Sandstone. The Lower Old Red Sandstone within the district comprises the upper 340 m of a mudstone sequence with some sandstone, the Black Nore Sandstone Formation. The formation represents an alluvial, upward-coarsening sequence in which the proportion of sandstone to mudstone increases in the upper part. Individual upward-fining units, up to several metres thick, that comprise a basal conglomerate, channel sandstone and mudstone top occur throughout, and fish debris is locally abundant. Distinctive dolomitic concretions and beds occur near the base and are interpreted as calcrete deposits formed in hot and arid conditions.

The Upper Old Red Sandstone is undivided in the Bristol district and comprises a sequence (about 250 m thick) of red sandstone known as the Portishead Formation, best exposed in the Avon Gorge and at Woodhill Bay. The junction with the underlying Black Nore Sandstone Formation is a sharp, erosive surface. The lowest member is the Woodhill Bay Conglomerate, a coarse, unsorted conglomerate, up to 9 m thick, that contains pebbles derived from an igneous and metamorphic terrain. The overlying sequence includes lenticular beds of green and red mudstone, siltstone and conglomerate. Important fish faunas have been collected from a siltstone bed about 32 m above the formation base and from the Sneyd Park Fish Bed, a conglomerate near the formation top. The Portishead Formation is an upward-fining sequence with marine passage beds at the top; these are Carboniferous in age, and transitional to the overlying thick limestones.

Carboniferous

The Dinantian (Lower Carboniferous) strata comprise the Avon Group and The Pembroke Limestone Group (previously known as the Carboniferous Limestone Supergroup). This is about 800 m thick in the district, and is subdivided into eight formations. The basal passage beds of the Avon Group, the Shirehampton Formation, are about 40 m thick and comprise coarse bioclastic and ooidal limestone with subordinate mudstone that has a brackish water fauna with Lingula; a reddened crinoidal limestone, the 'Bryozoa Bed' occurs at the top. The strata are overlain by greenish grey mudstone with interbedded black crinoidal limestone, the Maesbury Mudstone Formation (formerly Lower Limestone Shale), which is about 80 m thick in the Avon Gorge and around Portishead. The limestone beds have a rich shelly fauna although are poor in corals.

Transition into the overlying Black Rock Limestone occurs with upward increase in the proportion of limestone. The sequence consists of dark grey or black, well-bedded, poorly sorted crinoidal limestones that contain a rich fauna of conodonts, corals and foraminifera. Thickness decreases toward the north, from a maximum of more than 250 m in the Mendips; at Bristol, the Black Rock Limestone is about 150 m thick including about 30 m of secondary dolomite at the top.

The overlying succession is subdivided into lithologies of different facies. The lowest is the Gully Oolite Formation, a distinctive pale grey, medium- to fine-grained, cross-bedded oolite in which macrofossils are rare. The oolite weathers white and is massive with conspicuous vertical joints. The formation is 35 to 40 m thick on Broadfield Down, and includes 1 to 7 m of pale grey, well-sorted crinoidal limestone at the base ('Sub-Oolite Bed'). Pale grey, calcareous mudstones ('Clifton Down Mudstone Formation') interbedded with dark grey and brown shales, overlies an erosion surface at the top of the Gully Oolite. The calcareous mudstones weather whitish grey or buff, and are poorly fossiliferous. On the south side of Broadfield Down, about 12 m of basaltic lava and tuff occur in the lower part of the mudstone sequence. Absent from the Mendips, the mudstones are 30 to 60 m thick in the Bristol district. The overlying Goblin Combe Oolite Formation comprises 5 to 65 m of massive ooidal and crinoidal limestone. Well developed on Broadfield Down, the black colour and coarse texture distinguish it from the Gully Oolite.

The overlying Clifton Down Limestone Formation is a persistent unit and represents a progressive change to a more open-sea environment. Its type section is in the Avon Gorge (Plate 1) where it is about 190 m thick. The basal sequence comprises calcareous mudstones, overlain by shelly limestones with the coral Lithostrotion, together with algal beds ('Seminula Pisolite') and cross-bedded ooids interpreted as channel or bar deposits ('Seminula Oolite') in the upper part (Plate 1). The uppermost part comprises a thick calcareous algal mudstone sequence known as the 'Concretionary Beds' with a widespread erosion surface at the top.

The succeeding Hotwells Limestone Formation is best exposed in an almost continuous section along the east bank of the Avon Gorge between the Suspension Bridge and the Colonnade at Hotwells. The limestones are massive, grey, ooidal and crinoidal with an abundant coral and brachiopod fauna; they represent high-energy marine deposits characteristic of an open shelf environment. Although less than 100 m thick in the Avon Gorge, the Hotwells Limestone is about 225 m thick on Broadfield Down; the upper part of the formation is progressively replaced by sandy facies toward the north. The uppermost division of the Carboniferous Limestone in the district, comprises a poorly exposed sandy facies, the Upper Cromhall Sandstone Formation. The formation is known in detail from the Ashton Park Borehole [ST 5633 7146], just south of the Avon Gorge, which proved a thickness of 140 m of grey, coarse-grained sandstone, quartzitic sandstone, calcareous sandstone, sandy crinoidal and ooidal limestone, dark grey mudstone and grey, rootlet-bearing fireclay. Pyrite is common, the oxidation of which is responsible for the strong red colours, which characterise the formation at outcrop. The sequence is markedly cyclic, a typical cycle comprising limestone passing up into mudstone, siltstone and sandstone and overlain by a seatearth. The cycles represent a change from the open shelf environment of the Hotwells Limestone to marginal marine and estuarine deltaic conditions. Both the Hotwells Limestone and the Upper Cromhall Sandstone are absent from the west of the district, removed by erosion at the base of the overlying (Namurian) sequence.

The traditional lower (Namurian) division of the Upper Carboniferous is known as the Quartzitic Sandstone Formation (Millstone Grit Group) in the Bristol district. It overlies the Carboniferous Limestone and consists of grey mudstones and seatearths with thin carbonaceous or coaly beds and a preponderance of hard, pale grey quartzitic sandstone (pebbly in places), within the middle of the sequence. The formation has a distinctive cherty mudstone base, up to 15 m thick, and a marine fauna that includes fish fragments. The sandstones have erosive bases and are commonly conglomeratic with pebbles of white quartz, chert and ironstone. Although poorly exposed, the formation crops out between Kingsdown and Long Ashton, and forms the steep slopes of St Michael's Hill, Brandon Hill and Clifton Wood. The Quartzitic Sandstone Formation was proved to be 174 m thick in the Ashton Park Borehole.

The uppermost divisions of the Carboniferous (Westphalian) comprise a thick, mostly nonmarine or marginal marine rhythmic sequence of mudstone and sandstone, together with coals and their underlying seatearths. The basal part of the sequence, the Coal Measures Group, contains four marine bands, up to 7 m thick, each with a rich goniatite fauna that forms the basis of regional stratigraphical correlation. The principal divisions in the district are the Lower and Middle Coal Measures formations (Langsettian, Duckmantian and Bolsovian, formerly Westphalian A–C), which are about 150 and 375 m thick, respectively, and consist predominantly of mudstone and coal. The overlying Pennant Sandstone Group (Bolsovian to Westphalian D) is about 1000 m thick near Bristol, and comprises sandstone with subordinate mudstone and coal. There is further division of the Pennant Sandstone Group into six formations, of which the upper two are restricted to the Somerset coalfield. The nomenclature, in part, replaces earlier schemes in which the Pennant Sandstone Group was divided into two units. The lower unit, formerly known as the Pennant Measures or Pennant Sandstone, consists predominantly of sandstone. The upper unit, formerly known as the Supra-Pennant Measures is mudstone rich. Red measures, which include red sandstone and conglomerate associated with uplift and erosion, occur in both these sequences.

The coalfields of the district are geologically complex for two principal reasons. First, as a result of Variscan (late Carboniferous–early Permian) deformation, individual coalfields occur in a number of distinct areas or 'basins' (see (Figure 2)). The Bristol Coalfield includes the Kingswood Anticline and Coalpit Heath Syncline to the east and north-east of the city, while the Somerset Coalfield comprises the Pensford and Radstock synclines to the south-east of the district. The Avonmouth and Nailsea synclines contain coal basins that are structurally distinct. Second, the coalfields are in largely concealed by Mesozoic rocks and are known only from borehole or other subsurface data. Mesozoic strata, typically 30 to 50 m thick in south Bristol, overlie the coalfield which there subcrops at the base of Triassic sandstone. Close to the sub-Triassic unconformity, the coal seams have been extensively oxidised, and are otherwise uneconomic to extract.

A wide belt of Coal Measures is present at surface in the Kingswood Anticline. The coal seams are highly deformed and are traditionally called 'veins', from their similarity to the steeply dipping metalliferous mineralised zones found in south-west England (Figure 3). The sequence includes 20 extensively worked seams, 0.3 to 2 m thick, of which the Ashton Group within the Lower Coal Measures Formation is the lowest. The Ashton Little Coal (about 0.6 m thick) and Ashton Great Coal (0.9 m thick) are the two principal seams; borehole data show that they also subcrop along a line from Ashton Gate, through Canon's Marsh, Broadmead and St Pauls, although there is no evidence that they have ever been mined in this area. The Subcrenatum (Ashton Vale) Marine Band, 12 m below the Ashton Little Coal, was proved in the Ashton Park Borehole, and the Lower Coal Measures are 125 to 200 m thick in the district.

The overlying Middle Coal Measures Formation, above the Vanderbeckei (Harry Stoke) Marine Band, includes the Kingswood Little Coal, and the main economic seam, the Kingswood Great Coal (also known as the Bedminster Great Coal in south Bristol). The latter seam is 1.1 m thick, and is predicted to subcrop beneath Ashton Vale and extend north-eastward to Eastville. Farther north-east, the seam has been worked in the Harry Stoke Drift Mine where the Middle Coal Measures are concealed. Together with the Lower Five Coals, these seams all occur below the Aegiranum (Croft's End) Marine Band. The uppermost or Cambriense (Winterbourne) Marine Band has not been recognised in the Kingswood area, and the mapped base of the Pennant Sandstone Group is taken as the approximate top of the Middle Coal Measures (the Upper Coal Measures Formation is absent in the Bristol district). A number of good coking coals, including the Parrot, Buff, Rag and Millgrit have been worked from the basal part of the Pennant sequence.

The Pennant Sandstone Group is present at surface in the north-east of the district in the Coalpit Heath Syncline, where coals from the lower part of the sequence have been worked. The lower Downend Formation, is about 425–650 m thick, and comprises sandstone and mudstone with sparse coal seams. The overlying Mangotsfield Formation is about 425–1040 m thick, and consists predominantly of sandstone with coal seams in the lower part. The overlying mudstone sequence, the Farrington and Barren Red formations (350–500 m), were more productive with sandstone and coal seams near the base, and red mudstone and sandstone at the top. The erosional thickness of the undivided Farrington and Barren Red formations is typically about 320 m around Mangotsfield and Brislington, although the units are substantially thicker farther south.

The Pennant Sandstone Group also outcrops in the south-east of the district, along the northern edge of the Somerset Coalfield. There, the upper part of the succession comprises mudstones and muddy coals, the Radstock Formation, and the overlying barren mudstone sequence, the Publow Formation; together the formations are 300 m thick and known from the Pensford and Bromley collieries. The seven Bromley Coals, three of which are about 0.5 m thick, and the eight Pensford Coals, two of which have been extensively worked, are separated by about 200 m of barren measures.

The Nailsea Coalfield was abandoned in the 19th century and there are few stratigraphical data. The sequence comprises about 270 m of mudstones with 12 coals, thought to be Lower and Middle Coal Measures. Only White's Top (1.1 m) and the Dog (0.9 m thick) have been mined to any extent. A maximum of 330 m of Downend Formation overlies the sequence, from which Grace's Coal (0.9 m) has been worked. The Avonmouth Coalfield is almost completely concealed, beneath the Severn Estuary. Boreholes from the central part of the Avonmouth Syncline prove the existence of two workable coals, Avonmouth 1 and 2, from the lower part of the Farrington Formation, a sequence broadly comparable with that in the Coalpit Heath Basin.

Triassic and Jurassic

Triassic and younger strata lie with marked unconformity on the underlying Devonian and Carboniferous rocks (Plate 2). Erosion that began at least as early as the Permian accompanied Variscan deformation and stripped off part of the Coal Measures sequence to expose Carboniferous Limestone on Broadfield Down and along the Clevedon to Westbury ridge. Red mudstones, sandstones and breccias, the Mercia Mudstone Group, formerly the Keuper Marl, were deposited on this irregular landscape. Screes of locally derived, angular limestone and sandstone debris with a dolomitic sandstone cement, known as the Mercia Mudstone Marginal Facies (formerly the 'Dolomitic Conglomerate'; (Plate 3)), accumulated as outwash fans and valley infills. Individual boulders are up to several tons in weight and can be seen in Bridge Valley Road in the Avon Gorge and between Portishead and Clevedon; individual valley infills locally exceed 100 m in thickness. The marginal facies passes laterally into sandstones of early to late Triassic age and is considered to be diachronous.

Marginal facies of a comparable origin occur along the western edge of the Coal Measures outcrop in the north-east of the district. Red and buff calcareous and ferruginous sandstones, the Redcliffe Sandstone Formation, were deposited in an elongate depression between Bedminster and Winterbourne, and locally exceed 50 m in thickness. The Redcliffe Sandstone passes laterally into red mudstones and is locally interdigitated with Mercia Mudstone Marginal Facies. The best exposures of red sandstone are found in cliffs along the Avon in Redcliffe, the eponymous district of Bristol, and in the New Cut along Coronation Road, Southville.

The undivided Mercia Mudstone Group is the most widespread of the Triassic sequences and comprises red dolomitic siltstone and mudstone, commonly with small green patches, streaks and laminae. The strata rest unconformably on an irregular Carboniferous surface across much of the Redland area of central Bristol, as well as around the margins of the Bristol, Somerset and Nailsea coalfields. Although up to 500 m thick in the central Somerset basin, the Mercia Mudstone varies up to 300 m in the Bristol district and is locally absent where overlying formations overstep the remnant landscape.

The red mudstones contain scattered beds of grey sandstone and hard siltstone ('skerries') and, in the uppermost 8 to 25 m of the succession, thicker units of sandstone, which include the Stoke Park Rock Bed. The bed consists of hard greenish grey sandy and silty dolomitic mudstone and has been mapped north of the River Avon where it is overlain by a mudstone bearing celestite (strontium sulphate); termed the Severnside Evaporite Bed and is 0.5 to 2 m thick. The celestite is thought to replace the primary minerals gypsum and anhydrite, widespread in mudflat or sabkha-type environments where conditions are arid and there is a high rate of evaporation. In the south of the district, the Butcombe Sandstone Member, up to 4.8 m thick, occurs about 15 m from the top of the group and is also associated with celestite.

Red mudstones of the Mercia Mudstone Group pass upwards into a greenish or greenish grey mudstone sequence, the Blue Anchor Formation (formerly 'Tea Green Marl') with progressive decrease in the proportion of red beds. The formation is only about 3 m thick in the Filton area of north Bristol (seen outside the district at Aust Cliff [ST 5645 8920] near the Severn Bridge) and is largely absent farther south, where it is overlapped by the Penarth Group, south of Dundry. The Penarth Group is a thin but distinctive sequence associated with a marine transgression that comprises two formations, the Westbury Formation overlain by the Lilstock Formation. The Westbury Formation consists of very dark grey mudstones, 3 to 9 m thick, locally with a base of phosphatised and quartz pebbles. The mudstones have a rich fauna of gastropods and fish remains. The Lilstock Formation comprises two members: the lower or Cotham Member, consists of soft, greenish grey, silty calcite mudstones with a hard, 0.2 m-thick calcite mudstone ('Cotham Marble') at the top, and an upper, Langport Member (formerly known as the 'White Lias'), consists of pale grey to cream limestones with mudstone partings and a burrowed porcellanous limestone ('Sun Bed') at the top. Together, these units are about 4 m thick and are associated with a well-defined escarpment feature, both around Windmill Hill and Totterdown to the south of the Avon, and around Cotham, north-west of the city centre.

The overlying Lias Group, is a thick, mudstone-dominated sequence that contains important ammonite faunas. The basal division, the Blue Lias Formation, comprises a rhythmically alternating sequence, 17 to 20 m thick, of dark blue-grey mudstones and thin blue-grey limestones. The proportions of limestone and mudstone are comparable in the Bristol district but limestones form less than 25 per cent of the Central Somerset Basin succession. The Blue Lias is thickest on the lower slopes of Dundry Hill, around Whitchurch, Knowle and Hartcliffe, where a median argillaceous interval, the Saltford Shale Member, forms a mappable unit up to 8 m thick. The basal unit, the Wilmcote Limestone Member, comprises mudstones with flaggy limestones, in beds up to 0.3 to 0.4 m thick, that are shelly at the base and laminated at the top. Above the Saltford Shale, the Rugby Limestone Member, comprises mudstones with rubbly or nodular argillaceous limestones. Very dark grey mudstones, about 28 m thick, overlie the Blue Lias, and are succeeded by 40 m of grey calcareous mudstone and 54 m of silty mudstone, together the Charmouth Mudstone Formation. The Lias Group strata are largely known from the Dundry Borehole [ST 5636 6589] and the basal beds also occur on the hillcrest around Cotham Grammar School [ST 583 740] in north Bristol.

West of Dundry, the upper part of the Penarth Group and the Wilmcote Limestone Member of the Blue Lias Formation pass into coarsely shelly and conglomeratic rocks, formerly described as 'littoral facies' and now known as the Brockley Down Limestone. These beds, about 8 to 12 m thick, are thought to have accumulated as a sublittoral shell bank on a shoal of Carboniferous Limestone. The Brockley Down Limestone around Felton Hill [ST 520 650] is silicified, and the replacement cherts are known as the Harptree Beds. The restricted field occurrence and association with baryte and galena suggest that the source of the silica is hydrothermal.

The succeeding Dyrham Formation, 9 m thick in the Dundry Borehole, consists of siltstone and mudstone. It is overlain by the Beacon Limestone Formation, a shelly ferruginous limestone that is 1.4 m thick in the Dundry Borehole. The Whitby Mudstone Formation, comprises about 23 m of silty micaceous mudstone that rests on a ferruginous, ooidal and conglomeratic limestone (0.75 m thick). 'Middle' and 'Upper Lias' strata on Dundry Hill can be seen at the western end of the outlier but elsewhere are largely concealed by extensive landslips.

The youngest solid strata in the district, forming the escarpment of Dundry Hill, comprise Middle Jurassic limestones of the Inferior Oolite Group. The Lower and Middle Inferior Oolite consist of up to 6 m of 'ironshot' limestones, that is they are thinly bedded and ferruginous with abundant limonitic ooids; the strata are only preserved on the western end of the hill. The Upper Inferior Oolite consists of the Dundry Freestone and the Coralline Beds. The Dundry Freestone is about 4 m thick and comprises massive shelly limestone resting on a thin conglomerate. The Coralline Beds consist of a sequence of thinly bedded shelly limestones about 6 m thick, where preserved as a hill cap.

Quaternary

The unconsolidated or drift deposits of the district include a variety of glacial, periglacial, fluvial and estuarine sequences of Pleistocene and Holocene age (Plate 4).

Glacial deposits

Glacial deposits are sparse, largely because ice sheets were restricted to the Bristol Channel and only impinged on the coast at Clevedon. These deposits consist of poorly sorted glacial sand and gravel. The effects of increased sediment supply and runoff formed overdeepened channels filled with glacial outwash material. A 25 m-deep buried channel filled with glacial gravel and till was discovered during the construction of the M5 motorway through the Carboniferous Limestone at Court Hill, near Clevedon. Similarly, borehole data demonstrate that the Avon channel between Hotwells and Avonmouth is 12 to 20 m deep and filled with Pleistocene gravel.

River Terrace deposits

River Terrace deposits in the district flank the Rivers Avon and Kenn. These deposits consist of clay, silt and sand on a gravel base. Higher terraces, at about 15 and 30 m above OD, are patchily developed, and commonly contain Palaeolithic implements. Thin degraded terrace remnants may also be present in central Bristol, but their destruction and concealment through urbanisation makes identification difficult.

Alluvium

Alluvium is deposited by streams and rivers principally during flooding. It consists of clay, silt, sand, gravel and peat. The thickness of the deposit relates to the size of the watercourse and is usually less than 5 m. Alluvial deposits floor all major rivers and their tributaries in the district.

Tidal Flat and Peat deposits

Tidal Flat and Peat deposits, up to 20 m in thickness, border the Bristol Channel and Severn Estuary. The deposits comprise mud, peat, silt and sand on a gravel base; inland peat deposits can be up to 5 m in thickness.

At Avonmouth a 3 km-wide spread rests on the soft Triassic Mercia Mudstone Group. In the south-west of the district, around Nailsea and Kenn, flats of mud, peat, silt and sand form extensive moors; this is the northern limit of the Somerset levels. Along the coast south of Portishead, Raised Tidal Flat deposits occur in places at about 9 to 14 m above OD, consisting predominantly of silt and clay. The Burtle Beds occur in the south-west of the district to the west of Kenn. These consist of sand, gravel and shells and probably are the result of marine incursions of the Somerset levels.

Head

Widespread Head deposits, largely locally derived, consist of poorly sorted silt, sand and gravel. These heterogeneous deposits accumulated by the processes of downslope solifluxtion, hillwash and hill creep. The deposits are generally only a few meters thick. However, thin deposits of head are likely to be present on and at the foot of many slopes, because head deposits resemble the parent material from which, they are derived, they are unlikely to have been mapped comprehensively.

Landslip

Landslip and cambering have affected the rocks, which form the upper part of Dundry hill. Landslipped ground associated with the Lias mudstones forms a belt up to 1 km wide, while the cambered base of the Inferior Oolite falls steadily away from the scarp with associated outward flow of clay.

Artificial deposits and worked ground

The more extensive areas of artificial deposits and worked ground are shown on the 1:50 000 scale map but many minor occurrences have been omitted for clarity; 1:10 000 scale maps of the district (listed p.26) show more detailed distributions.

Made Ground

Made Ground is shown in areas where man-made or other material has been deposited on the ground surface (Figure 4). The most extensive areas of made ground are associated with the development of urban centres and major road construction, and with reclamation and development of port facilities and dock-land on the low-lying ground around Avonmouth, Portbury and Severnside. In the old established urban areas, the extent of made ground has been largely determined by information provided by site investigation data and the use of historic topographical maps dating back to 1826. In Bristol city centre and the Broadmead area, extensive raised ground is underlain by building rubble from wartime blitz debris. In many areas, developments, such as modern trading estates and motorways, have been built on raised ground to lessen flood risk.

Worked Ground

Worked Ground is shown where natural materials have been removed in quarries and pits, road and rail cuttings and for general landscaping. The construction of docks in Bristol, and at Portishead, Portbury and Avonmouth resulted in the excavation of large volumes of mainly Superficial Deposits. The development of Bristol Harbour also resulted in the diversion of the River Avon in 1809, through a major channel known as the New Cut, excavated in the Redcliffe Sandstone. Worked ground in the district is frequently associated with the extraction of limestone and sandstone for building stone and aggregate, and road construction such as the major M5 motorway cuttings through the Clevedon–Failand ridge. A significant part of the Avon Gorge has been quarried for limestone, and prominent quarried faces are present along the entire length of the eastern side of the gorge from Sneyd Park to the Clifton Suspension Bridge.

Infilled Ground

Infilled Ground is shown where the natural ground has been removed and the void partly or wholly backfilled with man-made deposits, the composition of which is generally not known. The types of fill are the same as those making Made Ground. Where quarries and pits have been filled and the ground restored and landscaped, built on or returned to agriculture use, there may be no surface indication of the extent of the backfilled area. In such cases, the boundaries of these sites are taken from archival sources such as local authority records and old topographical and geological maps.

Structure

The structure of the district is dominated by Variscan folds and thrusts that form a culmination within the Mendip Hills (Figure 2). Large-scale, asymmetric and north-verging folds with south-inclined axial surfaces, and south-dipping thrusts within the Mendip region have long been thought to indicate northward compressional deformation during the late Carboniferous to early Permian. The structures have been mapped and interpreted from Palaeozoic strata where they are exposed through the largely undeformed Mesozoic cover.

Structures that pre-date the Variscan deformation are preserved in the east of the district, and are thought to represent the southernmost extension of the Malvern Fault Belt, a north-south horst cored by Precambrian rocks. The Coalpit Heath Syncline is cored by Pennant Sandstone Group with the base of the Carboniferous at 2.3 km below OD in the axial region of the fold. Pennant Sandstone dips at about 10°E and 40°W in the west and east limbs, respectively. The Pensford and Radstock synclines are similar structures but symmetrical and more open in geometry. The base of the Carboniferous is 2800 to 3600 m below OD in the axial region of the folds.

The Mendip and Bristol thrusts have been described in terms of a 'piggy-back' sequence, that is, thrusts propagated from south to north with later thrusts developed in the footwalls of earlier thrusts (Williams and Chapman, 1986). In this way, displacement along the east–west-trending Farmborough Fault Belt (Green and Welch, 1965), just south of the district, occurred before thrust propagation around Bristol, but later than displacement on overlying thrusts that occur along the Mendip ridge, between Wells and Shepton Mallet. Total north– south shortening across the Mendip belt is estimated at 20 km (Williams and Chapman, 1986).

The south-dipping Farmborough Fault Belt defines the northern limit of the Radstock syncline and extends west as a concealed structure beneath the Chew valley. A small klippe or thrust outlier of Hotwells Limestone, the Churchill klippe [ST 442 602], occurs immediately south of the Farmborough Fault Belt and is underlain by the South-western Overthrust, a higher thrust that comes to surface along the Mendip ridge, about 7 km to the south.

A zone of thrusts, that include the Clevedon Thrust and the Avon Thrust (Kellaway and Hancock, 1983), extends from Clevedon to Bristol and continues beneath the Triassic cover to Kingswood. At Clifton, Visean rocks are repeated by the Avon Thrust, which can be seen, inclined 60°SE, about 250 m north of the Clifton Suspension Bridge on the east bank of the Avon Gorge (Plate 1). The hangingwall comprises Clifton Down Limestone (Seminula Oolite and overlying strata) while folded beds of Upper Cromhall Sandstone are present in the footwall. Farther west, several small klippen of Black Rock Limestone, the Gordano klippen [ST 471 740], are associated with the Avon Thrust and overlie Coal Measures in the footwall suggesting a minimum 1 km displacement along the thrust. Similarly, a 400 m-long outlier of dolomitised Black Rock Limestone, the Fore Hill Klippe [ST 463 758], occurs perched on a steep hillside overlooking Portishead. The strata above the thrust dip 20°SE and structurally overlie near-vertical Old Red Sandstone.

A concealed, north-north-west-trending, unnamed fault forms a splay of the Avon Thrust beneath Bristol and Stoke Gifford (Figure 2), and defines the western margin of the Coalpit Heath Basin. Also largely concealed, and with a similar orientation, the Ridgeway Thrust, extends from Portishead to Thornbury. The Ridgeway Thrust was formerly exposed in a 2 km-long sewage tunnel between the alluvial flats at Lawrence Weston [ST 5403 7855] and the Trym valley at Sea Mills [ST 5489 7683]. Inverted Gully Oolite and Black Rock Limestone in the hangingwall structurally overlie inverted Clifton Down Limestone and Pennant Sandstone at the margin of the Avonmouth Basin.

Normal and strike-slip faults of probable Variscan age also occur in the district. East of Bristol, the north-trending Whitefaced Fault, named for the associated alteration zone of white clay minerals, has a normal displacement of more than 100 m downthrow to the west. In the west, the offshore Denny Island Fault may represent a sinistral strike-slip fault with more than 3 km displacement that separates the western part of the Avonmouth Coalfield from the Avonmouth Basin. Denny Island comprises north-west dipping Black Rock Limestone that contains thrust surfaces together with sparse Triassic breccias.

Folding accompanied thrust displacement with the oldest and structurally uppermost thrust sheets containing the most tightly folded sequences (Figure 5). The Mendip ridge comprises four en échelon periclines, of which the Blackdown Pericline, underlain and generated by the Farmborough Fault Belt, is the westernmost. The fold is markedly asymmetrical with a near-vertical or overturned northern limb and a southern limb inclined 30°S. The South-western Overthrust forms a roof thrust to this fold but is now only preserved in the Churchill klippe.

Structures above the Avon Thrust are typically more open and concentric in geometry, and reflect the smaller thrust displacement. The Nailsea Syncline, cored by Coal Measures, is an open, concentric fold of this type, as is the associated Broadfield Down Anticline to the south. Strata are inclined in the range 20 to 40° in both folds. A number of sinuous, north-west-trending faults with down-to-the-south throws, which include the Brockley Fault, intersect the Broadfield Down Anticline and are now interpreted as backthrusts that propagated from the Avon Thrust. The Yanley Fault may have a similar origin. This is an east–west fault complex, which defines the southern edge of an area of simple structure that includes the Bedminster inlier [ST 572 706] and adjacent concealed coalfield. North of the fault, the Ashton and Bedminster seams dip uniformly 20°SE whereas south of the fault the Coal Measures and Pennant Sandstone sequence is highly deformed and largely unworked.

The intensely folded and thrusted Kingswood Anticline provides an exception to fold style typically associated with the Avon Thrust. The fold is an asymmetrical thrust culmination comprised of mudstone-dominated Lower and Middle Coal Measures. The mudstones formed a mechanically weak interval during deformation. Cross-sections constrained by subsurface mine data show disharmonic folds and at least four major reverse faults with local names such as the Northern Thrust and the Speedwell Thrust (the eastern extension of the Avon Thrust), that have vertical throws of 300 m and minimum horizontal displacements of 1 km. Folded Coal Measures in the Kingswood Anticline typically dip 50°N and locally 80oN in the northern limb and 30° to 40°S in the southern limb. The axial part of the fold contains a syncline known as the Swamp, developed in the footwall of the Northern Thrust and proved in workings of the Kingswood Great Vein.

The principal structure above the Ridgeway Thrust is the Westbury Anticline, well seen in exposures along the River Avon. Carboniferous rocks in the north-western limb of the fold are steep, typically inclined 70°NW, but also locally overturned and recumbent; the same strata in the south-eastern limb comprise the main gorge section (Plate 1) and dip about 35°SE. The two limbs are faulted together with no evidence of an anticlinal closure. Below the Ridgeway Thrust, the Avonmouth Syncline is largely concealed by Quaternary deposits and although subsurface data are sparse, appears to be a more gently dipping concentric fold that structurally overlies a translational fault farther north.

Faults that displace the Triassic and Jurassic sequences appear to be either related, or form extensions to earlier Variscan faults and probably represent reactivated structures.

Chapter 3 Applied geology

Mineral resources

The Bristol district has a wide variety of mineral resources (Figure 6), although most are no longer exploited. Coal was formerly the most important economic deposit, and together with access to the deepwater port in Bristol, provided the impetus for the early rapid urban expansion. There is archaeological evidence that coal was used at Bath in Roman times and documentary evidence that coal was worked in the district in the early 13th century. By 1547, port accounts for Bristol refer to coal exports. With the advent of new technology, steam-pumping engines were used to drain deep mines in the 18th century and the mining industry expanded rapidly through the 19th century. In the late 18th century, production from the Bristol and Somerset coalfields was estimated at 140 000 tons per annum, and by 1870 the figure had reached one million tons. Production peaked in 1920 at 1.5 million tons but subsequently, failing reserves led to a steady decline.

The major collieries within the district, south of the Avon, ceased operation by 1925. Here, the workings of South Liberty Colliery extended to over 800 m below surface in the Bishopsworth area. Speedwell Deep Pit [ST 632 744], in east Bristol, closed in 1936. Pensford Colliery [ST 618 627], which lies in the northern part of the Somerset Coalfield, closed in 1958. Following nationalisation, renewed exploration took place in the Bristol district, including drilling of the Avonmouth Coalfield and a detailed investigation of the Harry Stoke–Downend area. The latter resulted in the opening of a new, short-lived (1954–1963) drift mine at Harry Stoke [ST 618 786].

Metallic minerals are present but have not been worked on a large scale. Hematite (iron ore) has been worked since Roman times, and occurs as replacement bodies within the Carboniferous Limestone, Mercia Mudstone Marginal Facies and Pennant Sandstone commonly along faults and joints. Small workings have been recorded in Clifton around Cornwallis Crescent [ST 570 728], but the main mining in the district took place in the Frampton Cottrell area, where workings in hematite-filled vertical fissures in Pennant Sandstone extend to a depth of 146 m below surface. Clay ironstone was worked in association with the coal mining at Ashton Vale and Kingswood, but the deposits were not rich enough to give rise to a major industry. Several lead workings are known in Bristol, exploiting galena (PbS) veins in the Carboniferous Limestone. Small-scale mining took place around Henbury and Brentry, and on Durdham Downs where extensive linear surface workings are still visible [ST 566 743]. The most famous 'lead-mine', Pen Park Hole [ST 585 792], is actually a natural abandoned hydrothermal cave with little lead ore. None of the veins were very extensive, and most pinched out at depth. Zinc ore (smithsonite) from the Mendips, together with copper from Cornwall led to the development of a brass industry in Bristol.

Aggregate is the most important present-day mineral resource. Carboniferous Limestone and Quartzitic Sandstone have been extensively quarried as sources of lime and aggregate in the Avon Gorge, the Gordano Valley, Failand ridge and Backwell. Major active quarries in the Carboniferous Limestone include Durnford [ST 537 714], Stancombe [ST 503 687] and Freeman's [ST 510 672] (Figure 6). Approximately 0.4 million tonnes of marine sand, excavated from the sea bed of the Bristol Channel is shipped into the port of Bristol.

Other minerals include a wide variety of building stones that architects and builders have exploited to create the range of building style, colour and texture that does so much to enhance the city. The most ubiquitous is the 'Pennant Sandstone', easily quarried from the Frome and Avon gorges at Hanham, Fishponds and Stapleton, and used for the construction of many properties in east Bristol. It has also been extensively used for paving slabs and walls. Dressed Carboniferous Limestone is extremely weather resistant and has been used widely in the construction of 19th century properties, for example in Clevedon, Clifton, Redland, Westbury-on-Trym and Sneyd Park. The Jurassic 'Blue Lias' Limestone (Rugby and Wilmcote Limestone members) and 'White Lias' (Langport Member), displaying their characteristic flaggy nature, have been used locally as a building stone, and also for walling purposes and as paving slabs. The younger Jurassic ooidal limestones (Bath Stone and the Dundry Freestones) make excellent building stones, and have been used widely in building construction for lintels, sills and corner stones, being easily cut to shape. The Dundry Freestone was used in the construction of St Mary Redcliffe. They were extracted from surface quarries and underground workings around Bath and, within the Bristol district, at Dundry village [ST 554 667].

Various mudstones and shales have been used in the manufacture of bricks. Brickyards in Malago Vale [ST 579 699] and Stoke Gifford [ST 629 798] used the Mercia Mudstone, while mudstones of the Middle and Lower Coal Measures were worked in east Bristol and Kingswood, and are still extensively quarried at Cattybrook [ST 592 833], north of the district. The tidal flats have recently been worked in the Severnside area [ST 540 819] for brick making and the excavated pits subsequently used for landfill. Glass making was an important industry in Bristol, primarily used for bottling the thermal waters in the 18th century. The Redcliffe Sandstone provided the most likely source of sand for glass making, and it is probable that the 'caves' excavated into the Redcliffe Sandstone in the vicinity of Redcliffe Parade [ST 589 723] were dug originally for use in the nearby glassworks, and subsequently used for storage (Plate 5). Other mineral resources previously commercially exploited within the district include celestite (strontium ore) at Abbots Leigh, and red and yellow ochre at Winford [ST 534 638].

Water resources

The district has several major aquifers: the Triassic Redcliffe Sandstone, the Carboniferous Limestone and the Jurassic limestones. The combination of a narrow, easily defensible spur of land at Castle Green [ST 592 732], between the Rivers Frome and Avon, close to river crossing points, linked with a ready supply of water, both from the river and from shallow wells in the porous Redcliffe Sandstone provided a suitable site for the original Bristol settlement. Shallow wells dug in the easily excavated sandstone provided the earliest sources of potable water.

In time, contamination from neighbouring cesspits and burial grounds became a serious problem and caused recurrent epidemics. The water supply from these wells was augmented by a series of pipes and conduits feeding water from springs in and around the city. Many of these springs issue either from the Mercia Mudstone or the Lower Jurassic limestone beds, forming an important spring line at the foot of the lowest Jurassic scarp. Raven's Well near Temple Meads [ST 599 719] is a good example of such a spring, developed by an adit and piped to where it was needed. Another example is a spring near the top of Park Street that was piped to Quay Street. Several springs emerge from the Pennant Sandstone along the banks of the Avon Gorge and its tributaries, St Anne's Well [ST 622 725] is an example. Several significant springs emerge from the Carboniferous Limestone around Broadfield Down, the largest of which occurs at Chelvey Batch, near Backwell. Over the last century, boreholes into the Mercia Mudstone and the Carboniferous Limestone have augmented the water supply of the district. More recently, sources from outside the immediate district have provided the bulk of the water supply of the district. These include intakes from the Sharpness Canal and the large reservoir at Chew Stoke, fed from springs on the Mendips via a series of aqueducts and tunnels.

Many springs, some of them hot, issue from the Carboniferous Limestone along the Avon Gorge, some are in the intertidal zone. Both the limestone and the Mercia Mudstone Marginal Facies are karstic, the hydrology being dominated by conduit flow in caves, with little or no surface drainage. However, unlike the Mendips, the Carboniferous Limestone in Bristol contains few large caves, due to the limited recharge area and the lack of any allogenic drainage and swallets. Although not as famous as those at Bath, Bristol has five thermal springs: Hotwells spring is the best known, located about 200 m south of the Clifton Suspension Bridge [ST 5666 7262], while the others, St Vincents, Jacob's Well, Sion Well and the Mardyke spring are within a 1.5 km radius. The waters were considered a good cure for scurvy and consumption, and although best taken warm, were bottled and exported throughout Britain and overseas during the 17th and 18th centuries. The highest water temperature (24.4°C) occurs at the Hotwells spring, thought to be fed by 4000 m-deep groundwater along a fracture belt within the Carboniferous Limestone (Plate 6). The waters at Hotwells became red and turbid immediately after the 1755 Lisbon earthquake, consistent with deep structural control of the thermal spring. While the water at Hotwells emerges in the Avon Gorge at the lowest available point within the aquifer, the higher springs, such as Jacob's well, are diluted by cold groundwater percolating down from above and are cooler (13.8°C).

Ground conditions

The low lying areas around Ashton Vale, Bedminster and St Anne's have been prone to flooding, as was demonstrated in the major rainstorms of July 1968. In the 1960s and 1990s, Bristol City Council embarked on major tunnelling projects to channel storm water directly into the Avon and Severn. These have greatly reduced the flooding risks upstream of the Avon Gorge. Much of the Tidal Flat Deposits lies at less than 6 m above OD, and thus has been at risk from tidal inundation, especially when strong south-westerly gales coincide with spring tides. Coastal defences and sea walls parallel most of the Bristol Channel coast, and in areas of development such as Avonmouth and Portbury, much of the land is raised several metres by man-made deposits.

Coal mining activities have been responsible for a number of subsidence cases in recent years in the Bristol Coalfield. In particular, shaft collapses have occurred at regular intervals, often caused by the deterioration of capping material, but also by changes in water tables affecting the filling material of the shaft (Plate 7). In several instances, changes in the groundwater regime in east Bristol have been associated with the blocking of mine drainage levels by foundations for new building development and by sheet piling techniques (Figure 7).

In some areas, the Redcliffe Sandstone has become decalcified, losing its cement and becoming friable and difficult to handle when wet. Changes in groundwater regimes can be responsible for this decalcification process, which in turn can cause local subsidence. Weathered Redcliffe Sandstone, when used for fill material has also been known to provide poor support for buildings. Dissolution and piping also result in local subsidence within the Blue Anchor Formation.

Certain clay-rich formations contain significant amounts of smectite that can undergo volume change with variation in moisture content. The clays absorb water and expand during wet periods and lose water during droughts. This 'swell-shrink' effect can result in ground heave that causes structural damage if foundations are inadequate. Vegetation, especially trees, is a major factor in determining moisture content of the clay. The Lower Jurassic mudstones and part of the Triassic Mercia Mudstone Group contain moderate levels of smectite and have been known occasionally to cause ground heave subsidence in the Bristol district.

Major karst features are well known in the Mendip region, where the extensive cave systems of Cheddar and Wookey Hole occur just outside the district. Pen Park Hole [ST 585 792] and smaller cave systems at the Observatory, near the Clifton Suspension Bridge [ST 565 732], and on Broadfield Down, occur within the Carboniferous Limestone, but there have been no recorded instances of surface collapse resulting in property damage. However, development above Pen Park Hole is precluded by the very thin roof. Mass movement and slope stability has been an important development constraint along the Cotswold escarpment, and within the Bristol district around the flanks of Dundry Hill. Significant areas of landslipped ground have been mapped, and areas of gulls and cambering are present along the crest of the hill. During the July 1968 rainstorms, several small-scale mudflows occurred on the northern slopes of Dundry Hill. Where stratal dip and slope angle are coincident, for example along Hotwells Road (where the Carboniferous strata dip parallel to the slope surface at 35° southward), excavation by man has been responsible for destabilising the slopes, resulting in rock slides along water-saturated mudstone and clay surfaces. Rock falls have occurred on a number of occasions in the Avon Gorge, notably close to the former Black Rock Quarry at Seawalls Road [ST 561 746], and in the vicinity of the Clifton Suspension Bridge where a rockfall shelter tunnel has been constructed to protect the Portway trunk road (Plate 1).

Natural radon emissions are largely controlled by the underlying geology (Figure 7). Radon is a radioactive gas that is produced by the radioactive decay of radium, itself derived from the radioactive decay of uranium. Radon is released from rocks and soils and is quickly diluted in the atmosphere. Concentrations in the open air are normally very low and do not present a hazard. Radon that enters poorly ventilated enclosed spaces such as some basements, buildings, caves, mines and tunnels may reach high concentrations in some circumstances. Radon levels in individual buildings are influenced by the construction method and the degree of ventilation. Inhalation of the radioactive decay products of radon gas increases the chances of developing lung cancer. If individuals are exposed to high concentrations for significant periods of time, there may be cause for concern. In order to limit the risk to individuals, the Government has adopted an 'Action Level' for radon in dwellings of 200 becquerels per cubic metre. Within the Bristol district, there is evidence to suggest that more than 10 per cent of dwellings built on Carboniferous Limestone are likely to have radon concentrations greater than the Action Level (Miles et al., 1992; BGS/NRPB unpublished data). In contrast, however, less than 0.4 per cent of dwellings on the Mercia Mudstone were above the Action Level, and no houses were found to be above the Action Level on the largely impermeable Lower Coal Measures mudstones.

Conservation sites

The Bristol district has numerous Sites of Special Scientific Interest (SSSI's) and Regionally Important Geological Sites (RIGS), that reflect both the wide geological diversity and the contribution local people have made in the development of geology as a science. Thirteen geological SSSI's covering about 200 ha occur in the district, of which the Avon Gorge is the largest.

Information sources

Further geological information held by the British Geological Survey relevant to the Bristol district is listed below. It includes published maps, memoirs and reports. A Catalogue of geological maps and books is available on request. Enquiries concerning geological data for the district and advice of the geology should be addressed to the Manager, National Geological Records Centre, BGS, Keyworth.

Other information sources include borehole records, fossils, rock samples, thin sections, hydrogeological data and photographs, and further information is available at the web site: http://bgs.ac.uk.

Maps

• Geological maps
• 1:1 500 000
• Tectonic map of Britain, Ireland and adjacent areas, 1996
• 1:1 000 000
• Pre-Permian geology of the United Kingdom, 1985
• Geology of the United Kingdom, Ireland and the adjacent continental shelf (south sheet), 1991
• 1:625 000
• Geological map of the United Kingdom, South sheet, third edition, 1979 Quaternary map of the United Kingdom, South sheet, 1977
• 1:250 000
• Bristol Channel, solid geology, 1987
• Bristol Channel, sea bed sediments and Quaternary geology, 1986
• 1:50 000 and 1:63 360
• Sheet 250 Chepstow, 1972
• Sheet 264 Bristol, 2001
• Sheet 265 Bath, 1965
• Sheet 279 Weston-Super-Mare, 1980
• Sheet 280 Wells, 1984
• 1:25 000
• Clevedon and Portishead. Classic areas of British geology, 1968.
• 1:10 000
• For the most recent revision of the 1:50 000 Series Sheet 264 Bristol, the component 1:10 000 National Grid maps are listed below, together with the initials of the surveyors and dates of survey.

Surveyors K R Royse, P J Strange, A C Pople, R K Westhead, R A Edwards and A R Farrant. Copies of these maps are available for public reference in the libraries of the British Geological Survey in Keyworth and Edinburgh, and can be purchased as colour printed Versatec plots from Map Sales, BGS Keyworth.

• Geophysical maps
• 1:1 500 000
• Colour shaded relief gravity anomaly map of Britain, Ireland and adjacent areas, 1997
• Colour shaded relief magnetic anomaly map of Britain, Ireland and adjacent areas, 1998
• 1:250 000
• 51N 04W Bristol Channel, Aeromagnetic anomaly, 1988
• 51N 04W Bristol Channel, Bouguer anomaly, 1986
• 1:50 000
• Geophysical information maps; these are plot-on-demand maps which summarise graphically the publicly available geophysical information held for the sheet in the BGS databases. Features include Bouguer gravity and aeromagnetic anomalies and locations of data points, selected boreholes and detailed geophysical surveys.
• Geochemistry maps
• 1:625 000
• Methane, carbon dioxide and oil susceptibility, Great Britain, South Sheet, 1995 Radon potential based on solid geology, Great Britain, South Sheet, 1995 Distribution of areas with above the national average background concentrations of potentially harmful elements (As, Cd, Cu, Pb and Zn), Great Britain (South Sheet) 1995
• Hydrogeological maps
• 1:625 000
• Sheet 1 (England and Wales), 1977
• 1:100 000
• Groundwater Vulnerability Map, Southern Cotswolds, Sheet 37; produced by the Environment agency
• Mineral maps
• 1:1 000 000
• Industrial minerals resources map of Britain, 1996

Books

• British Regional Geology
• Bristol and Gloucester district, third edition, 1992
• The geology of Cardigan Bay and the Bristol Channel, 1994
• Colleries of Somerset and Bristol, first edition by J Cornwell, 2001
• Memoirs
• Geology of the Bristol district: the Lower Jurassic rocks (Bristol geological special sheet), 1984
• Geology of the Bristol district (Memoir for 1:63 360 geological special sheet), 1993
• Geology of the country around Monmouth and Chepstow (Sheets 233 and 250), 1961
• Geology of the Malmesbury district (Sheet 251), 1977
• Geology of the country around Wells and Cheddar (Sheet 280), 1965; second impression with amendments 1976
• Geology of the country around Weston-Super-Mare (Sheet 279 with parts of Sheets 263 and 295), 1983
• Reports
• Reports relevant to the district are arranged below by topic. Most are not widely available but may be purchased from BGS or consulted at BGS and other libraries.
• Geology
• Kellaway, G A. 1967. The Geological Survey Ashton Park Borehole and its bearing on the geology of the Bristol district: Bulletin of the Geological Survey of Great Britain, No. 27 (iii).
• Kellaway, G A, and Welch, F B A. 1955. The Upper Old Redstone and Lower Carboniferous rocks of Bristol and the Mendips compared with those of Chepstow and the Forest of Dean. Bulletin of the Geological Survey of Great Britain, No. 9.
• Mineral resources
• Harrison, D J, Buckley, D K, and Marks, R J. 1992. Limestone resources and hydrogeology of the Mendip hills. British Geological Survey Technical Report, WA/92/19.
• Edmunds, W M, And Morgan-Jones, M. 1970. Preliminary hydrogeochemical survey of strontium in the Bristol district: Sheet ST68, Tytherington. Institute of Geological Sciences, Hydrological Department Technical Report, WD/ST/70/12 Geophysics
• Evans, C D R. 1980. Report on the geological and geophysical investigations conducted in the Severn Estuary during 1979. Institute of Geological Sciences, Marine Geology Unit Report.
• Mouls, A S. 1984. Survey data report, Bristol Channel/Nymphe Bank: Marine Geophysics Unit project 81/06, RRS Shackleton. British Geological Survey. Marine Geophysics Research Programme, Computer Data Report 9.
• Nickless, E P F, Booth, S J, And Moseley, P N. 1976. The celestite resources of the area north-east of Bristol with notes on occurrences north and south of the Mendip hills and the vales of Glamorgan: description of 1:25 000 resource Sheet ST68 and parts of ST59, 69, 79, 58, 78, 67, 77.
• Mineral Assessment Report of the Institute of Geological Sciences, No. 25.
• Richardson, L. 1930. Wells and springs of Gloucestershire. Memoir of the Geological Survey of Great Britain.
• Simpson, B A. 1996. The Bristol Channel earthquake of 1 January 1994 (2.8ML). British Geological Survey, Seismology Series, Technical Report, WL/96/16.
• Tully, M C. 1971. Bristol Channel cruise report for m.v. Researcher, 14 May–7 June 1971, MGU project No. 71/03. Institute of Geological Sciences, Marine Geology, Marine Geophysics Reports, No. 20.

Documentary collections

Boreholes and shafts

Borehole and shaft data for the district is catalogued in the BGS archives (NationalGeological Records Centre) at Keyworth on individual 1:10 000 scale sheets. In the Bristol district there are sites and logs for about 2081 boreholes, for which index information has been digitised. For further information contact: The Manager, National Geological Records Centre, BGS, Keyworth.

Mine plans

BGS holds a collection of plans for underground mines. For further information contact: The Manager, National Geological Records Centre, BGS, Keyworth.

Geophysics

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

Hydrogeology

Wells and springs and water borehole records are held at the British Geological Survey, Hydrogeology Group, Maclean Building, Crowmarsh Gifford, Wallingford, Oxfordshire.

BGS Lexicon of named rock unit definitions Definitions of the named rock units shown on the 1:50 000 Series Sheet 264 Bristol are held in the Lexicon database. This is available on the web site http://www.bgs.ac.uk Further information on the database can be obtained from the Lexicon Manager at BGS, Keyworth.

Material collections

Palaeontological collection

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

Petrological collections

Hand specimens and thin sections are held in England and Wales Sliced rocks collection at BGS Keyworth. The Mineralogy and Petrology Group maintain a collection database at BGS Keyworth. The Group

Manager should be contacted for further information, including methods of accessing the database. Charges and conditions of access to the collection are available on request from BGS Keyworth.

Borehole core collection

The National Geosciences Records Centre, BGS Keyworth, holds samples and entire core from a small number of boreholes in the Bristol district.

BGS photographs

Copies of photographs, dated 1928 to 1972 are deposited for reference in the BGS Library, Keyworth.

Other relevant collections

Groundwater licensed abstractions, Catchment Management Plans and landfill sites.

The Environment Agency holds information on licensed water abstraction sites for groundwater, springs and reservoirs, Catchment Management Plans with surface water quality maps, details of aquifer protection policy and licensed landfill sites.

Coal abandonment plans

Coal abandonment plans are held by the Coal Authority, Mining Records Department, 200, Lichfield Lane, Mansfield, Nottinghamshire NG18 4RG.

References

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

Buckland, Wand Conybeare, W D. 1824. Observations on the south-western coal district of England. Transactions of the Geological Society of London, Series 2, Vol. 1, 210–316.

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

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, and Kellaway, G A. 1984. Geology of the Bristol district: the Lower Jurassic rocks. Memoir for 1:63 360 Bristol geological special sheet. Memoir of the British Geological Survey.

Green, G W. 1992. British Regional Geology: Bristol and Gloucester region (third edition). (London: HMSO for British Geological Survey).

Green, G W, and Welch, F B A. 1965. Geology of the country around Wells and Cheddar. Memoir of the Geological Survey of Great Britain, Sheet 280 (England and Wales).

Kellaway, G A. 1970. The Upper Coal Measures of south west England compared with those of South Wales and the southern Midlands. Sixth International Congress of Carboniferous stratigraphy and geology, Sheffield, 1967, Vol. 3, 1039–1055.

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).

Kellaway, G A, and Welch, F B A. 1993. Geology of the Bristol district: memoir for 1:63 360 geological special sheet (England and Wales). Memoir of the British Geological Survey.

Miles, J C H, Green, B M R, and Lomas, P R. 1992. Radon Affected Areas: Derbyshire, Northamptonshire and Somerset. Documents of the National Radiological Protection Board, Vol. 3, No. 4, 19–28.

Smith, W. 1815. A memoir to the map and delineation of the strata of England and Wales with part of Scotland. (London: John Cary.)

Vaughan, A. 1905. The palaeontological sequence in the Carboniferous Limestone of the Bristol area. Quarterly Journal of the Geological Society of London, Vol. 61, 181–307.

Vaughan, A. 1906. The Carboniferous Limestone series (Avonian) of the Avon Gorge. Proceedings of the Bristol Naturalists' Society, fourth series, Vol. 1, 74–168.

Williams, G D, and Chapman, T J. 1986. The Bristol–Mendip foreland thrust belt. Journal of the Geological Society of London, Vol. 143, 63–73.

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

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

(Index map)

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

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

Figures and plates

Figures

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

(Figure 2) Principal structural features of the district.

(Figure 3) Principal coal seams in the Bristol district.

(Figure 4) Schematic cross-section through central Bristol, including artificial and Quaternary deposits.

(Figure 5) Cross-section showing the main structural features of the district.

(Figure 6) Major mineral workings.

(Figure 7) Principal engineering geological units and their relevance to construction and development.

Plates

(Plate 1) Oblique aerial view of the Clifton suspension bridge above the Avon gorge. Limestones of the Clifton Down Limestone Formation beneath Observatory Hill are inclined about 35 degrees south-east (toward the lower right), and have been transported north-west on the Avon Thrust to structurally overlie younger sandstones and limestones of the Hotwells Limestone Formation within the wooded area in the top left of the picture. The rockfall shelter that protects the Portway trunk road can be seen beneath the bridge, adjacent to the narrow spread of tidal flats along the river (Photograph A R Farrant).

(Plate 2) Triassic Mercia Mudstone Marginal Facies resting unconformably on Lower Old Red Sandstone Black Nore Sandstone; Woodhill Bay, Portishead looking south, [ST 4500 7600] (A10733).

(Plate 3) Stone circle at Stanton Drew [ST 5900 6300]: the silicified Mercia Mudstone Group Marginal Facies (formerly Dolomitic Conglomerate) blocks are probably erratics (A11407).

(Plate 4) Tidal flats Portishead Point (looking north) Salt marsh deposits in the foreground formed by the colonisation of the mud flats by plants. In the middle distance, Portishead Point is composed dolomitised limestone (Pembroke Limestone Group). In the background is the Severn Estruary with Denny Island and the hills of Monmouthshire forming the skyline (A10740).

(Plate 5) Redcliffe Stone Mines. These 'caves' were excavated into the Redcliffe Sandstone to provide sand for glass making and the production of slip for glazing pottery, and subsequently used for storage. Parts of the mine have been backfilled with rubble from local potteries. (Photograph John Brewer).

(Plate 6) Plaque at the site of the Hotwells [ST 5666 7262] (Photograph G Stiles, Linkcheck)

(Plate 7) Collapsed mine shaft. An 18th century mine shaft, 116 m deep, uncovered in the front garden of a Kingswood property (Photograph Bristol Coalmining Archives Ltd).

(Front cover) Cover photograph. Neptune overlooking the docks. This lead statue is one of the last surviving relics of the Bristol Lead works that used Mendip lead. (Photographer H J Evans) (A11396).

(Rear cover)

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

Figures

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

(Figure 3) Principal coal seams in the Bristol district

(Figure 6) Major mineral workings

(Figure 7) Principal engineering geological units and their relevance to construction and development