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Geology of the Sidmouth district — a brief explanation of the geological map, sheets 326 and 340 Sidmouth

R A Edwards and R W Gallois

Bibliographic reference: Edwards, R A and Gallois, R W. 2004. Geology of the Sidmouth district — a brief explanation of the geological map. Sheet Explanation of the British Geological Survey. 1:50 000 Sheets 326 and 340 Sidmouth (England and Wales).

Keyworth, Nottingham, British Geological Survey, 2004. © NERC 2004 All rights reserved.

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

(Front cover) Otter Sandstone stacks at Ladram Bay, viewed from the west (Photographer R W Gallois; (MN39678).

(Rear cover)

(Geological succession) Summary of the geological succession at outcrop in the Sidmouth district.

Notes

The word 'district' refers to the area represented on the geological 1:50 000 series sheets 326 and 340 Sidmouth. National Grid references are given in square brackets. Lithostratigraphical symbols shown in brackets, for example (UGS), are those shown on the 1:50 000 published map. Numbers given with plate captions refer to the official BGS collection.

Acknowledgements

This Sheet Explanation was compiled by R J O Hamblin, largely from text supplied by R A Edwards and R W Gallois. Contributors include J D Appleton (natural radon emissions), A Forster (engineering geology), S Holloway (structure), B Humphreys (offshore geology), N S Jones (sedimentology), G Warrington (Triassic biostratigraphy), and M A Woods (Cretaceous biostratigraphy). The manuscript was edited by S G Molyneux. Cartography by R J Demaine, P Lappage and G Tuggey.

The authors' thanks are due to landowners, local authorities, and utility and site investigation companies who facilitated access to land and provided geological information. Thanks are also due to the professional fossil collectors of the Lyme Regis area for their generous stratigraphical advice on the Jurassic sequences.

Maps and diagrams in this book use topography based on Ordnance Survey mapping. © Crown copyright. All rights reserved. Licence Number: 100017897/2004.

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

(Rear cover)

The Sidmouth district is renowned for its coastal sections, exposing a succession from the Triassic Otter Sandstone to the Upper Cretaceous Seaford Chalk. They form a key part of the Dorset and East Devon Coast World Heritage Site. Inland, mudstone-dominated Triassic to Lower Jurassic strata form low ground beneath dissected escarpments of Cretaceous Upper Greensand and Chalk. The southward aspect of the coast, its mild climate and striking scenery have given rise to a thriving holiday industry. Inland the district is mostly rural and agricultural.

The oldest rocks exposed are Triassic sandstone and mudstone, derived from the weathering and erosion of Variscan mountain ranges. Inundation by the sea during the Late Triassic led to deposition of the marine Penarth Group and Lower Jurassic strata, but any Middle and Upper Jurassic strata deposited were removed by erosion during the latest Jurassic and Early Cretaceous. The succeeding Lower Cretaceous Gault and Upper Greensand formations and the Upper Cretaceous Chalk Group indicate further marine inundations.

Fluvial and lacustrine sands, gravels and clays may have been deposited on an eroded Chalk surface during the early Cainozoic (Tertiary), but solution of the underlying Chalk has caused these deposits to become mixed with flint from the Chalk and chert from the Upper Greensand to form the Clay-with-flints. No glaciers reached the district during the Pleistocene, but the climate during successive cold periods would have been periglacial, with permafrost developing at all topographical levels, and the formation of head, river terrace deposits and landslips.

Landslips are still active on the coast, and slope stability and coastal erosion have an impact on planning and land use. Rocks of the Penarth Group, the Upper Greensand Formation and Chalk Group, and to a lesser extent the Blue Lias and Charmouth Mudstone formations, are prone to relatively high radon levels. Some units are aquifers, notably the Sherwood Sandstone Group, which supplies much of the western part of the district from boreholes in the Otter valley. Other units have provided natural resources in the past, principally construction materials, but there is little present-day activity.

Chapter 1 Introduction

Coastal sections in the Sidmouth district form a key part of the Dorset and East Devon Coast World Heritage Site, exposing a succession from the Triassic Otter Sandstone to the Upper Cretaceous Seaford Chalk (Plate 1). Inland, the mudstone-dominated Triassic to Lower Jurassic succession forms low ground, while the Cretaceous Upper Greensand and Chalk form the remnants of a faulted, gently tilted plateau, capped by Clay-with-flints, which rises to a height of over 270 m in the north-west. Deeply incised valleys give rise to topography of high relief, with steep escarpments around the edges of the Cretaceous outliers.

The first geological map of the district, by De La Beche, was published in 1834. The district was surveyed on a scale of 1:63 360 in 1873–76, and was fully surveyed on a scale of 1:10 000 in 1987–2000. A listing of maps and other relevant British Geological Survey publications can be found under information sources.

Outcrops in the Exeter district to the west (Edwards and Scrivener, 1999), and deep boreholes throughout the Wessex Basin, suggest that Devonian and Carboniferous rocks, folded and faulted during the Late Carboniferous Variscan Orogeny, underlie the Sidmouth district. However, no Devonian or Carboniferous rocks crop out in the district, and no borehole has penetrated pre-Permian strata. Nevertheless, it is likely that a largely conformable succession of Permian, Triassic and Jurassic rocks, dipping eastwards at 2° to 5°, rests unconformably on Devonian and Carboniferous beds.

Permian breccias and sandstones pass up into Triassic sandstones and mudstones. This fining-upward sequence represents the steady wearing down of the Variscan mountain ranges to form an extensive peneplain. The peneplain was later inundated by shallow lakes and, in the Late Triassic, by a brackish sea in which the laminated mudstones and shallow-water limestones of the Penarth Group were deposited. The succeeding Jurassic mudstones indicate a deepening of the sea and a long period of shallow marine sedimentation that probably lasted almost to the end of the Jurassic Period, although only Lower Jurassic strata are preserved. Early Cretaceous erosion followed gentle folding and eastward tilting of the Triassic and Jurassic rocks, so that Cretaceous rocks overstep progressively older strata westwards. As a result, Jurassic rocks are absent west of the lower Axe and Yarty valleys. However, the subcrop pattern of Triassic and Jurassic rocks beneath the Cretaceous is complicated by north–south-trending major faults, most of which downthrow to the west, repeating parts of the succession.

Following the Early Cretaceous uplift and erosion, a trangressive sea deposited argillaceous Gault Formation over the eastern part of the district, and arenaceous Upper Greensand over the whole of the district. Further transgression and deepening resulted in deposition of the Chalk Group. The lowest Chalk formation, the Beer Head Limestone, is a condensed deposit that formed in relatively shallow water on a tectonic high starved of sediment. The succeeding chalks, up to the Seaford Chalk Formation, are present in their typical facies.

Much of the Chalk was removed by erosion and dissolution following regional uplift in the latest Cretaceous. Thin but extensive sheets of fluvial sands, gravels and clays were deposited throughout the district during the early Cainozoic (Tertiary), on an eroded Chalk surface. Tectonic activity in the Miocene produced gentle folds and a south-eastward tilt to the Cretaceous and Cainozoic strata, and resulted in renewed movement along north–south faults that had been intermittently active since Devonian times.

There is no evidence that any of the Pleistocene glaciations reached the district, but it was subjected to long periods of periglacial climate, in which permafrost caused deep weathering and seasonal meltwaters produced rapid erosion and extensive clastic deposition. Solution of the Chalk continued during cold phases and intervening temperate periods, and frost action mixed the Cainozoic clays and sands with Chalk flints and Upper Greensand cherts to form Clay-with-flints. Head and River Terrace Deposits were formed, and landslips were especially active during the cold periods. At least ten terraces occur along the valley of the River Otter, implying a long and complex history of denudation and aggradation. The climate warmed rapidly at the end of the last Pleistocene cold phase, and volumes of Spring runoff declined to a level at which rivers became meandering streams with low gradients and fine-grained sediment loads. Extensive spreads of colluvium (hillwash) formed during the Holocene, particularly following deforestation during the Bronze Age.

This Sheet Explanation contains an account of Holocene marine deposits, including currently mobile sea-bed sediments, but no account of the offshore solid geology is given. The latter is little different from the onshore geology along strike. A full account of the offshore geology of the English Channel may be found in Hamblin et al. (1992).

Chapter 2 Geological description

The geological succession at outcrop in the district is shown on the inside of the front cover. (Geological succession)

Permian and Triassic ('New Red Sandstone')

The oldest rocks proved in the district are 367 m of interbedded quartz-rich breccia, pale grey to purple-brown fine-grained sandstone and reddish brown mudstone in the Musbury No. 1 Borehole [SY 2670 9510]. They have been assigned to the Permian Exeter Group (Edwards et al., 1997), and are possible equivalents of the Alphington and Heavitree breccias of the Exeter district.

The oldest rocks at outcrop belong to the upper part of the Aylesbeare Mudstone Group (Ayb). They comprise reddish brown, slightly calcareous mudstone and muddy siltstone, with local beds of sandy silt. Greenish grey spherical spots, patches and thin beds are present throughout. The group is present beneath the whole of the district. Its full thickness of 541 m was proved in the Musbury Borehole, where it also includes a sandstone unit, the base of which lies about 236 m below the top of the group. The Aylesbeare Mudstone Group is thought to have been deposited in a sabkha-playa, a lake that periodically dried out precipitating evaporites. In the absence of definitive palaeontological evidence, the position of the Permian–Triassic boundary is not known, but possibly lies within the group.

The overlying Sherwood Sandstone Group comprises the Budleigh Salterton Pebble Beds Formation overlain by the Otter Sandstone Formation. Both formations crop out in western parts of the district, and seismic sections indicate that they are present in the subcrop of the rest of the district.

The Budleigh Salterton Pebble Beds Formation (BSP) forms disconnected, partly fault-bounded outcrops along the western border of the district, and produces a moderately pronounced escarpment. The formation is mostly between 20 and 30 m thick, consisting of brown, horizontally bedded gravel with subordinate lenticular beds of trough cross-bedded pebbly sand and sand. The gravel comprises well-rounded pebbles, cobbles and boulders in a coarse to fine-grained gravel and silty sand matrix. Up to about 90 per cent of the clasts are metaquartzite, with a few percent each of porphyry, vein quartz, tourmalinite and feldspathic conglomerate.

The Budleigh Salterton Pebble Beds lack indigenous macrofossils, but from their stratigraphical position are considered to be Early Triassic in age. The metaquartzite pebbles contain brachiopods, bivalves and trilobites indicating Ordovician to Devonian source rocks in Brittany, Normandy and south Cornwall. Sedimentary features show that the formation was deposited in the braided channels of a large fast river, flowing from south to north (Smith and Edwards, 1991). At the top, a thin bed (about 0.3 m thick) of reddish brown silty clay with polished, pitted and faceted pebbles (ventifacts) represents an ancient soil horizon (palaeosol), indicating a sedimentary break of uncertain duration.

The Otter Sandstone Formation (OS) crops out along the western side of the district and is exposed in cliffs between the mouth of the River Otter and Sidmouth (Front cover). The formation dips gently (less than 5°) to the east. Up to 210 m were proved in a borehole [SY 0916 8480] near Otterton, and the formation is 145 m thick in the Musbury Borehole. It consists mainly of reddish orange-brown, weakly to moderately cemented, cross-bedded, fine and medium-grained sandstone, with subordinate units of conglomerate and mudstone that occur as discontinuous lenses and sheets. The conglomerates are mostly intraformational, mainly less than 0.5 m thick, well cemented with calcite, and are interbedded at regular intervals (1–6 m) within the sequence. The mudstones are mainly reddish brown and up to about 2 m thick. Calcareous concretions are locally common, forming subhorizontal sheets, near-vertical cylinders and nodules. Vertical concretions may have precipitated around plant roots, and complex networks of calcareously cemented plant roots are prominent at many horizons. The sheets may represent cementation around ancient water tables (Purvis and Wright, 1991).

Sedimentary structures indicate that the lowest part of the Otter Sandstone at Budleigh Salterton (just west of the district) was deposited as wind-blown sand. However, at Foxenholes Quarry [SY 077 948] near Ottery St Mary, there are no aeolian deposits, and the basal 6 m of the formation consist of gravelly sand with large angular feldspar grains, granitic rock fragments and aureole rocks. The remainder of the Otter Sandstone was deposited in braided and meandering stream channels with highly variable flow rates in arid or semi-arid environments. Cross-bedding indicates that the rivers flowed from south to north.

Fossil vertebrates, including terrestrial reptile and amphibian remains in the sandstones and channel-lag gravels and fish in the mudstones, indicate a Middle Triassic, probably Anisian age (Benton and Spencer, 1995; Spencer and Storrs, 2002).

The Mercia Mudstone Group (MMG) is exposed in the cliffs between Sidmouth and Axmouth [SY 105 862] to [SY 270 894]. The group is about 450 m thick at outcrop in the district, but seismic surveys and the proximity of the Marchwood No. 1 Borehole [SY 3885 9880] 3 km east of the sheet boundary suggest that it locally exceeds 600 m in the subcrop area due to the addition of thick beds of salt (Gallois, 2003). The Sidmouth Mudstone (SiM) and Branscombe Mudstone (BeM) formations (Gallois, 2001a; (Figure 1)) consist of small-scale rhythms comprising fissile, brownish red mudstone overlain by reddish orange muddy siltstone (Plate 2). Each rhythm probably reflects a change from a wetter to a drier climate. Thin but laterally persistent beds of green mudstone are present, many of which are finely laminated or partially dolomitised. Green strata make up about 45 per cent of the sequence in the highest 19 m of the Branscombe Mudstone, giving rise to distinctive red and green striped beds. Thin beds of siltstone or very fine-grained sandstone occur at a few levels, and thin, fining-upwards beds of fine to medium-grained sandstone also occur at three stratigraphical levels in the Branscombe Mudstone. Gypsum is common throughout both formations, and is the dominant constituent of the 10 m thick Red Rock Gypsum Member of the Branscombe Mudstone.

The Dunscombe Mudstone Formation consists predominantly of green and purple mudstone with subordinate reddish brown mudstone and thin beds of pale grey calcareous sandstone, dark grey mudstone and mudstone breccia. Most of the succession is finely laminated and contains trace fossils indicative of deposition in brackish water. The breccias consist of angular mudstone clasts in a mudstone matrix and were probably formed penecontemporaneously by the repeated growth and dissolution of gypsum or anhydrite (autobreccias). Some of the breccias have a pervasive pinkish brown staining, indicative of the dissolution of evaporites (probably gypsum and possibly halite) long after deposition (collapse breccias). At outcrop, the collapse breccias are more porous and deeply weathered than the surrounding mudstone, and give rise to seepage lines and small landslips.

The red and green striped beds at the top of the Branscombe Mudstone represent an upward passage to the Blue Anchor Formation (BAn). The whole of the latter is exposed in cliff sections near Axmouth [SY 265 894] to [SY 275 893], and much of it is repeated at Charton Bay [SY 300 900]. It comprises 25 to 29 m of thinly interbedded, pale and medium grey siltstone, muddy siltstone and mudstone, with a few thin beds of dark grey mudstone and relatively common beds of very pale grey, finely laminated dolomitic limestone. A few thin beds of dark red-brown mudstone occur in the lower part. Nodular gypsum occurs at several levels, but is not common.

The Penarth Group (PnG) has not been subdivided into formations on the 1:50 000 Series map (Sheet 326). The Westbury Formation at the base overlies a burrowed and bored erosion surface cut into the Blue Anchor Formation, and comprises 6 to 8 m of dark grey, fossiliferous, pyritic, soft mudstone. When weathered, the formation is susceptible to landslip and there are few inland exposures. The basal bed contains whole and comminuted shells, and vertebrate remains including teeth and bone fragments of fish, ichthyosaurs and plesiosaurs. The invertebrate fauna, mostly thin-shelled bivalves and small gastropods, indicates deposition in a marine to brackish environment.

Following a sedimentary break, the Westbury Formation is overlain by another bone bed, and then by up to 1.5 m of grey and greenish grey, laminated and ooidal limestone with a few thin beds of green mudstone. Together, these constitute the Cotham Member of the Lilstock Formation. Loose slabs of 'Cotham Marble' or 'Landscape Marble', up to 0.1 m thick, have been recorded from time to time at Culverhole Point [SY 274 893], and are presumed to have come from the top of the sequence.

The Cotham Member is overlain by up to 9 m of very pale weathering, fine-grained limestone, previously known as the 'White Lias' but now forming the Langport Member of the Lilstock Formation (Warrington et al., 1980). Its full thickness is exposed at Charton Bay [SY 301 901] and Pinhay Bay [SY 318 908], where it contains several mineralised (hardground) surfaces of splintery grey porcellanous limestone. The member contains bivalves, brachiopods, gastropods, serpulids and rare corals, and was deposited in a marine environment. The top of the Langport Member is marked by a distinctive limestone, the 'Sun Bed', which is capped by an erosion surface from which numerous U-shaped burrows of the trace fossil Diplocraterion descend.

Lower Jurassic

Lower Jurassic rocks of the Lias Group crop out in the district. Accounts by Woodward and Ussher (1911), Lang (1914–36) and Hesselbo and Jenkyns (1995) provide a detailed account of the stratigraphy. Lang and subsequent workers used quarrying names for individual limestone beds, and Lang also allocated bed numbers to the whole of the succession. These have continued in use to the present day (Figure 2).

At the base of the Lias Group, the Blue Lias Formation (BLi) comprises 26 to 38 m of thinly interbedded limestone and mudstone. The limestone is hard and fine-grained ('blue-hearted'), and the mudstone includes both organic-rich and carbonate-rich varieties. The formation is fully exposed in the cliffs and foreshore around Lyme Regis (Plate 3). Head deposits and landslip debris obscure much of the outcrop inland, but the formation was exposed in quarries at Membury and near Axminster.

The lowest 18 m of the Blue Lias comprises tabular and nodular beds of limestone, mostly 20 to 40 cm thick, separated by beds of mudstone mostly 10 to 40 cm thick. Above this, about 8 m of mudstone contain seven tabular beds of limestone, 20 to 60 cm thick, with several horizons of discontinuous limestone doggers. The thickest mudstone is 1.8 m thick.

The Triassic–Jurassic boundary, marked by the first appearance of the ammonite Psiloceras planorbis, is taken at the base of Lang's Bed H25 at Pinhay Bay, about 2.5 m above the base of the Blue Lias. The fauna of the 'pre-Planorbis Beds' consists mostly of marine bivalves that are not diagnostic of age. Many of the Blue Lias limestones contain sheets and nests of oysters and other bivalves, brachiopods and echinoderm debris. The lower beds contain the ammonites Psiloceras and Caloceras, and the higher beds Metophioceras and Coroniceras, at some levels forming 'ammonite pavements'.

Above the Blue Lias Formation, the Charmouth Mudstone Formation (ChM), previously known as the Lower Lias Clay, consists of pale, medium and dark grey mudstone, calcareous mudstone and organic-rich mudstone. In contrast to the Blue Lias, there are few limestone beds; the formation may be divided into four members on the basis of gross lithology, but these have not been distinguished on the 1:50 000 Series map (Sheet 326). A lithologically distinctive stone band marks the base of each member (Figure 2).

The type section of the lowest member, Shales-with-Beef Member, is in the cliffs below Black Ven [SY 3570 9310] to [SY 3610 9307]. The lower part of the member is exposed in the cliffs eastwards from Pinhay Bay [SY 318 908] to [SY 332 915]. The member comprises thinly interbedded organic-rich and organic-poor mudstone, with numerous thin beds, mostly 0.02 to 0.2 m thick, of fibrous calcite or 'beef'. These beds are discontinuous over long distances and cannot be used for correlation. Thin limestone beds occur at a few levels, but most are also discontinuous. They are siltier and less well cemented than the Blue Lias limestones, and several are capped by a layer of 'beef'. Page (1992; 2003) has amplified the faunal succession.

The Black Ven Marl Member at Black Ven [SY 357 931] consists of calcareous and noncalcareous mudstone, with concentrations of thin beds of fissile, organic-rich mudstone at several horizons. Lang and Spath (1926) described lithologies and ammonite faunas. The member contains several laterally persistent tabular limestone beds and horizons of concretionary limestone, all of which form readily identifiable marker beds in the cliff sections.

The Belemnite Marl Member consists of 22 to 25 m of alternating, more and less calcareous, very pale grey and pale grey mudstone, mostly in couplets 35 to 40 cm thick. A persistent tabular limestone, the Hummocky Limestone, marks the base of the member, and two closely spaced beds up to 10 cm thick and crowded with belemnites mark its top. Belemnites are relatively common throughout. The full thickness of the member is exposed at Black Ven [SY 358 932] to [SY 350 931]. Westwards, the member is probably present on the western side of the Lim valley, but is everywhere obscured by drift deposits. The basal Cretaceous unconformity cuts out the Belemnite Marl west of the River Lim.

At its type section in the Bridport district, to the east of Sidmouth, the Green Ammonite Member comprises about 35 m of dark grey mudstone that passes up into paler, more calcareous mudstone (Hesselbo and Jenkyns, 1995). Its base is marked by the Belemnite Stone, 8 to 10 cm of soft muddy limestone crowded with belemnites. The full thickness of the Green Ammonite Member is present inland in the Sidmouth district, where it is poorly exposed, but the member is incomplete in the coastal sections. The Belemnite Stone and up to 6 m of deeply weathered, partially slipped grey mudstone, with one or more horizons of red ironstone nodules, are present beneath the Cretaceous unconformity at the eastern end of Black Ven [SY 3580 9323]. There is no undisturbed exposure here except for the Belemnite Stone, which forms a prominent ledge at the top of the Belemnite Marl cliff. Westwards, Cretaceous beds probably overstep the member before it reaches the Lim valley.

No Jurassic strata younger than the Green Ammonite Member occur in the coastal sections of the district, but about 50 m of the overlying Eype Clay Member (EyCl) of the Dyrham Siltstone Formation ('Middle Lias') are preserved in the north-east. The member comprises blue micaceous calcareous siltstone (marl) and siltstone with a concentration of impersistent, fine-grained, calcareous sandstones in the lowest part.

Lower Cretaceous

Cretaceous rocks underlie all the higher ground in the district, forming dissected plateau remnants that are capped by a Cainozoic (Tertiary) planation surface. The Cretaceous rocks have gently undulating easterly dips of generally less than 1°. From east to west they progressively overstep Lower Jurassic and Triassic rocks, dipping more steeply at 2° to 5° to the east.

The Gault Formation (G), at the base of the Lower Cretaceous, is poorly represented by a patchy, thin, sandy mudstone, which passes up into sandstone and calcarenite of the Upper Greensand Formation. The Gault and Upper Greensand were deposited in shallow and very shallow marine environments, and represent generally more turbulent and shallower conditions than their Albian correlatives in south-east England.

The patchy distribution of the Gault may be related to penecontemporaneous reactivation of faults. Its absence from a large part of the district is probably an original feature, and not the result of erosion before deposition of the Upper Greensand. Up to 5.3 m of Gault clay were formerly well exposed at Charmouth, where it has given rise to the extensive Black Ven Landslip. Pockets of sandy glauconitic clay at the base of the Undercliff Landslip between Axmouth and Lyme Regis [SY 270 896] to [SY 335 908], large coastal landslips at Seaton and Beer [SY 235 896], [SY 216 880], and extensive inland landslips in the steep-sided valleys around Uplyme [SY 327 932] and east of Axminster suggest the presence of at least a few centimetres of sandy Gault clay as far west as a line from Beer to Raymond's Hill [SY 32 96]. Reid (in Woodward and Ussher, 1911) recorded 5.3 m of fossiliferous clay and sandy clay, with a basal pebble bed resting on a burrowed surface of Lias clay, at the eastern end of Black Ven [SY 358 932]. The fossils from the Gault included numerous species of bivalves accompanied by gastropods, echinoids, foraminifera, rare belemnites and fish teeth, and ammonites indicative of a Mid Albian age. Up to 5 m of 'Gault' were recorded in a railway cutting [ST 195 010] east of Honiton (Woodward and Ussher, 1911), close to a major fault. This has not been confirmed, but the Gault may be represented by 0.55 to 0.65 m of very stiff, dark grey, silty clay in boreholes for the A30 trunk road improvement farther north [ST 200 049].

The Upper Greensand Formation (UGS) is well exposed in cliff sections between Sidmouth and Charmouth (Plate 4) [SY 150 878] to [SY 235 895]; [SY 257 898] to [SY 325 906]; [SY 355 933] where it varies between 45 and 55 m thick, and can be subdivided into the Foxmould Member (20 to 25 m thick), Whitecliff Chert Member (up to 32 m) and Bindon Sandstone Member (up to 8 m; Gallois, 2004a). Each member is separated by a mineralised erosion surface or hardground (Figure 3). The highest, chert-free part of the Bindon Sandstone in the Bindon Cliffs area may be the correlative of the Eggardon Grit, forming the highest part of the Upper Greensand in the adjacent Bridport district.

The members cannot be traced inland where exposure is poor, confined to one working quarry [SY 312 918] near Uplyme and the beds of the more deeply incised streams. The Upper Greensand consists of fine, medium- and coarse-grained calcareous sandstones, and calcarenites with variable amounts of silica, glauconite and comminuted shell debris. Glauconite is present throughout and is abundant at some levels, notably in the higher part of the Foxmould Member and at several well-defined levels in the Whitecliff Chert and Bindon Sandstone members (Figure 3). When fresh, the sandstones range from soft to very hard, and from faintly greenish grey to bright green. They weather to soft, yellow, brown and 'foxy' brown sand, with residual clasts of calcareous sandstone (Foxmould) or chert (Whitecliff Chert). Shell-debris beds and broken shells occur at many levels. Hardground surfaces are commonly overlain by scour hollows infilled with pebbly, shelly sand. Sedimentary structures indicative of deposition in current-agitated water are present throughout. These include troughs and planar cross-bedding, hummocky cross-stratification and ripple-drift bedding.

The Foxmould Member is generally finer grained and more siliceous than the other members, comprising fine to medium-grained sand with clay beds in the lower part. There is a facies change from calcareously cemented tabular beds and doggers in the east to siliceously cemented horizons (the Blackdown facies) in the north-west.

The Whitecliff Chert and Bindon Sandstone members comprise fine to coarse-grained sandstone and calcarenite, with many horizons of nodular and tabular chert in the former and lower part of the latter. There is an east to west decrease in overall carbonate content of both members, accompanied by a complementary reduction in chert content, with the result that chert is absent from the Bindon Sandstone west of Beer Head. The highest part of the Bindon Sandstone contains slumped beds, contortions and festoon-bedding structures. The highest Upper Greensand strata in the north-east of the district, around Membury and Storridge Hill, comprise buff-yellow, fine to coarse-grained calcareous sandstone, which is very hard, with a highly glauconitised upper surface.

The formation contains a rich and diverse bivalve fauna, with relatively common brachiopods, gastropods and echinoids but few ammonites. The latter are indicative of parts of the Upper Albian, and show the formation to be the correlative of the Gault of eastern England.

Upper Cretaceous

The Chalk Group (Ck) crops out in outliers that are the remnants of a once continuous cover (Figure 4). There is little inland exposure, but the succession is well exposed in cliff faces between Salcombe Regis and Beer [SY 150 878] to [SY 235 895] (Plate 4), and in the backface of the Undercliff Landslip complex between Axmouth and Lyme Regis [SY 270 896] to [SY 320 902], where it can be further subdivided into subgroups and members. The lowest unit, the Grey Chalk is Cenomanian in age and the overlying White Chalk is Cenomanian to Coniacian (Woods, 2002).

The Grey Chalk Subgroup can be traced throughout most of southern England, and consists of argillaceous chalk. It is represented in the Sidmouth district by the highly condensed Beer Head Limestone Formation (Jarvis and Woodroof, 1984; Cenomanian Limestone of early authors). This formed in a shallower marine environment than its more easterly equivalents in the Wessex Basin. At Beer Roads [SY 230 891], the Beer Head Limestone Formation comprises up to 0.6 m of dense, highly bioturbated, porcellanous limestone that sits with a marked sedimentary break on a hardened surface of Upper Greensand calcarenite. The limestone contains three prominent mineralised hardground surfaces, each of which is capped by a bed of green-coated pebbles. The formation is richly fossiliferous, with common ammonites, brachiopods, echinoids, gastropods and sponges. The ammonite assemblage is diverse and contains forms not known elsewhere in Britain (Mortimore et al., 2001).

In a narrow, fault-bounded area between the coast at Beer [SY 220 880] and the village of Wilmington [ST 210 000], the lower limestone beds of the Beer Head Limestone pass laterally into the Wilmington Sand Member. This member is up to 12 m thick and consists of glauconitic calcareous sandstone, deposited in a well-oxygenated, current-swept, shallow sea. It contains one of the most diverse Cenomanian faunas in Europe, especially rich in brachiopods, bivalves, echinoderms and crustaceans (mostly crabs), many of which have not been recorded elsewhere.

In the Membury (about [ST 27 02] to [ST 27 05] and Storridge Hill [ST 31 04]) outliers, the local correlative of the Beer Head Limestone comprises a highly condensed and winnowed, pebbly glauconitic sandstone and chalky sandstone (the 'Chalk Basement Bed' of Kennedy, 1970), with numerous derived Cenomanian ammonites in phosphatic pebbles and limestone cobbles.

The formations of the White Chalk Subgroup can be matched lithologically and palaeontologically with parts of the sequence elsewhere in southern England: representatives of the Holywell Nodular Chalk, New Pit Chalk, Lewes Nodular Chalk and Seaford Chalk formations are present, albeit with local lithological variations.

Throughout much of southern England, the Holywell Nodular Chalk Formation consists of laterally uniform, nodular, gritty chalk, rich in whole and broken shells of the bivalve Mytiloides. Although this is the dominant lithology in the Sidmouth district, the formation here also contains up to 15 hardground surfaces that represent sedimentary breaks, and these resulted in rapid local variations in thickness. In the eastern part of the district, the formation is up to 15 m thick, but this decreases locally to 5 m in the north and west, adjacent to major faults. Between Beer and Wilmington [SY 220 880] to [SY 210 005], a lenticular bed of comminuted echinoderm debris, up to 6 m thick, forms the Beer Stone.

At Beer Roads [SY 232 892], the New Pit Chalk Formation consists of 25 m of interbedded smooth-textured flinty chalk and marly chalk beds, with local nodular chalk beds. These beds pass laterally eastwards and westwards into similar thicknesses of relatively flint-free, smooth-textured, marly chalks that are lithologically similar to the New Pit Chalk of more easterly parts of the Wessex Basin. The formation is moderately fossiliferous, but inoceramid bivalves and the small brachiopod Terebratulina lata are the only common fossils at most horizons. The formation is susceptible to karstic dissolution with the result that there are few exposures inland, where it is everywhere overlain by Clay-with-flints.

The Lewes Nodular Chalk Formation, of late Turonian and early Coniacian age, is well exposed in cliff faces in the Sidmouth district. Most of the succession can be examined in huge blocks that have remained intact, despite having travelled up to 300 m, in coastal landslips. The full 35 to 40 m thickness of the formation is exposed in the cliffs at Beer Head [SY 226 879], at Allhallows [SY 290 899] and at Pinhay [SY 322 912]. The succession consists of nodular chalk and thick, distinctively weathering beds of thalassinoid chalkstone, with prominent iron-stained hardground surfaces and common burrowform and rare sheet flints. Many of the marker beds can be used for local and long-distance correlation. The formation is richly fossiliferous at many levels. The echinoid Micraster and brachiopods are especially common.

The Seaford Chalk Formation overlies the Lewes Nodular Chalk throughout much of southern England. In the Sidmouth district, the lower part of the Seaford Chalk crops out in the cliffs at Pinhay [SY 322 912] and is preserved in pockets at Beer Head [SY 226 879]. It consists of up to 20 m of soft white chalk with numerous courses of large flints and isolated very large flints, including paramoudras, and contains abundant fragments of the thick-shelled bivalve Platyceramus.

Cainozoic

Deposition of the Chalk was followed by a long period of uplift and erosion, resulting in the thin, isolated remnants of Chalk in the Sidmouth district. Farther east, in the Hampshire Basin, fluviatile and brackish marine Palaeocene and Eocene sediments overlie the Chalk. There is no proven correlative of these deposits in this district, but the local presence of laminated stoneless clays, quartz sands and well-rounded quartzitic pebbles within the Clay-with-flints suggests that a widespread veneer of 'Tertiary' deposits was present.

Where the estimated thickness of the Clay-with-flints exceeds 20 m, around Charton Cross [SY 303 913], Combpyne Hill [SY 301 928], Hartgrove Hill [SY 307 946] and north of Yawl [SY 321 949], up to 10 m of relatively undisturbed 'Tertiary' deposits may be present. At Combpyne Hill, Woodward (1902) recorded white sand and clay with chert and flint and many quartz pebbles, which Woodward and Ussher (1911, p.69) classified as 'Bagshot Series', comparable to the Eocene Gravels of east Dorset. Almost all sections within the Clay-with-flints contain pockets of clay, sand or gravel of probable 'Tertiary' origin, but later dissolution and Pleistocene cryoturbation effects have destroyed evidence of any stratigraphical relationships.

Clay-with-flints

The Clay-with-flints (Plate 5) may be up to 30 m thick and forms a gently undulating plateau that caps all the high ground. In eastern areas it rests locally on the Chalk with marked unconformity. Elsewhere it rests on Upper Greensand, which it progressively oversteps westwards. Spectacular sections showing Clay-with-flints resting on an irregular karstic surface of Chalk and Upper Greensand occur in the cliffs between Salcombe Regis and Beer Head [SY 150 877] to [SY 225 880].

The Clay-with-flints is laterally and vertically variable. The surface layers, mostly 1 to 5 m thick, consist of heterogeneous brown and red clay, sandy clay and sand, with numerous clasts of flint and/or chert ranging from granules to boulders. The great majority of clasts are angular and unworn, and many of the flints have retained their original patina and irregular shape. Flint clasts predominate on the Chalk outcrop, but elsewhere become increasingly less common and more stained and abraded. Chert clasts generally predominate where the Clay-with-flints rests on the Upper Greensand, but even in these areas flint remains a common constituent and testifies to the former presence of a continuous cover of Chalk.

The origin of the Clay-with-flints is complex, involving the interaction of several processes on a variety of sediments and rocks over long periods of time. The lithologies present in this district, together with their topographical distribution, suggest that the formation of clay-with-flints was preceded by uplift of the Cretaceous rocks and the formation of a subaerial fluvial planation surface during earliest 'Tertiary' times. This was followed by the partial redeposition and resorting of the insoluble residues derived from the Chalk and Upper Greensand to produce extensive sheets of flint and chert gravel, fluvial sands, and lateritic red clays, probably in a subtropical or tropical climate. The distribution of the deposits was subsequently modified by further dissolution during the 'Tertiary' and by periglacial activity during the Quaternary.

Mass-movement Deposits

Head and colluvium

Head and colluvium are widely developed in the district, and accumulated through processes of solifluction and slopewash, respectively. The principal types of deposit have been grouped into two categories on the geological map, largely on the basis of their topographical expression.

Colluvium and Valley Head occur in the valley bottoms and on the lower slopes of valleys. Valley Head formed by solifluction in periglacial conditions during the Devensian. Colluvium formed later, in part by slow downslope mass-movement such as soil creep but predominantly by water-assisted movement such as sheetwash. The formation of colluvium increased significantly during the Bronze Age, following deforestation. The deposits comprise up to 10 m of poorly stratified clay, sand and stones in heterogeneous mixtures. Clasts include chert and flint, some forming coarse-grained chert-rich gravels and deposits of angular chert clasts with variable but mostly low sand content. Sedimentary structures in sandy deposits include small washouts and channels.

Head deposits are widespread and commonly consist of unbedded, reddish brown or grey gravelly clay or clayey gravel, with clasts mainly of angular to nodular flint and some chert. Some chert clasts may be 0.4 m or more in diameter. The deposits are generally poorly sorted, but become progressively better sorted with down-slope movement. Thicknesses of at least 10 m recorded just south-east of Sidford are exceptional. Head has not been widely mapped on the outcrop of the Otter Sandstone, but its lithology may be similar to that of the weathered bedrock. Near Wiggaton, Straw and Hodgson (1988) identified at least 8.5 m of slope deposits derived largely from the Otter Sandstone. A few small patches of Head deposits at higher elevations represent the dissected remnants of former land surfaces. The most extensive example, near Stockland [ST 245 035], caps a ridge on the watershed between the River Yarty and the Corry Brook.

Landslip

Rotational landslips and translational mudflows and slides are common where the Upper Greensand Formation overlies argillaceous strata. Smaller rotational slips also occur in some of the more argillaceous strata where they crop out on steep slopes, including the Mercia Mudstone Group and Westbury Formation. All these types of mass movement are depicted as landslip on the published map. Landslips in the district cover areas that range from a few tens of square metres to several square kilometres.

Landslip mechanisms vary. In the larger landslips, initial movements in the argillaceous beds (Gault and Charmouth Mudstone formations) cause liquefaction of the lower part of the Foxmould Member, which initiates a complex sequence of rotational movement, flow and collapse of the overlying strata. In the coastal landslips, notably those at The Hooken [SY 220 880] (Plate 1), Bindon [SY 275 895] and Pinhay [SY 312 906], large blocks of competent Whitecliff Chert and Chalk have moved vertically and laterally without disruption of their internal bedding and sedimentary features. The susceptibility of each formation to landslip is summarised in (Figure 5).

In the eastern part of the district, the presence of the weak, impermeable Gault beneath the soft, water-bearing sands of the Foxmould Member seems to have been the controlling factor that initiated large areas of movement. The Undercliff landslip [SY 272 895] to [SY 335 910], running continuously for more than 8 km from the River Axe to Lyme Regis, contains active examples of all the principal types of mass movement, including rotational slides, mudflows, block (translational) slides, rock falls, debris flows, and liquefied sand flows. Almost all the individual landslips in the complex involve shear failure of the Gault and liquefaction of the Foxmould sands. Those between Culverhole Point [SY 270 895] and Lyme Regis, where the slipped area varies between 300 and 500 m in width, also involve the Lias Group and the Westbury Formation. West of Culverhole Point, where the landslipped area is 50 to 200 m wide, only Cretaceous rocks are involved. Detailed descriptions of individual landslips within the complex include those by Grainger et al. (1985) for Pinhay, and Pitts and Brunsden (1987) for Bindon.

The Black Ven landslip [SY 345 928] to [SY 360 930] encroaches onto the eastern margin of Lyme Regis (Gallois, 2001b). Arber (1973) and Brunsden (1969; 2002) have described individual movements, and Conway (1974) described the landslip mechanism. Shear failure in the Gault in the high part of the cliff causes collapse of the Foxmould, decalcified Whitecliff Chert and Clay-with-flints. The resulting sludge pours over the successive cliffs of Belemnite Marl, Black Ven Marl and Shales-with-Beef, in part mobilising the Lias Group mudstone as it does so. Tongues of mudflow pour over the lower cliffs onto the beach, and the process is temporarily arrested until they are removed by marine erosion. The landslip has been very active in the past 200 years.

The more competent rocks in the district, notably the Otter Sandstone, Mercia Mudstone, Upper Greensand and Chalk, give rise to rock collapses, including falls, topples and rock avalanches, where they crop out on the coastline.

Fluvial Deposits

River Terrace Deposits

Locally, terraces of river gravel border the River Otter and its tributaries at ten main levels, representing stages in the denudation history of the district. They represent deposits laid down on former floodplains, and range in height from about 10 to 100 m above the present-day floodplain. The deposits consist predominantly of gravel, locally overlain by sandy clay and sandy clay with gravel. The main types of clasts are angular chert and flint, derived from the Upper Greensand and Chalk, respectively, and rounded pebbles and cobbles, mainly of quartzite, derived from the Budleigh Salterton Pebble Beds.

River Terrace Deposits occur beneath the floor of the Axe valley and at three or more levels on the lower slopes, up to heights of 30 m above the present-day valley floor. The deposits are up to at least 15 m thick, and consist of coarse-grained gravels and boulder beds with clasts of dominantly Upper Greensand chert, with some flint, sandstone and Jurassic limestone (Plate 6). The angular nature of the clasts, the great thickness of the deposits, and the slope of the upper surface suggest deposition largely from meltwater sheet-floods during Pleistocene cold periods. Gravel pits in the deposits at Broom and Chard Junction have yielded more Palaeolithic handaxes and other implements than any other site in south-west England.

Alluvial Fan Deposits

Small areas of Alluvial Fan Deposits, mostly stony sand, occur in the Yarty and adjacent valleys, where small tributary streams meet the alluvial floodplain of the main river.

Alluvium

Alluvium is present along the valleys of the rivers Otter, Sid, Axe and their larger tributaries. The Axe, Otter and possibly the Sid are misfit streams, flowing within broad valleys that may have been formed by Pleistocene meltwaters. As a result, the Alluvium in each valley consists of a veneer of silt, silty clay and fine-grained sand, up to 2 m thick that rests on River Terrace Deposits. The floodplain of the River Otter is up to 500 m wide, and is underlain by up to 1.3 m of variable silty clay and silty sand, resting on chert and flint gravel that is normally less than 2 m thick. Local beds of soft, very dark brown, sandy peat are present within the Alluvium. The Alluvium of the Axe can be divided into two areas: the meander belt of the present-day river, in which sedimentation occurs during times of flood, and a slightly higher, outer zone in which sedimentation has now effectively ceased.

Marine and Coastal Zone Deposits

Sea bed sediments

Sea bed sediments are thin and discontinuous across much of Lyme Bay (Darton et al., 1981; Hamblin et al., 1992) and in the vicinity of Lyme Regis (Gallois and Davis, 2001). A thin sandy gravel lag deposit occurs across much of the western half of the district, between outcrops of bedrock. The deposits are concentrated in a belt about 4 to 8 km offshore that approaches the shore just west of Sidmouth and extends across the nearshore zone to Seaton. Separate accumulations of sand have been identified at five main offshore locations. Mud-rich sediments have been recorded nearshore at Lyme Regis.

Kellaway et al. (1975) reported submerged cliff-lines in Lyme Bay, but bathymetric data collected since then show that the sea bed has a relatively smooth, gentle gradient, although shallow steps and benches do occur below the 20 m isobath on sea bed surface profiles. No single definitive cliff-line can be recognised, nor is the sea-bed sediment cover sufficiently thick to hide such a topographical feature.

Salt Marsh Deposits

The estuaries of the rivers Otter and Axe between High Water Mark Median and Spring tides are underlain by salt marsh deposits, consisting of interlaminated marine mud and silt. Similar deposits, enclosed by sea-banks and reclaimed, underlie large areas of the seaward parts of both valleys.

Tidal River or Creek Deposits

Deposits in the active part of estuaries (below Mean High Water Mark) in the Otter and Axe valleys are classified as Tidal River or Creek Deposits, and comprise fine-grained sand and silt overlain by laminated mud.

Beach Deposits

Marine deposits of the modern shoreline consist mainly of shingle and sand. The thickness and position of the shingle varies, depending on local tide and current conditions, and the rate of cliff erosion can be related to the presence or absence of a protective belt of shingle at the cliff base. Most of the beaches in the district are shingle with local areas of sand.

An almost continuous strip of Storm Beach Gravel, locally exceeding 6 m in thickness, fringes the coastline, and the predominant direction of longshore drift is from west to east. Shingle ridges have developed across the mouths of the rivers Otter, Sid, and Axe. In front of Sidmouth town, the beach was augmented by gravel imported from the Budleigh Salterton Pebble Beds Formation during construction of the sea defences.

'Submerged Forest'

'Submerged Forest' deposits, representing vegetation drowned by the postglacial rise in sea level, are common around the coasts of south-west England. Traces of vegetation, including the in situ stumps of trees, have been recorded on the foreshore at low tide at Sidmouth and at the mouth of the River Char.

Chemical Deposits

Calcareous Tufa

Calcareous tufa, calcium carbonate precipitated on and encrusting plants or algae, forms a series of small waterfalls in a stream [ST 288 022] east of Membury. The carbonate is derived from the Langport Member of the Penarth Group. Between Sidmouth and Branscombe, the highest part of the Mercia Mudstone is thickly coated with tufa deposited by seepages from the base of the overlying calcareous Upper Greensand Formation.

Artificial Deposits and Worked Ground

Made ground and infilled ground are deposits of an unpredictable nature, extent and thickness which are likely to be present in urban areas. Small areas of Made Ground are also present in the rural areas of the district, mainly as road and railway embankments and as landscaping fill. The main areas of Infilled Ground in the district are infilled mineral workings and some sections of disused railway cuttings. No details of the infill material are available.

Worked ground is shown on the map where human activity has removed natural materials. It consists mainly of disused mineral workings (pits and quarries) and road and railway cuttings. Some Worked Ground is associated with landscaped (cut-and-fill) areas.

Structure

The Late Carboniferous Variscan Orogeny gave rise to a complexly folded and faulted succession that underlies much of the southern counties of England, including this district. The orogeny was followed by a period of lithospheric extension, producing fault-bounded basins in which the Permian Exeter Group was deposited. Later regional subsidence allowed the succeeding Permo-Triassic to Jurassic sedimentary sequences to overlap the faulted margins of these basins.

This was followed by uplift, accompanied by normal faulting and an eastward tilt of about 3°. The resulting unconformity beneath the Upper Greensand is an example of a post-rift unconformity, associated with the transition from extensional to post-extensional phases of basin evolution (Chadwick, in Hamblin et al., 1992). The field and seismic evidence indicates the dominant fault direction to be north–south, with only minor faulting in east–west and other directions (Figure 6). This is at variance with the dominantly east–west (Variscan) trend of faulting in the Wessex Basin to the east and the Crediton Trough to the west. The north–south-trending faults form part of a structural high that marks the western margin of the Wessex depositional basin. To the north, it joins the Mid-Dorset and Quantocks structural highs. It is presumed to overlie a major structure in the pre-Variscan basement that was reactivated in post-Variscan times. It is likely that at least some of the faults were active during the Triassic, since certain formations, for instance the Otter Sandstone, appear on seismic profiles to show thickness variations across faults. Many of the faults were intermittently active throughout the Jurassic and Cretaceous.

By the end of the Early Cretaceous, crustal extension in the North Atlantic had given way to active sea-floor spreading, leading to regional thermal subsidence, and to a series of transgressions as the Gault, Upper Greensand and Chalk successively overstepped one another westwards. This was followed in the Cainozoic by an episode of crustal compression and basin inversion, which led to uplift, a further eastward tilt of about 2°, and the reactivation of many of the normal faults in the district. (Figure 6) shows an interpretation of faulting cutting the Budleigh Salterton Pebble Beds Formation, based on seismic lines. Some of these faults did not penetrate the Mercia Mudstone Group because its ductility and low competence resulted in plastic rather than brittle deformation. More of these faults may have come to the surface than are shown on the 1:50 000 scale geological map, since it is difficult to plot their positions where they crop out beneath drift deposits and the throw at surface is wholly within the relatively uniform lithologies of the Mercia Mudstone Group.

Chapter 3 Applied geology

Mineral and energy resources

Construction materials

Building stone

Most of the Otter Sandstone is weakly cemented, but better cemented horizons, particularly intraformational conglomerates, have been widely used for building stone, especially around Sidmouth itself.

The large quarry in the Blue Lias Formation and Penarth Group at Tolcis [ST 280 010], now infilled, was worked for both building stone and lime, and Blue Lias limestones were worked on the foreshore immediately west and east of Lyme Regis from Medieval to Victorian times. Some of the older buildings in the town and the older parts of the sea wall are made of dressed blocks of these limestones. The original harbour [SY 339 917] dates back to the 13th century and was reputedly constructed of two rows of wooden piles with an infill of 'cowstones' (calcareous concretions) from the Foxmould Member of the Upper Greensand.

Chert-rich horizons in the Upper Greensand have been worked at only a few localities, but irregular and dressed blocks of chert are the most commonly used building material in the district. Much of this has been collected as loose blocks in ploughed fields, or was won from shallow pits (commonly referred to on maps as 'gravel pits') from the decalcified highest part of the Whitecliff Chert and Bindon Sandstone (Gallois, 2004b).

Quarries [SY 160 887] in the Bindon Sandstone at Salcombe Regis, where it is chert-free, provided the exterior stone for Exeter Cathedral. Much of the interior stone for this and other cathedrals and local churches came from the Beer Stone (Plate 7), a local development of echinoderm-debris calcarenite within the Holywell Nodular Chalk Formation. The stone is of such quality as a freestone that it has been worked continuously from Roman times until the present day in a mine [SY 215 894] and adjacent quarry [SY 215 898], and has been exported to other parts of England and to the USA.

Stones in the superficial deposits and soils, including Upper Greensand chert, Chalk flint, and pebbles derived from the Budleigh Salterton Pebble Beds, have been used in the construction of walls and buildings (Plate 7). Good examples of flint walls can be seen in Sidmouth.

Cement and lime

The Blue Lias was worked for cement manufacture in the cliffs and foreshore at Lyme Regis from early Victorian times until the First World War, to the detriment of the natural sea defences. The same beds were worked in large pits [ST 281 008] near Membury and at Coaxdon Quarry, Weycroft [ST 311 006], until the 1950s and 1940s, respectively. Limestone was worked for cement, lime mortar, plaster and agricultural purposes from the Triassic Langport Member at Uplyme [SY 322 938] and at Weybridge. The Chalk was extensively worked for similar purposes at Dunscombe [SY 160 882], Branscombe [SY 195 882], Beer [SY 215 897] and Uplyme [SY 313 918], and in small pits at numerous inland localities.

Brick and tile clay

The weathered clays of the Aylesbeare Mudstone, Mercia Mudstone and Lias groups have been used as brick and tile clays at several localities (Woodward and Ussher, 1911). Small amounts of Lias Group clay were worked for brick-making at Lyme Regis in Victorian times.

Building sand

Loose sand, decalcified from the Foxmould Member of the Upper Greensand, has been worked for building sand in numerous small pits throughout the district, notably at Lyme Regis. The Wilmington Sand Member, similarly decalcified (Plate 5), was worked for the same purpose at Wilmington Sandpit [SY 210 998] from the beginning of the 19th century until recently.

Sand and gravel

The Sherwood Sandstone Group contains large resources of sand and gravel. The Budleigh Salterton Pebble Beds and the lower part of the Otter Sandstone have been exploited in recent years at Foxenholes Quarry [SY 077 948] near Ottery St Mary. The River Terrace Deposits bordering the valley of the River Otter contain resources of gravel and sand, but most of the deposits are small, scattered and thin. River Terrace Deposits in the valley of the River Axe are currently being exploited in large pits at Chard Junction [ST 340 046], and were previously worked at Broom [ST 328 024] and Kilmington [SY 275 975].

Cherts from the Whitecliff Chert and Bindon Sandstone (Upper Greensand) have been used extensively for the construction of roads and tracks in the district. They continue to be worked for roadstone and concreting aggregates at Uplyme [SY 313 918].

Other materials

Marl and agricultural lime

Outcrops of the Aylesbeare Mudstone and Mercia Mudstone groups throughout the district are dotted with numerous small shallow pits, from which calcareous mudstone (marl) was worked for agricultural purposes. The marl was used to top-dress the leaner soils of the area. The small size of the pits suggests that they were used in the immediate vicinity.

Pyrites

A pyrite-rich band, the 'Metal Bed' in the Black Ven Marl at Black Ven, was exploited for use in the manufacture of sulphuric acid (Woodward and Ussher, 1911).

Gypsum

The Red Rock Gypsum Member of the Branscombe Mudstone Formation was mined at Littlecombe Shoot [SY 185 880], near Branscombe, in the 19th century for agricultural purposes. The remains of the grinding mill where it was processed are still visible at Branscombe Mouth.

Sharpening stones

Siliceous sandstones for sharpening stones, known as whetstones, scythe stones or 'Devonshire Batts', were formerly dug from horizontal adits extending into the side of the Upper Greensand escarpments (Stanes and Edwards, 1993), with evidence of old workings around the Hembury Fort spur of Upper Greensand.

Flint

The purer flints in the Lewes Nodular Chalk at Beer Head and in collapsed masses in the adjacent Hooken Landslip were intermittently worked from Neolithic to Victorian times (Skertchley, 1879), initially for axes and arrowheads, and subsequently for gun-flints.

Hydrocarbons

The Musbury No. 1 Borehole was drilled in 1986 to a depth of 1360 m, terminating in breccia of the Exeter Group (Permian). No hydrocarbon show was recorded.

Water resources

The Sherwood Sandstone Group forms a major aquifer, supplying much of the western part of the district from boreholes in the Otter valley, between Ottery St Mary and Otterton.

The Triassic and Jurassic rocks of the eastern part of the district, with the possible exception of the Triassic Langport Member, are almost entirely fractured aquicludes or aquitards that are unsuitable for water supply. In contrast, the Upper Greensand and Chalk are good aquifers. However, their position on high ground, mostly above the water table, has meant that they have only supplied water locally and on a small scale.

Springs issue from the bases of the Foxmould and the Whitecliff Chert at numerous localities in the district. Some of these provided the water supply for the major settlements in Victorian times, notably springs at Lyme Regis, Uplyme and Axmouth. Springs also emerge from Cretaceous masses at beach level on the west side [SY 293 896] of Charton Bay, and the Lyme Regis area receives much of its present supply from springs in the Undercliff Landslip at Pinhay, which emerge from the collapsed masses of Cretaceous rocks. Elsewhere individual large houses, for example at Ware and Charton Cross, obtained their supplies from wells sunk in the Upper Greensand and/or Chalk. A borehole at Uplyme, its exact site uncertain, gave a low yield, probably from the Langport Member (Woodward and Ussher, 1911).

Perched water tables are likely to occur in River Terrace Deposits where these rest upon impermeable solid strata, but none is known to contribute to the water supply. In the east of the district, water is imported to the Axe valley villages from Wimbleball Reservoir in north Devon.

Geotechnical considerations

Foundation conditions

The behaviour of geological materials in the Sidmouth district with regard to foundations, excavations, use as fill and stability in slopes is summarised in (Figure 7) for solid units and (Figure 8) for drift (superficial) deposits. More detail is given in Forster (1998).

Slope stability and coastal erosion

Much of the succession in the district is predisposed to slope instability, although inland slopes are generally mature and instability due to natural processes is rare. Large areas of the valley sides at Lyme Regis, Uplyme and Charmouth are in slipped ground, and smaller areas of landslip occur along the tributary valleys of the River Axe in the eastern part of the district. All these landslips probably formed during the late Pleistocene, and are stable except where they have been disturbed by more recent erosion or human activity.

In contrast, a range of active landslips and slope movements occur in the coastal zone (see Forster, 1998, for a more detailed analysis). The Undercliff and Black Ven landslips are active in response to continuous removal of their toe materials by marine erosion. The rate of landslip movement is directly related to changes in the rate or nature of erosion. An increase in the rate of erosion from the foreshore at Black Ven, and the resulting increase in the rate of inland migration of the back face, seems to be related to the loss of protective shingle from the foreshore. It is not known to what extent the loss of shingle is due to the obstruction of the Cobb wall (see below) or to a greater frequency of south-easterly and southerly gales, which tend to remove shingle from the foreshore. One result is that the landslip is encroaching onto the eastern edge of Lyme Regis and remobilising the older landslips there (Gallois, 2001b). Historical records suggest that the rate of retreat of the back face was greater during the 20th century than at any previous time.

Possible changes in the global climate pattern and consequent changes in patterns of erosion and sedimentation are a serious potential problem at Lyme Regis. There, the linking of the Cobb wall to the mainland in 1756 has contributed to the buildup at Monmouth Beach of a large mass of shingle that would originally have formed a protective apron for the whole of the Lyme Regis sea front (Gallois, 2001b). The loss of shingle from the Town Beach may have contributed to reactivation of the landslips in the Shales-with-Beef at Marine Parade.

Flood risk

The risk of flooding is high on low-lying ground in river valleys, particularly where underlain by Alluvium or River Terrace Deposits not far above river level. Protection measures include the construction of bunds, maintenance of drainage ditches, and the building up of land to be used for building construction and transport infrastructure.

Natural radon emissions

Approximately 60 per cent of the Sidmouth district, including parts of Axminster, Honiton, Lyme Regis, Sidmouth and Seaton are classified as Radon Affected Areas (Lomas et al., 1996). The Government recommends that houses in affected areas should be tested for radon.

A 1:250 000 scale study of geological radon potential indicates that the main bedrock units in the district prone to relatively high radon levels are the Penarth Group, Upper Greensand and Chalk. New properties built on these units may require radon protection measures. A few dwellings built on the Blue Lias and Charmouth Mudstone formations may have radon above the 'Action Level' of 200 becquerels per cubic metre; the few high concentrations recorded are believed to be associated with limestone beds within the mudstone.

Information sources

Sources of further geological information for the Sidmouth district are listed below. Enquiries concerning geological data or seeking geological advice should be addressed to the British Geological Survey at Keyworth, Exeter or Edinburgh (offshore data). Other information includes borehole records, fossils, rock samples, thin sections, geochemical samples, aeromagnetic and gravity data and hydrogeological data. Searches of indexes to some of the BGS data collections can be made on the Geoscience Data Index, available in BGS libraries and on the World Wide Web; this also allows access to the BGS Lexicon of named rock units and part of the photographic collection. The BGS catalogue of maps, books and other products is available on request.

Maps

• Geological maps
• 1:1 000 000
• Pre-Permian geology of the United Kingdom, 1985
• 1:625 000
• Solid geology map UK South Sheet, 2001; Quaternary geology, 1977
• 1:250 000
• 50N 04W Portland: Solid geology, 1983; Sea bed sediments, 1983
• 1:50 000
• Sheet 310 Tiverton, Solid and Drift, 1974
• Sheet 311 Wellington, Solid and Drift, 1976
• Sheet 312 Yeovil, Solid and Drift, 1973
• Sheet 325 Exeter, Solid and Drift, 1995
• Sheet 326/340 Sidmouth, Solid and Drift, 2002
• Sheet 327 Bridport, Solid with Drift, 1977; Solid and Drift, 1974
• Sheet 339 Newton Abbot, Solid and Drift, 1976
• 1:10 000
• The 1:10 000 scale sheets were surveyed between 1987 and 2000 by R A Edwards, R W Gallois, R J O Hamblin, R A Ellison, A J Newell and A C Pople.
• Copies of the fair-drawn maps are available for inspection at BGS offices. Uncoloured dyeline copies are available for purchase from the BGS Sales Desk at the Keyworth office.
• Digital geological map data
• In addition to the printed publications noted above, many BGS maps are available in digital form, which allows the geological information to be used in GIS applications. These data must be licensed for use. Details are available from the Intellectual Property Rights Manager at BGS Keyworth. The current availability of these can be checked on the BGS web site at:
• http://www.bgs.ac.uk/products/ digitalmaps/digmapgb.html
• Geophysical maps
• 1:1 500 000
• Colour shaded gravity anomaly map of Britain, Ireland and adjacent areas, 1997
• Colour shaded magnetic anomaly map of Britain, Ireland and adjacent areas, 1998
• 1:250 000
• 50N 04W Portland: Aeromagnetic Anomaly, 1978; Bouguer Gravity Anomaly, 1978
• Geochemical 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 national average background concentrations of potentially harmful elements (As, Cd, Cu, Pb and Zn), Great Britain (South Sheet), 1995
• Hydrogeological maps
• 1:625 000
• England and Wales, 1977
• 1:100 000
• Permo-Trias and other aquifers of SW England, 1982
• Minerals maps
• 1:1 000 000
• Industrial minerals resources map of Britain, 1996

Books

• British regional geology
• The Hampshire Basin and adjoining areas. Fourth edition. 1982
• South-west England. Fourth edition. 1975
• Offshore Regional Report
• The geology of the English Channel, 1992
• Memoirs
• Sheets 312 and 327 Bridport and Yeovil, 1958
• Sheet 325 Exeter, 1999
• Sheets 326 and 340 Sidmouth and Lyme Regis. Second edition. 1911*
• Sheet 339 Newton Abbot, 1984
• * out of print but photocopies may be purchased from the BGS Library, Keyworth
• Technical Reports
• Technical reports are available for the following 1:10 000 sheets:
• ST 00 SE, Plymtree and Payhembury: WA/89/2. R A Edwards.
• SY 08 NE, Newton Poppleford: WA/96/49. R A Edwards.
• SY 08 SE, Budleigh Salterton: WA/97/50. R A Edwards.
• SY 09 NE, Talaton: WA/89/1. R A Edwards.
• SY 09 SE, West Hill: WA/90/17. R A Edwards.
• SY 19 NW, Awliscombe, Ottery St. Mary and Gittisham: WA/99/60. R A Edwards.
• These reports contain more detail than appears in this Sheet Explanation, but may contain interpretations an stratigraphical terminology which have been superseded. Copies may be obtained from BGS offices at Keyworth and Exeter.
• Holiday Geology Guides
• Rocks and fossils around Lyme Regis, 1987
• Scenery and geology around Beer and Seaton, 1987
• These are printed on A3 card, in colour, laminated and folded twice, and are intended as popular guides to the area.
• Engineering Geology Reports
• Coastal terrain evaluation of the Charmouth-Lyme Regis area: EG 76/10. B W Conway.
• A regional study of coastal landslips in west Dorset: EG 77/10. B W Conway.
• Eastcliff landslip, Lyme Regis, Dorset. Summary 1974-78: EG 79/10. B W Conway.
• The engineering geology of the Exeter district: WN/91/16. A Forster.
• The engineering geology of the Sidmouth district: WN/98/1. A Forster.
• Biostratigraphical Reports
• Biostratigraphical reports are held as restricted reports under prefixes WH and PDL. Readers are recommended to contact the BGS Keyworth office for access to these reports and to the palaeontological collections.

Documentary collections

Boreholes

Borehole data for the district are catalogued in the BGS archives (National Geological Records Centre) at Keyworth on individual 1:10 000 scale sheets.

BGS photographs

Copies of these photographs are deposited in the BGS Library, Keyworth.

(A6387) – (AA6448) 1934

(A11416) – (A11431) 1971

(A12036) 1972

References

Most of the references listed below are held in the Library of the British Geological Survey at Keyworth, Nottingham. Copies of the references may be purchased from the Library subject to the current copyright legislation. BGS library may be accessed on line at: http://geolib.bgs.ac.uk

Arber, M A. 1973. Landslips near Lyme Regis. Proceedings of the Geologists' Association, Vol. 84, 121–133.

Benton, M J, and Spencer, P S. 1995. Fossil reptiles of Great Britain. Geological Conservation Review Series, No. 10. (London: Chapman and Hall.)

Brunsden, D. 1969. Moving cliffs of Black Ven. Geographical Magazine, Vol. 41, 372–374.

BS5930. 1999. Code of Practice for site investigations. (London: British Standards Institute.)

Brunsden, D. 2002. Geomorphological roulette for engineers and planners: some insights into an old game. Quarterly Journal of Engineering Geology and Hydrogeology, Vol. 35, 101–142.

Conway, B W. 1974. The Black Ven landslip, Charmouth, Dorset. Report of the Institute of Geological Sciences, No. 74/3.

Darton, D M, Dingwall, R G, and McCann, D M. 1981. Geological and geophysical investigations in Lyme Bay. Report of the Institute of Geological Sciences, No. 79/10.

Edwards, R A, and Scrivener, R C. 1999. Geology of the country around Exeter. Memoir of the British Geological Survey, Sheet 325 (England and Wales).

Edwards, R A, Warrington, G, Scrivener, R C, Jones, N S, Haslam, H W, and Ault, L. 1997. The Exeter Group, south Devon, England: a contribution to the early post-Variscan stratigraphy of northwest Europe. Geological Magazine, Vol. 134, 177–197.

Forster, A. 1998. The engineering geology of the Sidmouth district, 1:50 000 geological sheet 326/340. British Geological Survey Technical Report, WN/98/1.

Gallois, R W. 2001a. The lithostratigraphy of the Mercia Mudstone Group (mid to late Triassic) of the South Devon coast. Geoscience in south-west England, Vol. 10, 195–204.

Gallois, R W. 2001b. Field excursion to examine the geology and landforms of the Charmouth to Lyme Regis area, 3rd January 2001. Geoscience in south-west England, Vol. 10, 243–246.

Gallois, R W. 2003. The distribution of halite (rock salt) in the Mercia Mudstone Group (mid to late Triassic) in south-west England. Geoscience in south-west England, Vol. 10, 243–246.

Gallois, R W. 2004a. The lithostratigraphy of the Upper Greensand (Albian, Cretaceous) of south-west England. Geoscience in south-west England, Vol. 11.

Gallois, R W. 2004b. Large-scale dissolution features in the Upper Greensand (Cretaceous) in south-west England. Geoscience in south-west England, Vol. 11.

Gallois, R W, and Davies, G M. 2001. Saving Lyme Regis from the sea: recent geological investigations at Lyme Regis, Dorset. Geoscience in south-west England, Vol. 10, 183–189.

Grainger, P, Tubb, C D N, and Neilson, A P M. 1985. Landslip activity at the Pinhay water source, Lyme Regis. Proceedings of the Ussher Society, Vol. 6, 246–252.

Hamblin, R J O, Crosby, A, Balson, P S, Jones, S M, Chadwick, R A, Penn, I E, and Arthur, M J. 1992. United Kingdom offshore regional report: the geology of the English Channel. (London: HMSO for the British Geological Survey.)

Hesselbo, S P, and Jenkyns, H C A. 1995. A comparison of the Hettangian to Bajocian successions of Dorset and Yorkshire. 105–150 in Field geology of the British Jurassic. Taylor, P D (editor). (London: Geological Society.) ISBN 1-897799-41-1

Jarvis, I, and Woodroof, P B. 1984. Stratigraphy of the Cenomanian and basal Turonian (Upper Cretaceous) between Branscombe and Seaton, SE Devon, England. Proceedings of the Geologists' Association, Vol. 95, 193–215.

Jukes-Browne, A J, and Hill, W. 1900. The Cretaceous rocks of Britain. Vol. 1. The Gault and Upper Greensand of England. Memoir of the Geological Survey of the United Kingdom, (London: HMSO.)

Kellaway, G A, Redding, J H, Shephard-Thorn, E R, and Destombes, J-P. 1975. The Quaternary history of the English Channel. Philosophical Transactions of the Royal Society of London, Vol. 279A, 189–218.

Kennedy, W J. 1970. A correlation of the uppermost Albian and the Cenomanian of South-West England. Proceedings of the Geologists' Association, Vol. 81, 613–677.

Lang, W D. 1914. The geology of the Charmouth cliffs, beach and fore-shore. Proceedings of the Geologists' Association, Vol. 25, 293–360.

Lang, W D. 1924. The Blue Lias of the Devon and Dorset coasts. Proceedings of the Geologists' Association, Vol. 35, 169–185.

Lang, W D. 1932. The Lower Lias of Charmouth and the Vale of Marshwood. Proceedings of the Geologists' Association, Vol. 43, 97–126.

Lang, W D. 1936. The Green Ammonite Beds of the Dorset Lias. Quarterly Journal of the Geological Society of London, Vol. 92, 423–437 and 485–487.

Lang, W D, and Spath, L F. 1926. The Black Marl of Black Ven and Stonebarrow in the Lias of the Dorset coast. Quarterly Journal of the Geological Society of London, Vol. 82, 144–187.

Lomas, P R, Green, B M R, Miles, J C H, and Kendall, G M. 1996. Radon atlas of England. National Radiological Protection Board, Chilton. NRPB-R290.

Manual. 1991. Manual of contract documents for highway works. Volume 1, Specification for highway works. (London: HMSO for Department of Transport.)

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

Page, K N. 1992. The sequence of ammonite correlated horizons in the British Sinemurian (Lower Jurassic). Newsletters on Stratigraphy, Vol. 27, 129–156.

Page, K N. 2003. A review of the ammonite faunas and standard zonation of the Hettangian and Lower Sinemurian succession (Lower Jurassic) of the east Devon coast (south west England). Geoscience in south-west England, Vol. 10, 293–303.

Pettifer, G S, and Fookes, P G. 1994. A revision of the graphical method for assessing the excavatability of rock. Quarterly Journal of Enginering Geology, Vol. 27, 145–164.

Pitts, J, and Brunsden, D. 1987. A reconsideration of the Bindon Landslide of 1839. Procedings of the Geologists' Association, Vol. 98, 1–18.

Purvis, K, and Wright, V P. 1991. Calcretes related to phreatophytic vegetation from the Middle Triassic Otter Sandstone of south west England. Sedimentology, Vol. 38, 539–551.

Skertchley, B J. 1879. On the manufacture of gun-flints, the methods of excavating for flint, the age of Palaeolithic man, and the connexion between Neolithic art and the gun-flint trade. Memoir of the Geological Survey of Great Britain. (London: HMSO.)

Smith, S A, and Edwards, R A. 1991. Regional sedimentological variations in Lower Triassic fluvial conglomerates (Budleigh Salterton Pebble Beds), southwest England: some implications for basin geometry and basin evolution. Geological Journal, Vol. 26, 65–83.

Spencer, P S, and Storrs, G W. 2002. A re-evaluation of small tetrapods from the Middle Triassic Otter Sandstone Formation of Devon, England. Palaeontology, Vol. 45, 447–467.

Stanes, R G F, and Edwards, R A. 1993. Devonshire Batts: the whetstone mining industry and community of Blackborough, in the Blackdown Hills. Report and Transactions of the Devonshire Association for the Advancement of Science, Literature and Art, Vol. 125, 71–112.

Straw, A, and Hodgson, R L P. 1988. Periglacial slope deposits at Wiggaton in the Otter Valley, East Devon. Proceedings of the Ussher Society, Vol. 7, 104–105.

Woods, M A. 2002. The macrofossil biostratigraphy of the Turonian and Coniacian (Upper Cretaceous, Chalk group) of south-east Devon. Proceedings of the Geologists' Association, Vol. 113, 333–344.

Woodward, H B. 1902. Note on the occurrence of Bagshot Beds at Compe Pyne, near Lyme Regis. Geological Magazine, Vol. 39, 515–516.

Woodward, H B, and Ussher, W A E. 1911. The geology of the country near Sidmouth and Lyme Regis Second edition. Memoir of the Geological Survey, Sheets 326 & 340 (England and Wales). (London: HMSO.)

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

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

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

(Index map)

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.

(Index map)

Figures and plates

Figures

(Figure 1) Generalised vertical section for the Mercia Mudstone Group succession of the coastal outcrop (after Gallois, 2001a).

(Figure 2) Generalised vertical section for the Lias Group of the coastal outcrop.

(Figure 3) Generalised vertical section for the Upper Greensand of the coastal outcrop (after Gallois, 2004a).

(Figure 4) Distribution of outcrops of Chalk in the Sidmouth and adjacent Wellington districts.

(Figure 5) Landslip susceptibility of weathered formations.

(Figure 6) Contour map of the top of the Budleigh Salterton Pebble Beds Formation.

(Figure 7) Engineering characteristics of the solid rocks.

(Figure 8) Engineering characteristics of the drift (superficial) deposits.

Plates

(Plate 1) Hooken landslip seen from the east (MN39680).

(Plate 2) Sidmouth Mudstone Formation (Mercia Mudstone Group) below Salcombe Hill Cliff, Sidmouth [SY 143 876]. Height of section about 70 m (MN39685).

(Plate 3) Blue Lias Formation at Church Cliffs, Lyme Regis (MN39682).

(Plate 4) Upper Greensand Formation overlain by Chalk Group, view northwards from Beer to White Cliff (MN39681).

(Plate 5) Clay-with-flints in solution hollows on decalcified Wilmington Sand, at White Hart Pit, Wilmington (MN39679).

(Plate 6) River Terrace Gravels at Kilmington Gravel Pit (MN39684).

(Plate 7) Beer Stone and Chalk flints in use as building stones, Fore Street, Beer (MN39683).

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

(Front cover) Otter Sandstone stacks at Ladram Bay, viewed from the west (Photographer R W Gallois; (MN39678).

(Rear cover)

(Geological succession) Summary of the geological succession at outcrop in the Sidmouth district.

Figures

(Figure 5) Landslip susceptibility of weathered formations

(Figure 7) Engineering characteristics of the solid rocks

(Figure 8) Engineering characteristics of the drift (superficial) deposits