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Geology of the Wellington district —a brief explanation of the geological map. Sheet explanation 1:50 000 Sheet 311 Wellington
R C Scrivener, S J Booth, C E Burt, R A Ellison, R J O Hamblin, L M Hollick and K R Royse
Bibliographic reference: Scrivener, R C, Booth, S J, Burt, C E, Ellison, R A, Hamblin, R J O, Hollick, L M, and Royse, K R. 2014. Geology of the Wellington district — a brief explanation of the geological map. Sheet Explanation of the British Geological Survey. 1:50 000 Sheet 311 Wellington (England and Wales).
Keyworth, Nottinghamshire, British Geological Survey, 2014. © NERC 2014 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) Wellington Monument and the Upper Greensand escarpment (Photographer: Paul Witney; (P685544)).
(Rear cover)
(Geological succession)
Notes
The area covered by geological Sheet 311 Wellington is referred to as 'the district'. Ordnance Survey National Grid references are given in the form [0862 2333] or [086 233]. Unless otherwise indicated, all such references fall within the 100 km grid square ST. Symbols in round brackets after lithostratigraphical names are the same as those used on the geological map. The serial number given with the plate captions is the registration number in the National Archive of Geological Photographs, held at BGS. Boreholes are identified by their BGS registration number in the form (ST47SW/10), where the prefix indicates the 1:10 000 scale National Grid sheet. Colour is described using Munsell® colour names and notations e.g. yellowish brown (10YR 5.5/6).
Acknowledgements
The authors wish to acknowledge the readily volunteered advice and information provided by Mr Hugh Prudden and Dr Ramues Gallois during the course of the district mapping and the associated 1:50 000 scale map compilation and drafting of this sheet explanation. Contributors include J D Appleton (natural radon emissions), A Forster (engineering geology), M A Lewis (hydrogeology) and G K Lott (building stones). The typescript was edited by A A Jackson, D T Aldiss, and J E Thomas; the figures were drafted by Henry Holbrook and page setting was by Amanda Hill.
The National Grid and other Ordnance Survey data © Crown Copyright and database rights 2014. Ordnance Survey Licence No. 100021290.
Geology of the Wellington district — explanation of sheet 311 1:50 000 Series map (from rear cover)
(Rear cover)
The Wellington district is mostly rural and agricultural. It can be divided into four geomorphological areas related to bedrock geology: in the extreme north-west, rather rugged country is underlain by Devonian to Lower Triassic rocks; in the north, west and central area, gently rolling countryside is underlain by the Triassic Mercia Mudstone Group; in the east, rolling countryside is underlain by the Jurassic Lias Group; in the centre and south, a high dissected plateau (the Blackdown Hills) is underlain by the Cretaceous Upper Greensand Formation and Chalk Group.
Structurally, the district lies at the western margin of the Wessex Basin. The oldest strata are localised outcrops of Upper Devonian to Lower Carboniferous rocks in the extreme north west of the district. Isostatic uplift during the early Permian established an arid continental environment (the Halberton Breccia Formation) and extensive continental playa lake deposits (the Aylesbeare Mudstone Group). Regional subsidence took place during the Triassic and well into the Jurassic giving rise to nonmarine deposition of the Budleigh Salterton Pebble Beds and the Otter Sandstone Formation, and the Upper Triassic Mercia Mudstone Group. 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. The succeeding Lower Cretaceous Upper Greensand Formation and the Upper Cretaceous Chalk Group indicate further marine inundations. Only the lower parts of the Chalk are preserved, much having been removed by erosion and dissolution following regional uplift and tilting in the latest Cretaceous. Continental sediments were probably deposited across the whole district during the Palaeogene but have since been removed by erosion. Continued erosion and weathering accompanied by solution of the underlying Chalk caused remaining Palaeogene sediments to become mixed with flint and chert to form the clay-with-flints which caps much of the Blackdown Hills.
No glaciers reached the district during the Pleistocene, but successive cold periglacial periods resulted in the formation of head, river terrace deposits and landslides. The latter still have an impact on planning and land use. Some units are aquifers, notably the Sherwood Sandstone Group and others have provided natural resources in the past, principally construction materials.
Chapter 1 Introduction
This Sheet Explanation provides a summary of the geology of the Wellington district, the area covered by 1:50 000 scale geological Sheet 311 (England and Wales). It also provides directions to further geological information about the district.
The district includes parts of the counties of Somerset and Devon, within the local authority districts of Taunton Deane, South Somerset, Mid Devon and East Devon. The principal centres of population are the towns of Wellington and Chard, the southern outskirts of Taunton in the north and the western part of Ilminster in the east. Much of the district is occupied by the northern and central parts of the Blackdown Hills Area of Outstanding Natural Beauty (AONB), which was designated in 1991.
The topography of the district is dominated by the Blackdown Hills (front cover), a dissected plateau that rises steeply to a maximum elevation of 315 m above Ordnance Datum south of the gently undulating Vale of Taunton Deane. In the north-west the ground rises to around 160 m. North and east of the Blackdown Hills complex, rivers such as the Isle and Tone flow north-westwards and west towards the Bristol Channel. Within the south of the Blackdown Hills, the rivers Culm, Axe, Otter and Yarty drain southwards towards the English Channel (Figure 1).
The geological framework of the district was determined by the pioneering work of Henry de la Beche (1839) for the fledgling Geological Survey. This work was published as Old Series Sheet 21, which was revised at various times during the 19th century. Mapping by W A E Ussher et al., at the scale of one inch to one mile in the 1870s led to publication of the New Series Sheet 311 in 1892: a geological memoir for the district was also published (Ussher, 1906). The first detailed survey of the Wellington district, at the 1:10 000 scale, began in 2000 and was completed in 2003. A listing of maps and other relevant British Geological Survey publications can be found under Information Sources.
This district lies at the western margin of the Wessex Basin, a major zone of linked sedimentary basins extending eastwards across southern England. It is floored, at least in this district, by the folded Upper Palaeozoic strata of the Variscan orogenic belt. Small outcrops of Lower Carboniferous limestone and Upper Devonian to Lower Carboniferous mudrock and sandstone, mapped along the north-west margin of this district, mark the edge of this basin. The Lower Carboniferous limestones, in this and the adjacent Tiverton district to the west, are interpreted as calcareous turbidite deposits, derived from the north. Late in the Carboniferous, probably in Westphalian times, the region was subjected to intense north–south directed compression, the culmination of the Variscan Orogeny, which formed a mountain chain extending eastwards into the northern part of the Bohemian Massif. Rapid early Permian isostatic uplift established a continental environment accompanied by accumulation of substantial red-bed deposits of the 'New Red Sandstone'.
In this district, the earliest Permian deposits (the Halberton Breccia Formation) are poorly sorted, locally derived sedimentary breccias, deposited under arid conditions as alluvial fans and debris flows. Continued erosion of the Variscan orogenic belt led to the development of an extensive peneplain with sheet-flood sands and gravels, and aeolian sands, represented by the Tidcombe Sand Member. By Early Triassic times, extensive continental playa lake deposits of the Aylesbeare Mudstone Group formed over much of the region. The district lay at the western margin of the Wessex Basin, an area of subsidence that persisted throughout the Triassic and well into the Jurassic. Following Middle Triassic movement on north–south trending fault zones, detritus was carried into the district from the Armorican Massif of what is now northern France. The Budleigh Salterton Pebble Beds Formation represents high-energy fluvial channel deposits formed at this time. These were succeeded by the sand-dominated fluvial braidplain sediments of the Otter Sandstone Formation, and then, in a prolonged period of deposition in extensive ephemeral playa lakes, by the muds and silts of the Upper Triassic Mercia Mudstone Group. A brackish sea eventually covered the peneplain towards the end of Triassic times, recorded by the green and grey mudstone and limestones of the Penarth Group. Fully marine conditions persisted throughout most of the Jurassic period, but only Lower Jurassic strata are preserved in the district. Initial deepening of the sea saw the deposition of a succession of alternating limestone and mudstone, the Blue Lias Formation, followed by the mudrock-dominated Charmouth Mudstone Formation. Conditions of shallower water followed, with deposition of the siltstone and fine-grained sandstone of the Dyrham Formation and finally of an ooidal iron-rich limestone, the Beacon Limestone Formation overlain by the sand-rich Bridport Sand Formation.
A period of erosion in the Late Jurassic to Early Cretaceous was followed by some tectonic activity, with eastward tilting, faulting and gentle folding of the Triassic and Jurassic rocks. A marine transgression during the Albian stage of the Early Cretaceous is recorded by the littoral deposits of the Upper Greensand Formation. These overstep progressively older strata westwards and largely conceal the boundary between Jurassic and Triassic strata. Continued sea-level rise and a reduction in the input of terrestrial sediment led to deposition of the Chalk Group in the Late Cretaceous. Only the lower parts of the Chalk are preserved, much having been removed by erosion and dissolution following regional uplift and gentle south-eastward tilting in the latest Cretaceous at about 65 Ma. The history of the district through the Palaeogene and Neogene periods (65 to 2 Ma) is uncertain although it seems likely that continental sediments were deposited across the whole district in the Palaeogene and have since been largely removed by erosion. Continued erosion and weathering produced the relict clay-with-flints capping of the Blackdown Hills.
The Pleistocene glaciations did not reach as far south as the Wellington district, but it was subject to periods of intense permafrost and of solifluction (soil flow) movement of superficial debris. Head deposits produced in this way mantle much of the district, and are at their greatest thickness along the lower valley sides. Landslides were particularly active during these cold periods, and there was some development of river terrace deposits. The climate warmed at the end of the Pleistocene (10 000 years ago), the amount of seasonal surface water declined, and extensive spreads of colluvium developed along valley floors, mostly by reworking of head. Some landslide activity continues to the present day, particularly along the active spring lines at the base of the Upper Greensand.
Chapter 2 Geological description
The geological succession at outcrop in the district is shown on the inside of the front cover.
Concealed strata
The only deep borehole in the district was drilled near Chard [ST 3430 0653], for geothermal purposes in 1983 and reached 301.66 m.
On the Bouguer gravity anomaly map, a high in the north-east corner of the Wellington district coincides with the margin of the Quantock Hills, terminated by the Cothelstone–Hatch Beauchamp Fault (CHF) (Figure 2). This fault merges eastwards with the Barrington Fault in the Yeovil district (Wilson et al., 1958). This configuration is supported by the interpretation of the seismic reflection data in the east of the district (Sheet 311, cross-section 2).
A prominent gravity lineament trending west–north–west to east–south–east (L1) is coincident with the Lopen–Coker Fault. This fault is mapped in the Yeovil district (Wilson et al., 1958) and appears on seismic reflection profiles in the south-east part of the Wellington district. It appears to merge with the Barrington Fault in the Yeovil district and the Cranbourne Fault within the Wessex Basin.
Prominent east–west linear gravity lows (GL1 and GL2) are associated with low-density Permo-Triassic rocks that infill the Crediton and the Tiverton troughs respectively. The linear east–west gravity low across the Wellington district itself (GL3) has a similar gravity expression to the Tiverton Trough and may represent the eastern continuation of that structure at depth. A local north–south gravity low (GL4) in the southern part of the Wellington district is probably a local sedimentary basin bounded by one or more north–south trending faults.
The Crediton and Tiverton troughs lie within a regional gravity high attributed to the presence of a thick sequence of relatively dense Palaeozoic mudstones and shales. Local linear east–west highs superimposed upon the regional high relate to Variscan fold structures and faulting. The gravity map suggests that some of these structures may continue at depth beneath the Wellington district.
The magnetic anomaly map reflects deeper basement features, which appear broad and poorly defined (Figure 3). The Wellington district lies within a regional magnetic low (ML1) where magnetic rocks lie at a significant depth. Just north of the district a linear west–north–west to east–south–east magnetic high (MH1) follows the outcrop trend of the Middle and Upper Devonian rocks of the Brendon Hills and Exmoor where the anomaly becomes more sharply defined. This suggests that magnetic igneous rocks within or beneath the Devonian sequence have been brought nearer to the surface by faulting or folding.
Devonian and Carboniferous
Small outcrops at the north-western margin of the district represent deformed Variscan strata, which form the basement at the margin of the Wessex Basin. The Pilton Mudstone Formation (PLT) spans the Devonian–Carboniferous boundary, and includes grey, cleaved mudstone with interbeds of sandstone and some thin calcareous horizons. The overlying Doddiscombe Formation (DDC), probably of Tournaisian age, is formed from hard, black or very dark grey, laminated mudstone. The Westleigh Limestone Formation (WST) is of Visean age, and includes thickly bedded limestone turbidites, minor limestone conglomerates and thin, grey mudrock interbeds. At outcrop in the Tiverton district, folds in the Devonian and Carboniferous strata trend roughly east–west. The outcrops in this district are overlain, with marked unconformity, by Permian red beds.
Permian and Triassic
In the north-western part of the district, outcrops of sedimentary breccia and weakly cemented red sandstone are classified, on lithological grounds, in the Exeter Group (Edwards et al., 1997) represented by the Halberton Breccia Formation (HlBr); this terminology replaces the 'Lower Sandstone and Breccia' of Ussher (1902). The older breccia unit of the formation is composed of roughly bedded, poorly sorted breccia, with subrounded to subangular clasts of Carboniferous sandstone, vein quartz and minor Devonian sandstone and chert. The matrix is of fine and medium-grained red-brown sand, silt and clay, described by Ussher as 'loam'. A maximum thickness of about 40 m of breccia is observed in the district, significantly less than the approximately 270 m identified within the Exeter district (Sheet 325) to the south-west. Towards the top of the breccia unit, well-cemented channel features form topographical ridges, for example [ST 0862 2333] north of Poleshill. Typical channel-fill lithologies include angular clasts of dark sandstone, pale quartzite, crystalline quartz and red quartzite, with medium to coarse-grained red-brown sand, overlain by beds of red-brown silty clay up to 2 m thick. Between 30 m and 50 m thickness of the Tidcombe Sand Member (TdS) of the Halberton Breccia Formation rests with an unconformity on the breccias. It is predominately composed of decimetre-scale cross-bedded, fine to medium-grained sandstone, with well-rounded grains and some silt. Local occurrences of pebbly sandstone, with some angular fragments, occur at the base of small (less than 1 m deep) channel forms. Near to Greenvale Farm [ST 0833 2363], an exposure of more than 2.5 m of fine-grained red sand with cross-bedding and intraformational erosion surfaces can be seen, indicative of an aeolian depositional environment.
Stiff red clays and silty clays of the Aylesbeare Mudstone Group (AyB) conformably overlie the Tidcombe Sand Member. These were formed as continental ephemeral lake deposits. Exposures are rare, and the outcrops are characterised by wet, wooded moderately low-lying land. Soils developed over the Aylesbeare Mudstone are mostly brown or reddish brown clay loams, with some local patches of heavy silty clay and some locally derived gravel in places. A thickness of approximately 80 m of Aylesbeare Mudstone is present around Langford Heathfield [ST 0970 2323], much less than the 400 m recorded in the Exeter district. Beds of red-brown, fine-grained, silty sand 20–30 m thick, with angular sandstone fragments up to 5 mm across, are present in the upper part of the Aylesbeare Mudstone; these beds form well-defined ridges around Wellisford [ST 0936 2156] and south of Langford Budville [ST 1050 2216]. It is probable, though not definitely proved, that the Permian/Triassic boundary lies within the lower part of the Aylesbeare Mudstone.
The Triassic Sherwood Sandstone Group (SSG) marks a return to high-energy, coarse-grained clastic sedimentation. The Budleigh Salterton Pebble Beds Formation (BSP) succeeds the Aylesbeare Mudstone, and includes some 20 to 40 m thickness of conglomerates, distinguished, at least in the southern part of the outcrop, by ovoid cobbles and pebbles of Ordovician quartzite, metabasite, vein quartz and Carboniferous sandstone. These are similar to the pebble lithologies at the type locality, on the coast, in the Newton Abbot district (Sheet 339) to the south-west. The matrix of the conglomerate is red-brown silty sand, and at Town Farm Pit [ST 079 168], the relatively high content of matrix sand and silt indicate debris flows and sheet-flood deposits (Plate 1), rather than the low-sinuosity braided river sediments of the type locality. At Town Farm Pit, the beds of conglomerate show some channelling into the underlying silty sandstone of the Aylesbeare Mudstone, where the base of the Pebble Beds is locally marked by a bed of yellow-brown sandy conglomerate. The northern outcrops of the Pebble Beds differ markedly in that clasts of Carboniferous limestone are locally dominant. In some beds the Ordovican grey quartzite is absent, and other horizons are composed almost exclusively of Carboniferous limestone. This could indicate that the source areas for the clasts within the conglomerate were changing during deposition, and that syndepositional faulting caused changes in the local pattern of erosion and source supply routes. The Budleigh Salterton Pebble Beds Formation is considered to be of Early Triassic age (Induan–Olenekian), with the succeeding Otter Sandstone Formation (OS) being of Mid Triassic (Anisian–Ladinian) age. In the district, the Otter Sandstone includes some 100 m of fine to medium-grained, red-brown, planar-bedded and cross-stratified sandstone with locally varying amounts of silt. Layers of small pebbles locally form 'lags' at the base of cross-beds. The sandstone beds are generally weakly cemented and slightly calcareous in places, for example near Higher Cross at [ST 0845 1482]. In Nynehead Hollow [ST 1409 2285], an excellent exposure of the Otter Sandstone is seen in the walls of a down-cut lane (Plate 2). It is composed of decimetre-scale cross-bedded channel sands with pebbly lags at the erosional base of many of the channels. The varying thicknesses of the cross-bedded sandstone units and the steeply angled sides of the channels indicate that deposition occurred in a shallow environment, with variable current directions, most likely attributable to shifting channels of braided rivers. Near the base of the Otter Sandstone, a section near Lovelynch Farm just outside the Wellington district at [ST 112 249] shows red sand varying from coarse to medium grained with many pebbles. The grain size decreases upwards through channels and well-developed cross-beds, which dip towards the north. This indicates a southerly provenance and also suggests a fining-upwards sequence, which could be characteristic for the Otter Sandstone as a whole. The contact with the underlying Budleigh Salterton Pebble Beds is sharp, and locally affected by normal faulting.
The Mercia Mudstone Group (MMG) (formerly Keuper Marl) forms a broad outcrop, and is estimated at about 450 m in thickness. In the Sidmouth district, excellent coastal exposures have permitted the subdivision of the group into formations (Edwards and Scrivener, 1999; Gallois, 2001; Edwards and Gallois, 2004), and with the exception of the central part of the district these distinctions are mapped in the Wellington district.
The bulk of the Mercia Mudstone Group is red-brown mudstone and silty mudstone, calcareous or dolomitic in places, weathering to produce stiff red clay soils. Subordinate beds and lenses of grey-green mudstone and pockets of white, cemented dolomitic material are also present, and there are beds of mostly reddish brown or buff-coloured sandstone in places. The overall strike in the Wellington district appears to be north–south or north-east–south-west with gentle bedding dips towards the east or south-east. In the northern part of the district, the base of the Mercia Mudstone is difficult to define, as there is a gradation from the sands of the underlying Otter Sandstone, through stiff clay, e.g. sands to silty and sandy clay. At the top of the group, the unconformity at the base of the Upper Greensand is rarely observed due to cover by landslide and solifluction deposits.
The Mercia Mudstone Group is of mid to Late Triassic age and was formed in a mainly arid terrestrial environment. The mudstones are interpreted as having been deposited within short-lived lakes and also by sheet flooding events, while the sandstones were deposited in fluvial channels. Towards the top of the Mercia Mudstone Group wetter conditions are marked by the incoming of the grey-green mudstones with subordinate algal dolomitic siltstones of the Blue Anchor Formation: these are the precursors to the establishment of fully marine conditions recorded in the succeeding Penarth Group.
The Sidmouth Mudstone Formation (SiM) consists mainly of brownish red mudstones, but contains hard sandstone bands or 'skerries', which form steep ridges running north–south. The Sidmouth Mudstone Formation is calculated as about 200 m thick, rather more than present in the Sidmouth district to the south.
The skerries, which are rarely exposed, appear to be mostly grey or buff-coloured sandstones. They are difficult to map in detail due to the effects of faulting and lack of exposure, but it is most probable that there are four main skerries, only one of which, between Bradford-on-Tone [ST 173 228] and West Buckland [ST 175 203], is distinguished on the face of the map. This sandstone unit is probably about 3 m thick and forms a distinct hill feature with a steep west-facing scarp slope that is prominent below West Buckland Church [ST 174 205] and also along the west side of Bradford-on-Tone from Heatherton Park [ST 168 219]. Whilst the sandstone is not exposed, angular brash of grey sandstone is evident in surrounding streams. Other ridges relating to thinner sandstone units run northwards from the Upper Greensand escarpment: from near Wrangcombe [ST 129 170] to [ST 128 182]; above Park Farm [ST 133 175] to [ST 131 183]; below Gidland's Farm [ST 143 172] to [ST 144 178]; and above Ford Street [ST 158 176] to [ST 155 183]. No such ridges were observed in the southern half of the district.
The base of the Sidmouth Mudstone Formation is marked by stiff, red-brown sandy clay, with abundant white mica flakes, which grades down into the underlying Otter Sandstone Formation. This sandy part of the Sidmouth Mudstone is perhaps 20 m thick in the southern part of the district, but increases to about 50 m in the north around Wellington.
The Dunscombe Mudstone Formation (DuM) was described (Gallois, 2001) from exposures on the coast to the east of Sidmouth, where it includes some 35 m of interbedded green and purple laminated mudstones, dolostones and breccias. In this district, the correlative has a different lithofacies but is nevertheless included in the Dunscombe Mudstone, comprising up to 35 m of grey, thin-bedded sandstones with interbedded grey-green mudstones and occasional pockets of hard-cemented dolostone. It crops out east of Wellington, where it forms a crest running approximately north to south from Chelmsine [ST 189 180] to the northern limit of the district. The best exposures of the Dunscombe Mudstone are in a road cutting at Lipe Hill [ST 186 215]. Here, a measured section (Gallois, oral communication) shows red-brown and purplish red mudstone of the Sidmouth Mudstone, overlain by about 6 m of green and grey mudstone, at the base of the Dunscombe Mudstone. Succeeding are about 3 m of buff and pale grey sandstone beds with interbeds of mudstone (Plate 3), and these are overlain by up to 14 m of green and grey mudstone. The top of the formation is not seen. Sandstone and dolostone are present at The Maze, above Higher Comeytrowe Farm [ST 195 231]. In the northern part of the district, the top of the Dunscombe Mudstone is marked by a distinct grey mudstone bed observed as fragments in ploughed fields to the north of Hamwood Farm [ST 198 213] and also to the west of Chilliswood Farm [ST 196 218]. To the east and south of Luppitt [ST 170 068] in the southern part of the district, several small outcrops of Dunscombe Mudstone have been mapped beneath the Upper Greensand escarpment: it has not been identified in the central part of the district.
About 180 m of the Branscombe Mudstone Formation (BcM) have been mapped in the district. The lithology of this formation is mainly red-brown mudstone and silty mudstone, with minor beds of grey and grey-green mudstone. Gypsum-rich mudstones have been recorded around the Hatch railway cutting [ST 301 207] at the base of the Blue Anchor Formation, and also in boreholes.
Thin beds of sandstone and siltstone (skerries), which form topographical ridges, are less common in the Branscombe Mudstone Formation than in the lower parts of the Mercia Mudstone Group. A distinct ridge, formed by a thin sandstone unit, is present from east of Poundisford [ST 222 208] to Cotlake Hill [ST 224 224]. The sandstone forms a prominent feature at Cotlake Hill but does not continue to the northern boundary of the district, and is assumed to be a channel-fill. Another distinct sandstone feature is mapped from Green Lane [ST 229 202] towards the north–north–east, to Duddlestone [ST 233 212] where it dies out. The sandstone ridge which runs north-eastwards to the northern margin of the district, from Thorn Hill [ST 2865 2332], is interpreted as correlative to the North Curry Sandstone of the Taunton district (Warrington and Williams, 1984).
The Blue Anchor Formation (BAn) (formerly the 'Tea Green Marls' and 'Grey Marl') is the youngest formation of the Mercia Mudstone Group and is 20 m to 35 m thick in the district. The base of the unit has been mapped at the concave break in slope at the base of the escarpment formed by the Penarth Group. Lithologies include green to dark greenish grey silty mudstones and siltstones overlain by dark grey mudstones and shaly mudstones, together with greenish grey and buff grey silty mudstones and siltstones, which are dolomitic in part. Gypsum-bearing mudstones have been recorded in boreholes in the Blue Anchor Formation. The formation is not well exposed in the district, being mostly obscured by head and slipped debris from the overlying strata.
The Penarth Group (formerly 'Rhaetic Beds') occurs at the top of the Triassic succession, and forms a prominent escarpment in the north-eastern part of the district. The Westbury Mudstone Formation is up to 13 m in thickness. Much of the formation is of black, dark grey and greenish grey fissile mudrocks (shale), with thin interbeds of rubbly, locally fossiliferous, grey limestone. The Cotham Formation includes up to 3 m of grey-green to brownish grey, calcareous, fissile mudrock with thin beds of limestone and siltstone. In the course of the recent mapping, the lack of exposure and extensive superficial cover precluded distinguishing the Westbury and the Cotham formations hence they are combined on the map where they are denoted WbCt. The White Lias Formation (WLi) (formerly Langport Member) is 9 m thick and forms the cap of the Penarth Group escarpment. Its base is marked by beds of porcellanous pale grey limestone, interbedded with mudstone. Towards the top of the White Lias, Ussher (1906) describes a limestone 18 cm in thickness, with a bored surface and characteristic disturbed appearance, which he correlates with the 'Sun Bed' (Figure 4) of the Sidmouth district. Penarth Group sections are recorded at Hatch Beauchamp [ST 3012 2076] and along the track to Smith's Farm [ST 3478 2363], and the White Lias in particular is exposed at Brockfield [ST 3000 0561].
Jurassic and Cretaceous
Lower Jurassic Lias Group
The base of the Jurassic is taken at the lowest occurrence of the ammonite Psiloceras planorbis (Figure 4). However, the Lias Group also includes just over a metre thickness of latest Triassic 'pre-Planorbis' beds. Lias Group rocks crop out at the top of the north-west-facing Upper Triassic escarpment in the central and north-eastern parts of the district. The sequence dips gently to the south-east, forming an undulating scarp and vale landscape.
Outside the district, Lang (1914–1936), Lang and Spath (1926), Palmer (1972), Hesselbo and Jenkyns (1995) and Callomon and Cope (1995) have undertaken detailed work on individual Jurassic limestone beds (Figure 4). Because of the paucity of exposures, differentiation to member level has not always been possible during this survey, which has relied heavily upon gross lithological characteristics allied to landscape features (the scarp and vale or dip slope topography) and some fossil information kindly provided by Mr Hugh Prudden with identifications by Drs Mike Simms and Kevin Page. At the base of the Lias Group, the Blue Lias Formation (BLi), comprises 45 m of thinly interbedded argillaceous limestones and mudstones. The lower part comprises tabular and nodular beds of limestone, 10 to 30 cm thick, separated by beds of mudstone mostly up to 40 cm thick. Above this, mudstone dominates with discontinuous limestone doggers. The Blue Lias limestones are hard and fine-grained, 'blue-hearted' when first exposed whilst weathered blocks turn a rusty orange-blue, and the mudstones include both organic-rich and carbonate-rich varieties. The formation is rarely exposed because head deposits and landslide debris obscure much of the outcrop particularly on the northern flanks of the Blackdown Hills near Staple Fitzpaine [ST 2645 1829]. Many Blue Lias limestones contain sheets and nests of oysters and other bivalves, brachiopods and echinoderm debris. The Bucklandi Zone dominates ammonite faunas towards the top of the sequence; according to Hesselbo and Jenkyns (1995), two closely spaced limestone beds referred to as 'Grey Ledge' mark the uppermost part of the Blue Lias. Above the Blue Lias Formation, the Charmouth Mudstone Formation (ChM), consists of around 95 m of pale, medium and dark grey mudstones, calcareous mudstones and organic-rich mudstones. In contrast to the Blue Lias, there are fewer limestone beds and the scarp and vale landscape is more subdued.
The Shales-with-Beef Member has here been surveyed combined with the succeeding Black Ven Marl Member, as they are difficult to differentiate district-wide, the combined package being around 70 m thick. The Shales-with-Beef are so-called because within mudstone sequence there is an abundance of calcite-rich beds, mostly 0.02 to 0.2 m thick. These comprise 'veins' of vertically oriented fibrous calcite crystals with a central parting of mudstone or shale, the structure faintly resembling the fibrous texture of beef steak. Minute selenite crystals and/or pyritic nodules also occur along some bedding planes especially in weathered mudstones. Thin discontinuous limestones are present. The Black Ven Marl Member consists of dark grey to black, calcareous and noncalcareous mudstone, with concentrations of beds of fissile, organic-rich mudstone. Polished surfaces or 'slickensides', a characteristic of the fissile units, give the mudstones as a whole a propensity to slip when loaded or when pore pressure is increased. The member contains several laterally persistent tabular limestone beds and horizons of concretionary limestone. These form marker beds and are described in south coast cliff sections (Lang and Spath, 1926) but have not been identified in the district. The overlying Belemnite Marl Member (BM) consists of 20 to 25 m of alternating, more and less calcareous, pale grey micaceous mudstones. In the field, the pale colour contrasts strongly with the darker hues of the underlying strata, and the greater permeability of the Belemnite Marl commonly results in spring issues along its base. In coastal sections of the Sidmouth district, belemnites are relatively common, especially towards the top of the member, but belemnites were rarely seen in the sequence cropping out within this district. West of the north–south trending valley east of Chard, the Cretaceous unconformity cuts out the upper part of the Belemnite Marl and the overlying Green Ammonite Member (GA). The latter comprises up to 15 m of dark grey mudstone which crops out only along parts of the eastern district boundary beneath extensive head cover. Regionally, this member appears to vary significantly in thickness and has a localised occurrence (Hesselbo and Jenkyns, 1995). The Green Ammonite Member is so named because the sequence includes nodules commonly containing the ammonite Androgynoceras lataecosta, whose internal cavities are infilled with green-tinged calcite.
The 'Middle Lias' Dyrham Formation (DyS) and the overlying Beacon Limestone Formation (BnL) (sometimes known as the Junction Bed or Marlstone Rock Bed) crop out locally in the east of the district. The Dyrham Formation, where weathered at outcrop, mostly comprises yellowish-red, oxide-stained, micaceous siltstone and sandy mudstone up to 70 m thick. The Beacon Limestone Formation comprises up to 2 m of rubbly, ferruginous, reddish cream limestone with abundant ammonite fragments.
The Bridport Sand Formation (BdS) caps the Jurassic sequence within the district, occurring at one locality at Sprays Hill [ST 360 111]. It comprises grey, weathering yellow or brown, micaceous silt to very fine-grained sand, locally with calcite-cemented sandstone beds and lenses. It is approximately 10 m thick.
Lower Cretaceous
The Upper Greensand Formation (UGS), unconformably overlies the Lias, Penarth and Mercia Mudstone groups. The sediments represent littoral and shallow marine deposits of the Albian to earliest Cenomanian transgression. The Upper Greensand is about 35 m in thickness in the north-west of the outcrop and thickens to about 60 m in the south-east (Figure 5). Gallois (2004a) has divided the Upper Greensand of the Devon coast into three members, the lowest of which, the Foxmould Member (up to 47 m thick), is the equivalent of the 'Cowstones' and 'Foxmould' of Jukes-Browne and Hill (1900). The overlying Whitecliff Chert Member was formerly known as the 'Chert Beds', and the succeeding Bindon Sandstone Member as the 'Top Sandstones'. In this district a full succession, similar to that recorded at the coast, has been described from the area around Chard (Ussher, 1906; Kennedy, 1970), with representatives of the Whitecliff Chert (about 12 m thick) and Bindon Sandstone (about 4 m thick) present beneath Chalk at Snowdon Hill Quarry [ST 313 089]. However it has not been practical to delineate these members on the 1:50 000 map. In the west of the district only the Foxmould Member is present, capped by extensive spreads of chert-rich debris (see clay-with-flints). The upper part of the Upper Greensand has been lost through dissolution of calcareous cements and the younger part of the formation is now represented as derived chert fragments in the overlying superficial deposits (Gallois, 2004a).
The dominant lithology of the Foxmould Member is fine-grained glauconitic sand, green when fresh but weathering rapidly to iron-stained orange when exposed. In some places the sandstone beds have retained their original calcareous cement, but this has mostly been removed by weathering and dissolution. The lower part of the member contains grey clay interbeds, similar to the Gault (Edwards and Gallois, 2004, p.9), as for example north-east of Higher Luxton [ST 214 114] (Plate 4) and north of Knapp Farm [ST 227 104]. The more persistent of these clay beds give rise to springs. The Foxmould Member is not well exposed due to cover by head and landslide deposits, but there is better exposure where valleys ('goyles') have been cut through the plateau for example the deep gorge north of Evaleigh Manor [ST 248 112] and a sunken track [ST 2306 1675] near Feltham. Towards the top of the member, 35 to 40 m above the base of the formation and most notably in the Blackborough area, silica-cemented fine-grained sandstone beds and lenticular concretions crop out on the upper slopes of the escarpment. These beds contain well-preserved and diverse silicified fossils of Albian age, including ammonites and age-indicative bivalves, the famous 'Blackdown fauna' (Downes, 1882; Owen in Gallois, 2004a). Some of the silicified sandstone beds and concretions were formerly excavated at crop or mined in short adits, for the production of whetstones (Chapter 3).
The Foxmould Member passes up into the Whitecliff Chert Member. At Snowdon Hill Quarry [ST 313 089] this comprises up to 12 m of variably cemented calcareous sandstone with interbeds and lenticular developments of chert. Elsewhere it is poorly exposed and its presence is inferred from the abundance of chert debris in brash. The highest part of the Upper Greensand, the Bindon Sandstone Member, has been described in detail from Snowdon Hill Quarry (Kennedy, 1970). It was also recorded at Northay Quarry [ST 281 112], where it was formerly worked as building stone known locally as the 'Calcareous Grit'. At Snowdon Hill, the basal part of the Bindon Sandstone includes 1 m of silty, cross-bedded sandstone, overlain by 1.7 m of nodular sandstone and 0.3 m of coarse-grained calcareous sandstone.
Upper Cretaceous
Outcrops of the Chalk Group (Ck), Grey Chalk Subgroup are limited to the south-east quarter of the district, near to the town of Chard. The Group overlies the Upper Greensand Formation and is capped by the clay-with-flints. It crops out mainly as outliers within faulted blocks or across the tops of hills as remnants of a former continuous cover. The Chalk is only well exposed in old quarries and pits and has been recorded up to a maximum thickness of 18 m. The main lithology in the district is a firm white to cream coloured, blocky weathering chalk with some marl present. At Snowdon Hill Quarry [ST 313 089], a phosphatic hardground, the 'Chalk Basement Bed' of Kennedy (1970) overlies the Upper Greensand. It is 0.25 m thick and has yielded a rich fauna of Cenomanian age. This hardground is overlain by about 0.3 m of nodular chalk, which in turn passes up into typical blocky-weathering Grey Chalk. Other noteworthy exposures in the district include an old quarry north of Scrapton [ST 297 106], where up to 15 m of a hard, white, blocky chalks with some grey marly layers are exposed. The most extensive outcrop of Chalk lies north and east of Weston Farm [ST 291 090], where in extensive former shallow pits the Chalk is seen to dip towards the east. Chalk between 6 and 18 m in thickness has also been recorded in boreholes for a covered reservoir north of Pole Rue Farm [ST 296 115].
Superficial deposits
Palaeogene to Quaternary
Deposition of the Chalk was followed by a long period of uplift and erosion, leaving thin, isolated remnants of Chalk in the south-east of the district. Farther east, in the Hampshire Basin, fluviatile and brackish marine Palaeogene sediments overlie the Chalk. None of these are present in the Wellington district, but locally within the clay-with-flints, generally at the base, there is brown and yellow stone-free clay, and red-brown sandy clay with a few cobbles that may be a remanié (residual) veneer of Palaeogene deposits, e.g. near Churchinford [ST 213 125] (Ussher, 1906, p.45) and Burnworthy [ST 180 159] (Ussher, 1906, p.49). Siliceous sandstone (silcrete) ('sarsens') are commonly found at the surface around Staple Fitzpaine [ST 2645 1829], at the back of the Greyhound public house (Prudden, 2001, pp.90–92). These sarsens, similar in lithology to those commonly found on the Chalk downland farther east, are probably the fragmented remnants of Palaeogene silcretes formed in hot and arid conditions.
Clay-with-flints is the name used in this survey to replace the earlier 'Plateau Deposits' and 'Clay with Flints and Chert' of Ussher (1906) and Woodward and Ussher (1911), and thus reconciles the terminology with that of the adjacent Sidmouth district. The origin of clay-with-flints is complex, it having been derived from Palaeogene sediments combined with the debris from dissolution and erosion of the underlying Chalk and Upper Greensand (Waters, 1960). These materials have been combined into a heterogenous mélange by lengthy periods of weathering, local fluvial processes, and periglacial solifluction and frost heaving.
The clay-with-flints caps all the high ground of the gently undulating, dissected plateau of the Blackdown Hills. The plateau surface falls gently southward from around 300 m in the north to 250 m in the central part of the district, rising again to 283 m at Black Down in the south-west. The base of the deposit approximately coincides with the unconformity at the base of previously existing Palaeogene deposits, on Chalk locally in the east and overstepping onto the Upper Greensand to the west. The base is likely to be irregular due to dissolution, particularly of the Chalk (see schematic section on map marginalia of 1:50 000-scale Sheet 311), and although there are no good exposures in the district, typical karst development is spectacularly displayed in the coastal cliffs of the Sidmouth district to the south, between Salcombe Regis and Beer Head [SY 150 877] to [SY 225 880] (Gallois, 2004b). Evidence from the field surveys and from boreholes suggests a weathered horizon of Upper Greensand, around 1 m in thickness, occurs beneath the clay-with-flints.
The deposit is laterally and vertically variable: where exposed it varies from 3 to 6 m thick, whilst borehole records indicate a thickness range from 2.4 m [ST 239 101] to 15.5 m [ST 218 076]. It is likely that the deposit is 'draped' across the Upper Greensand topography, which locally appears to have been affected by Palaeogene-age faulting, e.g. at Crawley [ST 268 080], at Bewley Down [ST 286 062] and at Wambrook [ST 290 085]. This last feature was noted by De la Beche (1839).
The lithologies of the clay-with-flints include heterogeneous brown and red clays, sandy clay and sand. Abundant clasts of chert and/or flint, from granules to boulders in size, are present. Locally, purple-brown ironstone is also recorded. The majority of clasts are unworn and many of the flints have retained the patina and shape that they display in situ in the Chalk. Flint clasts predominate on the Chalk outcrop but elsewhere become less common and are more stained and abraided indicating that the Chalk was largely eroded away prior to deposition of Palaeogene strata and subsequent development of the clay-with-flints (Gallois, 2004a). Chert clasts dominate where the deposit rests on the Upper Greensand in the west of the district, but even in these areas flints are locally common. Other clasts include well-rounded quartz and quartzite pebbles and well-rounded dark grey and black clasts of metasediment (oral communication R J Merriman, 2004).
Quaternary
The landmass of Great Britain may be divided into two distinct geomorphological provinces ('glaciated' and 'non-glaciated'; (Figure 6)), based on landscape evolution during the Quaternary and the nature and distribution of superficial deposits across the country. The district falls well within the 'non-glaciated province' which lies south of the known limit of glaciations in England — demarcated by a line drawn approximately from the Isles of Scilly north-westwards, fringing the coast of north Cornwall and Devon, the southern shores of the Bristol Channel and Severn Estuary inland to north of Gloucester and then across country south-eastwards to the northern outskirts of London and into Essex to the North Sea.
Landscape in the non-glaciated province is a product of numerous geomorphic cycles initiated during the Neogene (about 24 Ma ago), but mostly influenced by the multiple climatic changes that occurred throughout the Quaternary (from 2.6 Ma), ranging between periglacial (i.e. Arctic 'tundra') to cool and to warm temperate. The present-day topography is in essence a relict landscape resulting primarily from sediment reworking associated with periglacial processes, in situ chemical and physical weathering (deeply weathered regolith or residual deposits) and mass movement.
Aside from the Palaeogene clay-with-flints (see above) no residual deposits or loessic deposits (wind-blown silt prevalent during cold climate conditions) have been distinguished on the 1:50 000-scale Sheet 311 although both will certainly be present locally.
Mass movement deposits
Mass movement (synonymous with mass wasting) occurs in all climatic regimes due to the overriding influence of gravity; however, it is in periglacial environments that the multicyclic processes of solifluction, slope failure and slope wash, and the resulting deposits, are most significant. The principal types of deposit have been divided on the geological map into three categories (head, landslide deposits and colluvium, and 'valley head', respectively), largely on the basis of their topographical expression and to some extent lithological composition.
Head is a term introduced by De la Beche (1839) to describe unsorted, unconsolidated 'rubble' which blankets much of the landscape of south-west Britain. Head deposits are interpreted to have accumulated through various periglacial mass movement processes (frost creep, gelifluction and active-layer detachment failures; collectively referred to as solifluction). This deposit is generally only a few metres thick but can vary significantly in thickness over a short distance; it is widespread in the district, covering much of the bedrock formations. It is especially well developed on outcrops of the Mercia Mudstone and Lias groups. Adjacent to the Upper Greensand escarpment, it commonly drapes the lower slopes, where it is mostly derived from, and interdigitated with, landslide debris. Elsewhere, it occurs locally above the level of landslide at the foot of the main Upper Greensand escarpment, where it consists mainly of clay and chert debris derived from the clay-with-flints. In many places these deposits form a planar surface below the main escarpment, sloping at 4° to 6°, e.g. in the valley at Moor Farm [ST 235 155]. Individual solifluction flows are seen in places, some of them richer in chert debris than others. The more gravel-rich examples form low ridges up to 50 m across. Head has not been widely mapped over the outcrop of the Otter Sandstone as its lithology is very similar to that of the weathered bedrock thus making certain distinction difficult.
Head is a matrix-supported diamicton, i.e. a poorly sorted mixture of sand, silt and clay enclosing clasts of variable size. Its composition is almost entirely dependent on local bedrock lithology. Consequently, in the west of the district, sand grains and quartz and quartzite pebbles dominate, derived from the Permo-Triassic formations, whereas in the central part of the district, coincident with the Upper Greensand and Mercia Mudstone outcrops, head typically consists of chert fragments in a red-brown sandy clay matrix. Chert derived from the Upper Greensand, being the most resistant type of clast, is the most conspicuous component of head even on the relatively low-lying ground in the north of the district. On the Lias Group bedrock, head comprises extensive 'blanket' spreads of silt-rich clay with a matrix enclosing clasts of dark grey mudstone and siltstone, locally with abundant limestone clasts (see schematic section on map marginalia of 1:50 000 scale Sheet 311).
Head gravel occurs on isolated hilltops between the River Culm and the Blackdown Hills in the west of the district and north of the Blackdown Hills. It is most extensive in shallow valleys and low-lying interfluves of the River Isle and its tributaries north of Chard. These exposures generally occur on planar or slightly convex slopes, locally these spreads exhibit bench-like interfluve landforms similar in appearance to river terraces, for example around Ashill [ST 322 173], Broadway [ST 322 154] and Ilton [ST 352 174].
The deposits are poorly sorted, sandy, clayey and silty chert-rich gravels, generally characterised by a noticeably higher proportion of subangular gravel compared to 'normal' head or colluvium and valley head. A small proportion of well-rounded clasts less than 5 cm in diameter of black tourmalinised metasediment, and subangular flint presumably derived from the clay-with-flints, occur also. The exposures in the west of the district contain well-rounded clasts of quartz and quartzite, with rare flint. In comparison with head, head gravel tends to be less variable in thickness — generally less than 3 m.
The origin of head gravel is poorly understood. Possibly it is the result of multiphased rapid thaws giving rise to relatively higher energy slope processes. Its deposition probably spanned a period from pre-Devensian to late Devensian times. The occurrences north and east of the Blackdown Hills fall progressively in altitude away from the Upper Greensand escarpment. They are interpreted here as the eroded remnants of two or more spreads of sheet-wash and solifluction debris derived from erosion and landslide of the escarpment. Close to the Upper Greensand escarpment above West Buckland, two patches of head gravel rest on two distinct sloping surfaces, with the lower [ST 190 188] bevelled into the upper [ST 189 181]. Reconstruction of the surfaces on which these head gravel remnants rest suggests they were at a higher elevation, and therefore predate, the main apron of head and landslide deposits associated with the escarpment (see schematic section on map marginalia of 1:50 000 scale Sheet 311).
The outcrops of head gravel in the west of the district also appear to be the remnants of formerly extensive slope deposits on the flank of the Blackdown Hills. In contrast to the other outcrops, these include clasts derived from the Budleigh Salterton Pebble Beds.
Colluvium and valley head, mapped together, occur in valley bottoms and the lower slopes of valleys (see schematic section on map marginalia of 1:50 000 scale Sheet 311). The valley head component is formed by solifluction in periglacial conditions during the Devensian and tends to floor the valleys, whilst colluvium formed later, by downslope mass movement such as soil creep and hillwash. The formation of colluvium probably increased significantly following deforestation in the Bronze Age between 4000 and 2500 years BP (Brown, 1997). Typically the deposits comprise up to 3 m of poorly stratified, heterogeneous clay, silt, sand and gravel, the composition being variable and dependent on the local bedrock and superficial deposits. The gravel clasts vary in grade from fine to coarse, and may include angular to subangular flint and chert in the east of the district, and quartzite and Culm (Carboniferous) sandstone in the west.
Landslide deposits
Rotational landslides and translational mudflows are common where the Upper Greensand Formation overlies argillaceous strata, and generally obscure the base of the Upper Greensand. Areas mapped as landslide deposits are characterised by boggy ground with numerous springs and irregular hummocky topography, with ridges and mounds up to several metres high and several hundred metres long. In most areas it is difficult to distinguish individual slips as they are a complex of multiple slide blocks and coalesced slides. The downslope extent of landslide deposits is commonly difficult to determine precisely, particularly where the landslide toe commonly passes into or interdigitates with head and colluvium (see schematic section on map marginalia of 1:50 000 scale Sheet 311).
The mechanism for the development of landslides is well known from documentation of the Dorset coast cliffs where the same geological succession as in the present district is exposed. Groundwater percolating through the Upper Greensand is intercepted at the boundary with underlying impermeable Lias or Mercia Mudstone group strata resulting in development of springs. An effective head of groundwater is maintained in the Upper Greensand, keeping the contact zone saturated with a significantly reduced shear strength.
Where Upper Greensand overlies Mercia Mudstone, landslides tend to involve the entire thickness of the Upper Greensand failing above a shear surface in the top part of the Mercia Mudstone. However, where the strata are saturated from springs emanating at the base of the Upper Greensand over Lias clays, initial slip movement causes a severe reduction in strength and liquefaction of the lower part of the formation, which initiates a complex sequence of rotational movement, flow and collapse of the overlying strata. On the actively eroding Dorset coast cliffs, fresh landslides in the Upper Greensand produce a series of successive rotational blocks, but in the present district, the landslides have not been subjected to such active erosion and have largely weathered in situ with the typical hummocky features becoming increasingly subdued.
The angle of rest of the resultant landslide deposits depends largely on the thickness of Upper Greensand on the escarpment and the proportion of silicified sandstone and clay-rich beds in the succession. Near Buckland Hill [ST 168 175] the overall slope of the landslide deposits is 8.9°, whereas at Black Down Common [ST 1147 1617] it is 6.6°, the difference being due to a thinner Upper Greensand succession at Black Down Common, and consequently a smaller volume of landslide material. Around Blackborough [ST 0930 0925], the combination of siliceous-cemented Upper Greensand and the easterly dip of strata taking groundwater away from the escarpment are factors that have mitigated landslide development.
In areas where the Upper Greensand overlies the Lias Group, principally between Adcombe Hill [ST 225 175] and Bishopswood [ST 260 128] and around the north-east edge of the Blackdown Hills to Chard, landslides are in places very extensive and the mechanism complex as units within the Charmouth Mudstone Formation are themselves susceptible to landslides. Also, in the valley north-east of Staple Hill [ST 2475 1665], there is a broad arcuate embayment in the Upper Greensand escarpment and rotational blocks of Blue Lias Formation rest in the valley floor, about 450 m from the escarpment.
Landslides on oversteepened Lias clay slopes, between 5 and 10°, occur in a number of localities e.g. (Plate 5). These slides also involve rotational and translational movement, exacerbated where there are springs emanating at the base of the Blue Lias. Small areas of landslide deposits occur also on slopes of about 20° in the Mercia Mudstone on the Penarth Group escarpment, for example south-east of Corfe [ST 2332 1840].
Many of the areas mapped as landslide deposits are currently not active slips. Whilst their age is not known for certain, they have probably been active in the district throughout the Quaternary. The landslides seen today range from 'relict' (movement within the historic timescale 100–1000 years) to 'fossil' (movement in early historic to prehistoric times associated with significantly different climatic regimes than present day). Their last significant active phase was probably during the cold, wet, freeze–thaw periglacial conditions of the Younger Dryas (Loch Lomond) stadial between about 12 800 and 11 500 years BP, equating to landslide events on the Dorset coast (Conway, 1974). Other later major phases of landslide activity, corresponding to warm and wet periods, are from 7500 to 6000 BP, 5500 to 3000 BP and 2500 to 2000 BP (Jones and Lee, 1994). Individual 'recent' to 'active' landslides can be initiated by even short-lived wet periods; currently active landslides may be observed in a number of locations, for example at Park Gate [ST 2382 1828] and Hayne [ST 238 172], both of these causing minor damage to roads.
Fluvial deposits
River terrace deposits represent the dissected remnants of former floodplains. These are seen as one or more elevated landform surfaces within the main valleys flanking and more or less parallel to the present-day stream or river channel. Within the district, river terrace development is noticeably restricted in comparison to terrace development in the neighbouring drainage basins of the rivers Exe and Culm to the east, and of the River Otter to the south. A possible reason for this is attributed to the blanketing effects of the enormous volumes of mass movement deposits (head and head gravel), which have obscured or modified the drainage profiles, particularly in the eastern and central parts of the district. River terraces recorded in the district comprise two terraces in the River Tone [ST 085 215] catchment and one terrace in the River Culm [ST 085 135] basin. These terraces have been mapped on the basis of their bench-like landform position with respect to the river and associated gravel-rich soils. (See schematic section on map marginalia of 1:50 000 scale Sheet 311).
Alluvium includes fluvial channel deposits of gravel and overbank spreads of sand, silt and clay. The level ground flanking the rivers and larger streams of the district, e.g. the Rivers Culm and Tone, may be underlain by up to 5 m thickness of alluvium. This typically comprises up to 3 m of crudely stratified silt, sand and clay, with local gravel seams and organic-rich lenses, overlying up to 2 m of poorly sorted cobble gravel. The thickness along the minor streams is likely to be less and the deposits may be partly covered by, and interdigiate with, colluvium.
Artificial deposits and worked ground
Made ground and infilled ground are anthropogenic deposits of variable nature, extent and thickness. They are most likely to be present in urban areas, or where mineral workings have been infilled. Examples of the latter include the area around Poole [ST 151 215], where the former clay pits dug for the brick and tile works have been turned into a landfill site, and the extensive former workings for sand, gravel and aggregate within the Sherwood Sandstone Group outcrops between Burlescombe [ST 076 166] and Thorne St Margaret [ST 098 211]. No details of infill materials are available. A number of ancient barrows and tumuli are also present, mainly on the plateau tops and notably around the area of Robin Hoods Butts [ST 230 144]. Worked ground is shown on the map where human activity has removed natural materials. Examples include mineral workings dating from the 18th, 19th and early 20th centuries, and road and railway cuttings. Some worked ground is associated with landscaped (cut-and-fill) areas.
Structure
The Devonian to late Carboniferous Variscan Orogeny gave rise to folded and faulted strata that form the structural 'basement' to southern England. This basement crops out in the extreme western part of the district but elsewhere lies beneath Permian and younger cover, with increasing depth towards the east. Late Carboniferous north–south compression was followed by orogenic collapse and regional extension with the deposition of the Permian Exeter Group in east–west-trending basins. Further subsidence resulted in the accommodation of considerable thicknesses of eastward-dipping Triassic and Jurassic sedimentary rocks. The suite of north–south trending faults seen in the Sidmouth district (Edwards and Gallois, 2004) continues northwards into the Wellington district (Figure 7). These faults were established during Triassic sedimentation, and their early topographical expression may have controlled the sediment supply for the Budleigh Salterton Pebble Beds. The north–south faults were re-activated at times throughout the later geological history of the district, and displace strata ranging in age up to the Cretaceous e.g. at [ST 169 065].
It appears that the north–south faulting terminates against the prominent west–north–west to east–south–east-trending faults in the northern part of the district (Figure 7), (Miliorizos and Ruffell, 1998). Of these, the Cothelstone–Hatch Beauchamp Fault (CHF), which displaces the Lower Lias outcrop some 2.5 km to the south-east at Hatch Beauchamp, is considered by Whittaker (1972) to be a splay from the regionally important Watchet–Cothelstone strike-slip fault system to the north-west (Prudden, 2005, fig. 1). The CHF appears to extend eastwards to link up with the Barrington Fault (Wilson et al., 1958) of the Yeovil district (Sheet 312). Faults of similar trend to the CHF cross the district near Staple Fitzpaine [ST 264 183], and to the south of Wellington: the latter marks the boundary between the quartzite pebble-dominated facies of the Budleigh Salterton Pebble Beds to the south, and the limestone pebble facies to the north. The west–north–west to east–south–east-trending fault near Staple Fitzpaine is believed to be the western extension of the Lopen–Coker Fault (LCF) which also links with the Barrington Fault of the Yeovil district to the east (Wilson et al., 1958; Miliorizos and Ruffell; 1998).
Structures regarded as superficial in origin
Tightly folded and disrupted strata are seen in a number of incised stream sections within the Lias Group. Such structures occur where layered sequences of competent limestones are interbedded with shales and considerbly weaker mudstones. The disruption of these strata is thought to be associated with 'valley bulging' allied to localised cambering (see schematic section on map marginalia of 1:50 000 scale Sheet 311).
Valley bulging takes the form of a broad 'doming' deformation of argillaceous strata underlying a valley floor, with the fold axis running roughly parallel with the valley axis. It is generally considered that these features developed under periglacial conditions but may also result due to valley overdeepening and the associated stress release as overlying material is removed. The interpretation is not unequivocal as undoubtedly bedding is also disturbed by localised slips and by reactivated head or colluvium movement. One such stretch where a range of contrasting bedrock dip directions were observed is in the stream section between [ST 280 185] to [ST 285 185] near Bickenhall.
Chapter 3 Applied geology
Geological factors can give rise to variations in ground conditions over small areas and should be considered during the planning and development processes. Diversity in geotechnical properties may, within a single rock unit, be exacerbated by the superimposed effects of weathering or periglacial processes. Where mineral deposits have been quarried, the legacy may also be areas of derelict land, with highly variable geotechnical and geochemical characteristics. By considering possible problems associated with the local geology, appropriate remediation or mitigation measures can be taken prior to development.
Mineral resources
Building stone
Typically, prior to the 19th century, houses were constructed of locally quarried stone. As transportation improved in the mid 19th century, building stone such as Devonian slates, Carboniferous limestones and sandstones, and Jurassic Bath, Cotswold and Portland limestones became common in larger towns such as Taunton, Wellington, Ilminster and Chard.
In the north-western corner of the district, considerable use has been made of Carboniferous limestone in the construction of buildings and walls, for example at Greenham [ST 079 205], Burlescombe, Taunton and Wellington. There are disused quarries at Whipcott [ST 075 186], now infilled, and near Wiseburrow Copse [ST 077 194].
Pebbly, cross-bedded, red Triassic sandstones from the Budleigh Salterton Pebble Beds and Otter Sandstone are commonly used as rubblestone in cottages and as ashlared blocks in church towers , for example in Wellington, Langford Budville [ST 109 203] and Taunton. Ussher (1906) records the use of 'the thin band of conglomerate which crops out below the Lower Marls (i.e. the Aylesbeare Mudstone Group), east of Bathealton'. The 'North Curry Sandstone' of the Mercia Mudstone Group with small outcrops near Thornfalcon [ST 283 238], Staplehay [ST 218 216] and Bradford-on-Tone [ST 173 229] was an important source of building stone. Farm buildings around Thornfalcon show extensive use of large ashlared blocks of this greenish grey sandstone.
Stone from the White Lias Formation (Penarth Group) is seen in buildings in Hatch Beauchamp and Fivehead [ST 350 230]. The pale grey limestone is durable and of good quality producing large blocks used for quoins and window framing. However, it has been relatively little used compared with the grey micritic limestones of the succeeding Blue Lias Formation. Quarries at Beer Crocombe [ST 323 207], Curry Mallett [ST 320 216], Thurlbear [ST 265 212] and Hayne [ST 237 171] supplied wall stone and paving for numerous villages and cottages in their local area. The Blue Lias limestone has a tendency to weather to a yellow-brown colour, due to the decomposition of finely disseminated iron pyrite. The Beacon Limestone Formation, a rich brown, ferruginous limestone, was quarried in the Ilminster–Donyatt–Moolham area, and has been utilised in the village of Broadway and in Ilminster. Some buildings in Broadway show decorative interbanding of Beacon Limestone Formation and Blue Lias limestones.
Chert and flint nodules from the Upper Greensand, Chalk and clay-with-flints have been extensively used as building materials: Ussher (1906) mentions, in particular, the workings for flint on Snowdon Hill [ST 313 089]. An important source of building stone was the Bindon Sandstone Member of the Upper Greensand Formation, which occurs in the south-eastern part of the district. De la Beche (1839) refers to the use of this well-cemented sandstone in the construction of the Wellington Monument (cover picture); this was quarried at Northay [ST 281 112]. Similar material was formerly worked from Snowdon Hill Quarry [ST 313 089]. Large blocks of this sandstone are commonly seen in quoins, and in door and window frames. Older cottages and houses in the hamlets and villages of the Blackdown Hills and its adjacent area invariably have rubblestone fabrics of small, rounded, white and brown nodules of chert or flint commonly with quoins, windows and door frames in contrasting locally made red brick. Perhaps the most decorative form of their use is as precisely squared, knapped flints as seen in a number of buildings in Chard.
Iron ore
Hutchinson (1872) and Griffiths and Weddell (1996) cite evidence for localised iron smelting in the Blackdown Hills. The source of the ironstone may be the clay-with-flints or the underlying Upper Greensand, possibly enriched by ferruginous waters percolating from the overlying Palaeogene cover.
Brickmaking materials
There were numerous brickpits in the area around Taunton and Wellington (Issac and Parrott, 1995), which worked sandy and silty clays, mostly from the lower part of the Mercia Mudstone Group. All such workings are now closed and many have been infilled and built over. The last brickmaking operation, at Poole [ST 151 217], which had produced a wide range of terracotta ware as well as brick and tile products over a period of 150 years, ceased production in 1992. Parts of the Lias Group were also worked, on a small scale, for brickmaking in the Ilminster and Chard areas.
Dwellings, agricultural buildings and walls of 'cob' are common in the western part of the district. Cob is based on clay-rich weathered material, dug close to the site of construction, in this district either from the more argillaceous parts of the Permian and Triassic formations, or from head deposits derived from these formations. The clay-rich material is thoroughly mixed with water and a proportion of straw or animal manure to act as a binder. Cob walls are built up on stone foundations using wooden formers, and successive layers are allowed to dry before adding the next. As long as the tops of the walls are kept dry, cob buildings are very durable, and the combination of cob walls and thatched roofs is a key element of the architectural heritage of Devon and west Somerset.
Lime and agricultural materials
Stone from the limited outcrops of the lower Carboniferous limestone at the western margin of the district was formerly burnt for lime. The remains of a limekiln can be seen close to the entrance of the former Wiseburrow Quarry [ST 077 194].
The northern part of the outcrop of the Budleigh Salterton Pebble Beds is rich in pebbles and cobbles of Carboniferous limestone. These were formerly quarried and burnt for lime: there is a typical quarry and the remains of an old kiln near Langford Budville [ST 107 223]. Lime was also obtained from limestones within the Penarth Group and the Blue Lias, with the preserved remains of a kiln near Bishopswood [ST 254 132].
Numerous old pits in the outcrop of the Mercia Mudstone are the result of the former practise of 'marling' soils, that is decreasing soil acidity by spreading calcareous clay dug from weathered mudstone. Such pits are particularly common close to areas of acid sandy soil which needed such top dressing.
Sand, gravel and crushed rock aggregate
These commodities are produced at the present day from the Budleigh Salterton Pebble Beds and the lowest parts of the Otter Sandstone. Extensive pits in the outcrop of the Budleigh Salterton Pebble Beds yield sand and gravel, while the coarse pebbles and cobbles are crushed for aggregate. The high proportion of quartzite in the crushed fractions gives products suitable for concrete, road construction and general aggregate. The principal working at present is Town Farm Pit at Burlescombe [ST 089 168], where beds of poorly sorted conglomerate, up to 3 m in thickness, alternate with units of red-brown silty sand up to 2 m thick. The thickness of the Budleigh Salterton Pebble Beds here is about 24 m, and the base of the formation is marked by a bed of yellow-brown sandy conglomerate which in turn overlies the red-brown clay of the Aylesbeare Mudstone. Several former pits between Burlescombe and White Ball [ST 096 119] have been used for landfill and the reinstated or landscaped surface has been returned to agricultural use.
Sand was formerly obtained from numerous pits in the outcrops of the Otter Sandstone, Upper Greensand and parts of the clay-with-flints. Sand and gravel has been obtained from river terrace deposits; most of these workings were small, and many have been infilled. Extraction is currently undertaken from the terrace gravels of the River Axe near Chard Junction, though most of the workings are in the adjacent Sidmouth district.
Whetstones (scythe stones)
Whetstones or scythe stones, locally known as 'Devonshire Batts', were mined from tunnels driven into the upper part of the Foxmould Member of the Upper Greensand along the escarpment between Sheldon [ST 119 085], Ponchydown [ST 092 087] and Hembury Fort [ST 112 031], a north to south distance of some 7 km (Jukes-Browne and Hill, 1900). The extracted material occurred as irregular siliceous concretions within beds of sand. The concretions were removed from the mine, cut and shaped into whetstones by axes and grinding whilst fresh and relatively soft, and then allowed to harden by drying in the air prior to sale. The industry flourished for over two centuries, before the introduction of synthetic carborundum sharpening tools and the depletion of the natural resources led to a reduction in activity and, eventually, the closure of the last mine in about 1929 (Stanes and Edwards, 1993).
Strong (1992) states that the two samples examined are 'chert-cemented and slightly micaceous very fine-grained sandstones, composed of detrital quartz (approximately 5–10 per cent), some opaque grains, and minor Rhaxella-type spicules, tightly cemented by chert'. The chert cement shows a complex history of silica dissolution and reprecipitation.
Water resources
Hydrology
Local average annual rainfall varies from 1250 mm on the high ground in the central and southern part of the district to 815 mm over the surrounding low-lying land. The average potential evapotranspiration is 610 mm/a but average actual evaporation is 540 mm/a (Thompson et al., 1981).
The Blackdown Hills create a surface water divide that splits the area almost equally into two. The headwaters of the rivers Culm and Otter, and the smaller Yarty, are fed by springs issuing from the Upper Greensand Formation. These rivers mostly flow over the Mercia Mudstone Group and drain southwards into Lyme Bay. The River Tone rises outside the area on Carboniferous strata and then flows east over Permo-Triassic sandstones in the extreme north-west before crossing the Mercia Mudstone Group. The River Isle flows north over Lias strata in the east and north-east of the district. Both form tributaries of the River Parrett, which drains northwards into the Bristol Channel.
Water supply
The pre-Permian rocks and Permian Exeter Group provide limited supplies of water with groundwater flow and storage essentially restricted to poorly developed fractures. The succeeding Aylesbeare Mudstone Group is regarded as an aquiclude, but small supplies are derived from the interbedded sandstone horizons.
The Sherwood Sandstone Group is an important aquifer, yielding some 780 000 m3/a. The Group includes the Budleigh Salterton Pebble Beds and the Otter Sandstone Formation which are generally considered to be in hydraulic continuity, and the majority of water boreholes in the area penetrate both formations. However, groundwater from the Budleigh Salterton Pebble Beds is commonly undersaturated with respect to Fe2+, while groundwater from the Otter Sandstone is commonly oversaturated with respect to Fe3+. Artesian boreholes are known to facilitate the mixing of the two water types resulting in elevated concentrations of iron.
The average daily licensed abstraction for the Otter Sandstone Formation is 205 m3/d. According to records in the National Well Record Archive the average yield from boreholes in the Sherwood Sandstone Group of this area is 2.1 l/s. However, these yields are generally either less than 1 l/s, or between 5 and 10 l/s, suggesting that fracture flow is dominant in this well-cemented and faulted part of the aquifer.
The Mercia Mudstone Group accounts for a large proportion of the licensed sources. It also provides baseflow to the rivers. Fractured skerries may form multilayered confined aquifers that can give rise to artesian boreholes. Skerries are responsible for the good yields obtained in the areas south of Wellington and between Wellington and Taunton. A deep borehole in Taunton [ST 2126 2421] obtained a supply of 13 l/s for a drawdown of 42 m after a two-day pumping test; water being struck in four distinct horizons. A borehole near West Buckland [ST 161 203] was artesian and overflowed at a rate of 1.4 l/s. However, where major skerries are not penetrated, yields typically range from 0.25 to 0.4 l/s. While the majority of good yields are obtained from the area around Wellington, many such yields obtained in the south of the area are from boreholes less than 500 m from the base of the Upper Greensand Formation. The abstractions are undoubtedly tapping water that has drained from the overlying Upper Greensand Formation and are exploiting the storage capacity of the upper weathered horizon of these predominantly argillaceous strata.
Limestones occur within the Penarth Group and Blue Lias. As a consequence, many abstractions from the Lower Lias are located near the contact with the underlying Penarth Group, which may also contribute to supplies. As in the case of the Mercia Mudstone Group, there are a number of abstractions from the Lias Group within about 500 m of its boundary with the Upper Greensand Formation. A well and borehole in West Hatch [ST 2752 2093] supplied 1.9 l/s for a drawdown of 6.7 m, during a 12-hour test. The borehole penetrated strata in the Blue Lias and possibly the Penarth Group. However, even yields from the Blue Lias are generally less than 1 l/s.
The Upper Greensand Formation includes permeable sands and sandstones overlying low permeability clays and sands. Groundwater drains to issue as springs along the flanks of the outcrop, and these provide most of the groundwater supplies from the formation. Large yields are also obtained from boreholes located near the outcrop of the base of the formation, for example a 4.1 m-deep large-diameter well near Chard [ST 3321 0629], yielded a supply of 24 l/s for a drawdown of 3.3 m, after pumping for seven days. Abstractions are less common towards the centre of the Upper Greensand outcrop possibly because groundwater levels in the formation fluctuate by up to 3 m seasonally, and recharge may be inhibited by the overlying clay-with-flints. The Chalk is generally unsaturated in this area, due to its elevation and it being underlain by the porous Upper Greensand.
Large areas of the Upper Greensand and Chalk outcrops are covered by clay-with-flints. This is predominantly of low permeability and limits recharge to the underlying aquifers. Where sandier horizons are present, perched water bodies may be encountered and small supplies may be obtained. Run-off from the clay-with-flints is relatively acidic compared to groundwater from the Chalk so it is possible that along the margins of the clay-with-flints, recharge to the underlying Chalk concentrates dissolution resulting in karst features.
The alluvium and river terrace deposits may provide small supplies of water. However, yields are not likely to be large due to the limited thickness of these deposits.
Groundwater quality
Water from the Lias Group is likely to be hard due to the presence of calcium-rich limestone horizons, whilst that from the Mercia Mudstone Group could be mineralised by sulphate/chloride salt enrichment. Residence times are likely to be high. Water in the Upper Greensand Formation is usually soft but may be ferruginous. (Figure 8) shows examples of groundwater quality for the major aquifers in the area.
Geotechnical considerations
Foundation conditions
The physical characteristics of the geology determine the suitability of the ground to support structural foundations, the ease of its excavation and its worth in engineered earthworks and fills. Geotechnical issues are summarised in (Figure 9) for bedrock units and (Figure 10) for superficial deposits. Geological structure, slope stability, natural weathering and potential for flooding are also locally important. Variable man-made conditions encountered in landfill sites and areas of made ground, are a further potential problem with respect to severe differential settlement.
Significant slope instability is associated with the Upper Greensand escarpment and with valleys cut into the Black Ven Marl Member of the Charmouth Mudstone (see Landslide deposits). Whilst extensive areas of landslide are mapped in the district, the survey observed localised occurrences of landslide too small to be shown at the 1:50 000 scale. Construction projects in areas underlain by these formations should exercise due care with open faces of excavations especially if these are on the downslope side of any construction. This applies particularly to head deposits derived from the Charmouth Mudstone, as the head is likely to contain relict shears that will reactivate if loaded or if water is introduced (e.g. by an excavation or fractured drainage pipe) thus reducing the effective stress. During periods of intense rainfall, head may become saturated and could remobilise resulting in landsliding.
Any area underlain by clay-bearing strata should be considered as being potentially prone to 'shrink-swell' behaviour, which can cause foundation problems.
Construction in low-lying areas underlain by alluvium may experience foundation problems due to 'compressible' strata such as peat. Construction on made ground may also encounter compressible material in the infill.
Chalk is prone to dissolution by acidic rainwater and groundwater, particularly in the near-surface vadose zone. This may result in cavities and pipes that become infilled with overlying material commonly derived from the clay-with-flints (see schematic section on map marginalia of 1:50 000 scale Sheet 311).
The risk of flooding tends to be high on low-lying ground within river valleys, particularly ground underlain by alluvium (floodplains). The Environment Agency has protective measures in place including the construction of bunds and the maintenance of drainage ditches. Flash floods may occur within confined valleys should unseasonably high rainfall be experienced (see also the slope stability columns in (Figure 9) and (Figure 10)).
Toxic residues
Artificial (man-made) deposits may be potential sources of pollution. Significant sites in this district would include areas of landfill and quarry spoil, industrial complexes, railway sidings and sewage works. In old or modern landfill sites with inadequate containment, leachate migration into surface watercourses and groundwater could occur if developed on permeable superficial deposits or bedrock.
Landfill gas can include methane, formed by the degradation of organic matter in landfill sites under anaerobic conditions. It can migrate from the landfill site through permeable substrates, or along faults and joints, both vertically and laterally. As a general rule none of the bedrocks of the district should be considered for the disposal of degradable waste material without suitable arrangement for its safe containment. Radon is a naturally occurring radioactive gas found in small quantities in all rocks and soils, although the amount varies from place to place. Geology is the most important factor controlling the source and distribution of radon (Appleton and Ball, 1995). Approximately 99 per cent of the Wellington district is classified as a Radon Affected Area (Green et al., 2002). The Government recommends that houses in Radon Affected Areas should be tested for radon. Radon protective measures may need to be installed in new dwellings (and extensions to existing ones) in some parts of the district (BRE, 1999).
A 1:250 000 scale map of 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, especially where the latter two formations are overlain by clay-with-flints. It is thought that limestone bands in the Blue Lias and Charmouth Mudstone formations are also prone to high radon levels, especially in the northern part of the district.
Advice on potential radon hazard and measures for the alleviation of radon build-up in properties can be obtained on application to the Enquiries Desk at the British Geological Survey, Keyworth.
Information sources
Sources of further geological information held by the British Geological Survey relevant to the Wellington district and adjacent areas are listed here.
Information on BGS publications is given in the current BGS Catalogue of Geological Maps and Books, available on the BGS website (www.bgs.ac.uk). BGS maps, memoirs, books, and reports relevant to the district may be consulted at BGS and some other libraries. They may be purchased from the BGS Sales Desk, or via the bookshop on the BGS website. This website also provides details of BGS activities and services, and information on a wide range of environmental, resource and hazard issues.
Searches of indexes to some of the materials and documentary records collections can be made on the BGS website.
Geological enquiries, including requests for geological reports on specific sites, should be addressed to the BGS Enquiry Service at Keyworth. The addresses of the BGS offices are given on the back cover and at the end of this section.
Maps
- Geological maps
- 1:1 500 000
- Tectonic map of Britain, Ireland and adjacent areas (1996)
- 1:1 000 000
- Industrial minerals resource map of Britain (1996)
- Pre-Permian geology of the United Kingdom (South) (1985)
- 1:625 000
- Bedrock geology of the United Kingdom: South Map (2007)
- Quaternary geology: South sheet (1977)
- 1:50 000
- Sheet 294 Dulverton, Solid and Drift, 1974
- Sheet 296 Glastonbury, Solid and Drift, 1973
- Sheet 310 Tiverton, Bedrock and Superficial Deposits, 2012
- Sheet 311 Wellington, Solid and Drift, 1976, 2013
- 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, Bedrock and Superficial Deposits, 2005
- 1:10 000
- Relevant parts of the component 1:10 000 scale maps for the Wellington district were surveyed between 2002 and 2004 by the following BGS geologists: S J Booth, C E Burt, R A Ellison, R J O Hamblin, L M Hollick, K R Royse and R C Scrivener.
- Copies of maps from these and earlier large-scale surveys are available for reference in the BGS Libraries at Keyworth and Edinburgh, and at the BGS London Information Office in the Natural History Museum Earth Galleries. Copies for purchase are produced on a print-on-demand basis.
- Digital geological map data
- In addition to the printed publications, many BGS geological maps are available in digital form. Details are given on the BGS website. National coverage of digital geological map data (DiGMapGB) is derived from geological maps at scales of 1:625 000, 1:250 000 and 1:50 000. Selected areas also have digital geological data derived from 1:10 000 scale geological maps. Digital geological data for offshore areas is derived from 1:250 000 scale geological maps.
- Geophysical maps
- 1:625 000
- Gravity anomaly map of the UK: South sheet (2007)
- Magnetic anomaly map of the UK: South sheet (2007)
- Hydrogeological maps
- 1:625 000
- England and Wales (1977)
- 1:100 000
- Groundwater vulnerability of the Somerset Coast (Sheet 42), (1990)
- Groundwater vulnerability of East Devon and South Somerset (Sheet 50), (1998)
- Soil maps
- 1:250 000
- Sheet 5 South-west England Soil Survey of England and Wales
Books
- British regional geology guides
- South-west England (Fourth Edition), (1975)
- Memoirs and sheet explanations
- Geology of the country around Taunton and the Quantock Hills (Sheet 295), (Memoir, 1985)
- Geology of the country between Wellington and Chard (Sheet 311), (Memoir, 1906*)
- Geology of the country around Bridport and Yeovil (Sheets 312 and 327), (Memoir, 1958)
- Geology of the country around Exeter (Sheet 325), (Memoir, 1999)
- Geology of the country near Sidmouth and Lyme Regis (Sheets 326 and 340), (Memoir, 2nd Edition, 1911*)
- Geology of the Sidmouth district – a brief explanation of the geological map (Sheets 326 and 340), (Sheet Explanation, 2004)
- The Cretaceous rocks of Britain. Vol. I. The Gault and Upper Greensand of England. (Memoir of the Geological Survey of the United Kingdom, 1900*)
- *these memoirs are out of print but facsimile copies may be purchased from the BGS Sales Desk.
- 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.
Documentary records collections
Detailed geological survey information, including large scale geological field maps, is archived at the BGS. Enquiries concerning unpublished geological data for the district should be addressed to the Manager, National Geoscience Data Centre (NGDC), BGS Keyworth.
Borehole and trial pit records
Borehole records for the district are catalogued in the NGDC at BGS Keyworth. Index information, which includes site references, names and depths for these boreholes, is available through the BGS website. Copies of records in the public domain can be ordered through the same website, or can be consulted at BGS Keyworth.
Hydrogeological data
Records of water wells, springs, and aquifer properties held at BGS Wallingford can be consulted through the BGS Hydrogeology Enquiry Service.
Geophysical data
These data are held digitally in the National Gravity Databank and the National Aeromagnetic Databank at BGS Keyworth. Seismic reflection data from coal and hydrocarbon exploration programmes is available for the north of the district. Indexes can be consulted on the BGS website.
BGS Lexicon of named rock units
Definitions of the stratigraphical units shown on BGS maps, including those named on Sheet 311 (Wellington), are held in the BGS Stratigraphical Lexicon database, which can be consulted on the BGS website. Further information on this database can be obtained from the Lexicon Manager at BGS Keyworth.
BGS photographs
The photographs used in this Sheet Explanation are part of the National Archive of Geological Photographs, held at BGS in Keyworth and Edinburgh. Part of the collection can be viewed on the BGS website. Copies of the photographs can be purchased from BGS.
Materials collections
Information on the collections of rock samples, thin sections, borehole samples (including core) and fossil material can be obtained from the Chief Curator, BGS Keyworth. Indexes can be consulted on the BGS website.
External collections
Fossils collected from the Lias Group by Mr Hugh Prudden and identified by Dr Mike Simms and Dr Kevin Page are held by Museum of Wales, Cardiff.
References
Most of the references listed here can be consulted at the BGS Library, Keyworth. Copies of BGS publications can be obtained from the sources described in the previous section. The BGS Library may be able to provide copies of other material, subject to copyright legislation. Links to the BGS Library catalogue and other details are provided on the BGS website.
Appleton, J D, and Ball, T K. 1995. Radon and background radioactivity from natural sources: characteristics, extent and the relevance to planning and development in Great Britain. British Geological Survey Technical Report, WP/95/2.
BRE. 1999. Radon: guidance on protective measures for new dwellings. Building Research Establishment Report, BR 211.
Brown, A G. 1997. Clearances and clearings: deforestation in Mesolithic/Neolithic Britain. Oxford Journal of Archaeology, Vol. 16, 133–146.
BS5930. 1999. Code of practice for site investigation. (London: British Standards Institute.)
Callomon, J H, and Cope, J C W. 1995. The Jurassic geology of Dorset. 51–103 in Field geology of the British Jurassic. Taylor, P D (editor). (London: The Geological Society).
Conway, B W. 1974. The Black Ven landslip, Charmouth, Dorset. Report of the Institute of Geological Sciences, No. 74/3.
De la Beche, H T. 1839. Report on the geology of Cornwall, Devon, and West Somerset. (London: Longman, Orme, Brown and Green.)
Downes, W. 1882. The zones of the Blackdown Beds and their correlation with those at Haldon, with a list of fossils. Quarterly Journal of the Geological Society of London, Vol. 38, 75–94.
Edwards, R A, and Gallois, R W. 2004. Geology of the Sidmouth District. Sheet Explanation of the British Geological Survey, Sheets 326 and 340 (England and Wales).
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 north-west Europe. Geological Magazine, Vol. 134, 177–197.
Gallois, R W. 2001. 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. 2004a. The stratigraphy of the Upper Greensand (Cretaceous) of south-west England. Geoscience in south-west England, Vol. 11, 21–29.
Gallois, R W. 2004b. The development and origin of karst in the Upper Greensand Formation (Cretaceous) of the south-west England. Geoscience in south-west England, Vol. 11, 30–36.
Gallois, R W. 2009. The lithostratigraphy of the Penarth Group (Late Triassic) of the Severn Estuary area. Geoscience in south-west England, Vol. 12, 71–84.
Green, B M R, Miles, J C H, Bradley, E J, and Rees, D M. 2002. Radon Atlas of England and Wales. National Radiological Protection Board Report, NRPB-W26.
Griffiths, F, and Weddell, P. 1996. Ironworking in the Blackdown Hills: results of a recent survey. Bulletin of the Peak District Mines Historical Society, Vol. 13, No. 2 27–34. Winter 1996. Historical Metallurgy Society Special Publication: the Archaeology of Mining and Metallurgy in South West England.
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: The Geological Society.)
Hutchinson, P O. 1872. Iron pits. Report of the Transactions of the Devon Association for the Advancement of Science, Vol. 5, 47–50.
Issac, L, and Parrott, I. 1995. Brickmaking in Wellington. Second edition. (Wellington: Wellington Museum Society.)
Jones, D K C, and Lee, E M. 1994. Landsliding in Great Britain. (London: HMSO.)
Jukes-Browne, A J, and Hill, W. 1900. The Cret-aceous rocks of Britain. Vol. 1.The Gault and Upper Greensand of England. Memoir of the Geological Survey of the United Kingdom. (London: HMSO.)
Kennedy, W J. 1970. A correlation of the uppermost Albian and the Cenomanian of South-West England. Proceedings of the Geologists' Association, Vol. 18, 613–677.
Lang, W D. 1914. The geology of the Char-mouth cliffs, beach and foreshore. Proceedings of the Geologists' Association, Vol. 25, 293–360.
Lang, W D. 1917. The Ibex Zone at Charmouth and its relation to the zones near it. Proceedings of the Geologists' Association, Vol. 28, 31–36.
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 Char-mouth 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.
Miliorizos, M, and Ruffell, A. 1998. Kine-matics of the Watchet–Cothelstone–Hatch fault system; implications for the fault history of the Wessex basin and adjacent areas. 311–330 in Development, evolution and petroleum history of the Wessex Basin. Underhill, J R (editor). Geological Society of London Special Publication, No. 133.
Palmer, C P. 1972. Revision of the zonal classification of the Lower Lias of the Dorset coast. Proceedings of the Dorset Natural History and Archaeological Society, Vol. 93, 102–116.
Pettifer, G S, and Fookes, P G. 1994. A revision of the graphical method for assessing the excavability of rock. Quarterly Journal of Engineering Geology, Vol. 27, 145–164.
Prudden, H C. 2001. Geology and landscape of Taunton Deane — a geological exploration of south-west Somerset. (Taunton: Taunton Deane Borough Council.)
Prudden, H C. 2005. Strike slip faulting in Somerset and adjacent areas. Geoscience in south-west England, Vol. 11, 158–161.
Stanes, R G, 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.
Strong, G E. 1992. Chert-cemented glauconitic sandstone (Upper Greensand) from Blackborough, Devon. British Geological Survey Mineralogy and Petrology Short Report, MPSR/92/2.
Thompson, N, Barrie, I A, and Ayles, M. 1981. The Meteorological Office Rainfall and Eva-poration System: MORECS. Meteorological Office Hydrological Memorandum, No. 45.
Ussher, W A E. 1902. The geology of the country around Exeter. Memoir of the Geological Survey of Great Britain, Sheet 325 (England and Wales).
Ussher, W A E. 1906. Geology of the country between Wellington and Chard. Memoir of the Geological Survey of Great Britain, Sheet 311 (England and Wales).
Warrington, G, and Williams, B J. 1984. The North Curry Sandstone Member (Late Triassic) near Taunton, Somerset. Proceedings of the Ussher Society, Vol. 6, 82–87.
Waters, R S. 1960. The bearing of superficial deposits on the age and origin of the upland plain of east Devon. Transactions of the Institute of British Geographers, Vol. 28, 89–97.
Whittaker, A. 1972. The Watchet fault — A post-Liassic transcurrent reverse fault. Bulletin of the Geological Survey of Great Britain, No. 41, 75–80.
Wilson, V, Welch, F B A, and Robbie, J A. 1958. Geology of Bridport and Yeovil. Memoir of the Geological Survey of Great Britain, Sheet 312 (England and Wales).
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 of Great Britain, Sheets 326 and 340 (England and Wales).
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 Office at the Natural History Museum, and from BGS approved stockists and agents.
Northern Ireland maps can be obtained from the Geological Survey of Northern Ireland.
Figures and plates
Figures
(Figure 1) Topography and drainage of the Wellington district. NEXTMap Britain elevation data from Intermap Technologies.
(Figure 2) Bouguer gravity anomaly map. Scale 1:750 000. Bouguer gravity anomalies in milligals (mGal) calculated against the Geodetic Reference System 1967, referred to the National Gravity Reference Net, 1973. Variable Bouguer reduction density. The anomalies are shown as a colour shaded relief presentation. The shaded topographic effect has been created using an imaginary light source located to the north. Contour interval 2 mGal. (1 mGal = 1 x 10−5 m/sec2). Based on data in the BGS National Gravity Databank. Station distribution is approximately 1 station per square kilometre. The inset frame indicates the extent of the Wellington district. Key to abbreviations: CHF Cothelstone–Hatch Beauchamp Fault. TF Timberscombe Fault. L1 A prominent gravity lineament trending WNW–ESE coincident with the Lopen-Coker Fault. GL1, GL2 Prominent east–west linear gravity lows outside the Wellington district which are associated with low density Permo-Triassic rocks that infill the Crediton and Tiverton troughs respectively. GL3 Has a similar gravity expression to the Tiverton Trough and may represent the eastern continuation of the Tiverton Trough structure at depth beneath the Mesozoic cover. GL4 A local north–south gravity low in the southern part of the Wellington district which is probably related to a local sedimentary basin bounded by one or more north–south trending faults.
(Figure 3) Total field magnetic anomaly map. Scale 1:750 000. Total field magnetic anomalies in nanotesla (nT) relative to a local variant of IGRF90. The anomalies are shown as a colour shaded relief presentation. The shaded topographic effect has been created using an imaginary light source located to the north. Contour interval 10 nT. Based on data in the BGS National Aeromagnetic Databank. Flown at a mean terrain clearance of 305 metres on north–south flight lines 2 kilometres apart with east–west tie lines 10 kilometres apart. The inset frame indicates the extent of the Wellington district. Key to abbreviations:ML1 A regional magnetic low where magnetic rocks if present lie at a significant depth beneath non-magnetic Culm, Permo-Triassic and Mesozoic sediments. MH1 A linear NNW–SSE magnetic high which follows the outcrop trend of the Middle and Upper Devonian rocks of the Quantock Hills and Exmoor where the anomaly becomes more sharply defined. This suggests the presence of magnetic igneous rocks within or beneath the Devonian sequence brought nearer to the surface by faulting or folding.
(Figure 4) Generalised vertical section for the Lias Group based on coastal sections in the adjacent Sidmouth district (Sheet 326 and 340).
(Figure 5) Outcrop of the Upper Greensand Formation (after Gallois, 2004a).
(Figure 6) Quaternary provinces.
(Figure 7) Fault network mapped within the Wellington district. NEXTMap Britain elevation data from Intermap Technologies.
(Figure 8) Selected groundwater analyses in the Wellington district
(Figure 9) Engineering characteristics of bedrock in the Wellington district.
(Figure 10) Engineering characteristics of the superficial (drift) deposits in the Wellington district.
Plates
(Plate 1) The Budleigh Salterton Pebble Beds Formation at Town Farm Pit [ST 0797 1678]. Height of face about 1.5 m. Photographer S Parkhouse (by permission of David Roche GeoConsulting) (P781352).
(Plate 2) The Otter Sandstone Formation at Nynehead Hollow [ST 1410 2287]. Soil auger is 1.2 m long (P781353).
(Plate 3) The Dunscombe Mudstone Formation at Lipe Hill [ST 186 215]. Detail of alternating beds of hard, fine-grained sandstone and grey dolomitic mudstone (car keys for scale) (P781354).
(Plate 4) Grey clay band in weathered Foxmould Member, Upper Greensand, west of Bishopswood [ST 248 128] (P781355).
(Plate 5) Landslide in Lias clays near Otterford Mill (P781356).
(Index map) Index to the 1:50 000 Series maps of the British Geological Survey
(Front cover) Wellington Monument and the Upper Greensand escarpment (Photographer: Paul Witney; (P685544)).
(Rear cover)
(Geological succession) Summary of the geological succession at outcrop in the Wellington district.
Figures
(Figure 8) Selected groundwater analyses in the Wellington district
| Location | Chard* | Forches Corner† | SampfordArundel* | Payton* | Wellington* | Bradford-on-Tone* | Trull† |
| National Grid | [ST 3203 0830] | [ST 1830 1710] | [ST 1156 1986] | [ST 1088 2072] | [ST 1220 1976] | [ST 1629 2361] | [ST 1968 2107] |
| Type of source | Shallow well | Shaft and bore | Spring | Spring | Borehole | Borehole | Borehole |
| Aquifer | Upper Greensand | Upper Greensand | Otter Sandstone | Budleigh Salterton
Pebble Beds |
Otter Sandstone and Budleigh Salterton Pebble Beds | Mercia Mudstone Group | Mercia Mudstone Group |
| Date of analysis | 20/7/1959 | 15/03/2005 | 16/08/1978 | 15/09/1977 | 19/07/1977 | 14/09/1977 | 03/03/2005 |
| pH | 7.0 | 6.65 | 7.5 | 7.35 | 7.18 | 7.45 | 7.4 |
| Electrical conductivity µmho/cm | 104 | 530 | 520 | 486 | 1840 | 1063 | |
| Total dissolved solid mg/1 | 320 | ||||||
| Alkalinity (CaCO3) mg/l | 225 | 10 | 218 | 22 | 252 | 165 | 448 |
| Sulphate (SO42-) mg/1 | 10 | 19.2 | 28 | 14 | 115.3 | 93 | |
| Chloride(Cl-) mg/1 | 15.0 | 17.8 | 20.8 | 19 | 17 | 21 | 39.1 |
| Fluoride (F-) mg/1 | <0.05 | 0.15 | 0.28 | 0.48 | 0.385 |
||
| Nitrate (NO3-N) mg/1 | 2.4 | 2.72 | 7.93 | 7.3 | 0.14 | 2.4 | 9.34 |
| Calcium (Ca2+) mg/1 | 5 | 105.6 | 105 | 70 | 380 | 101 | |
| Magnesium (Mg2+) mg/1 | 2.76 | 3.9 | 4.4 | 20.1 | 69 | 78.3 | |
| Sodium (Na+) mg/l | 8.9 | 9.0 | 9.0 | 10.2 | 16 | 17.5 | |
| Potassium (K+) mg/l | 0.71 | 3.5 | 3.3 | 4.95 | 9.15 | 3.03 | |
| Iron (total Fe) mg/l | <0.03 | 0.22 | 0.004 | 1.5 | 0.028 | <0.03 | |
| Manganese (total) Mn mg/l | <0.01 | 0.0002 | 0.078 | 0.0025 | <0.01 | ||
| Silica (SiO2) mg/l | 14.7 | 22.9 | |||||
| (*) Data from National Well Record Archive
(†) Environment Agency Information © Environment Agency and database right. Data supplied by the Environment Agency, South West Region |
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(Figure 9) Engineering characteristics of bedrock in the Wellington district.
| Engineer- ing unit | Geological units | Geotechnical description* | Foundation conditions | Excavatability† | Use as fill‡ | Class | Natural slope stability |
| Moderately weak to moderately strong rock | Chalk Group | Generally moderately weak to moderately strong limestone, but properties are dependent on weathering grade. Probably highly fractured or reduced to soft to firm soil in near-surfaced zone | Generally good bearing capacity, with low rapid settlement on fresh material. Possibility of chalk workings, solution cavities and karstic rockhead | Hard digging to easy ripping. Flints may cause significant wear on plant. Stable, depending on weathering grade and fracture spacing | Suitable for general fill. Care needed to avoid over- compaction | 3 | Not generally prone to slope instability, but rockfall on steep faces |
| Strong limestone and mudstone | White Lias Formation
Blue Lias Formation Beacon Limestone Formation |
Strong limestone interbedded weak to moderately strong mudstone | Generally good bearing capacity. Potential shrinkage or swelling behaviour from mudstone | Hard digging to hard ripping or hydraulic breaker depending on thickness of limestone beds and fracture spacing | Suitable for general fill if crushed and graded and moisture content is controlled | 2C | Not generally prone to slope instability but rock-fall possible on steep faces |
| Weak conglomerate | Budleigh Salterton Pebble Beds Formation
Halberton Breccia Formation |
Weak sandstone and conglomerate weathering to medium dense to very dense silty sandy gravel with well-rounded pebbles, cobbles and boulders of quartzite | Generally good bearing capacity with low, rapid settlement | Good where dry. Possible running conditions below water table or if perched water table present | Suitable for general fill | 1A | No known stability problems. Slopes generally less than 12º except river banks up to 25º |
| Weak mudstone | Aylesbeare Mudstone Group, Mercia Mudstone Group, Dyrham Formation | Heavy overconsolidated mudstone and siltstone. Weathers to fissured, low to intermediate plasticity clay | Generally good bearing capacity with low but slow settlement | Easy to hard digging. Stable, depending on weathering grade, fractures (fresh) or fissures (weathered) | Suitable for general fill. Class depends on weathering grade and moisture content | 2B
2A |
Natural slopes generally stable, but instability occurs in Aylesbeare Mudstone Group due to springs at base of overlying Budleigh Salterton Pebble Beds |
| Charmouth Mudstone Formation, Cotham Formation, Westbury Mudstone Formation, Exmoor Group | Heavily overconsolidated mudstone and siltstone with some limestones. Weathers to fissured silty clay of intermediate to high plasticity | Generally good bearing capacity with low but slow settlement. Prone to shrinkable clay subsisdence | Easy to hard digging. Stable, depending on weathering grade, fractures (fresh) or fissures (weathered) | Suitable for general fill. Class depends on weathering grade, moisture content | 2B
2A |
Slopes prone to landslide. Landslide activity dormant (ancient) landslips may be present | |
| Weak sandstone | Otter Sandstone Formation Upper Greensand Formation
Sandstone in the Aylesbeare Mudstone Group |
Very weak to weak sandstone, weathering to loose to very dense sand | Generally good bearing capacity with low rapid settlement. Frost heave possible in very fine-grained sand or silty material | Easy digging. Stable where dry. Possible running conditions. Very weak below water table (especially Upper Greensand) or if perched water table is present | Suitable for general fill | 1A | No known stability problems unless formations harbour perched aquifers overlying clays or mudstones. e.g. landslide common feature of the Upper Greensand escarpment |
| Note: Predicted engineering behaviour is for fresh material and will change in the near-surface zone as bedrock material becomes weathered. Rocks in this area may be deeply weathered and be transformed to an engineering soil.
* Based on BS 5930, 1999. † Based on Pettifer and Fookes, 1994. ‡ Based on Manual, 1991. |
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Figure 10 Engineering characteristics of the superficial (drift) deposits in the Wellington district.
| Engineer- ing unit | Geological unit | Geotechnical characteristics* | Foundation conditions | Excavatability† | Use as fill‡ | Class | Natural slope stability |
| Non-cohesive | River terrace deposits | Loose to very dense but generally medium dense, well-graded sand and gravel. May contain buried channels and lenses of clay, silt and peat | Generally good bearing capacity, with low and rapid settlement, except where buried channels and pockets of silt, clay and peat are present | Easy to medium digging. Moderately stable. Less stable below water table due to high inflow rate and running sand conditions | Suitable | 2C | Areas of natural, unstable slopes unlikely |
| Organic | Peat | Very soft, highly compressible. Laterally highly variable | Extremely poor. Low bearing capacity. High, rapid settlement. Possible acidic, sulphate-rich groundwater | Easy digging. Very unstable when saturated. Stable when dry, depending on type | Not suitable | Poor when saturated. May slide off underlying stratum on slope; can flow | |
| Mixed (cohesive and non-cohesive) | Alluvium | Soft to stiff, normally consolidated, silty or sandy clay of low to high plasticity, and sandy gravel | Low bearing capacity, with medium to high compressibility | Easy digging. Moderately stable, may need support; possibility of running sand | Not suitable | Areas of natural, unstable slopes are unlikely except for banks of streams or drainage channels, which may be sensitive to loading or undercutting | |
| Clay-with-flints | Variable composition, medium dense/stiff material, ranging from inorganic silty clay of intermediate to high plasticity to clayey gravel and cobbles | Variable, may be prone to shrinkable clay subsidence | Easy to medium digging. Generally stable | Generally suitable | 2C | Areas of natural, unstable slopes are unlikely | |
| Head (partly unmapped) Colluvium and valley head. Head gravel | Highly variable. Cohesive and non-cohesive soil and clasts | Highly variable | Easy digging. Potentially unstable | Generally unsuitable | Potentially unstable. Contains relict shear planes capable of reactivation | ||
| Landslide deposits | Highly variable. Cohesive and non-cohesive soil and rock masses | Potentially unstable ground determines suitability on site-specific basis | Easy digging for soils, hydraulic breaker for rocks. Unstable | Generally unsuitable | Highly unstable. Contains relict shear planes capable of reactivation. Fissures in rock masses | ||
| Artificial | Made ground, Infilled ground | Very variable grain size, compaction and composition. Potential contamination | Determine suitability on site-specific basis | Easy digging except for large blocks of rock. Moderate to unstable | Determine on site-specific basis | Determine on site-specific basis | |
| Worked ground, Landscaped ground | Reworked, remoulded. Residual strength values and uneven bearing capacity | Determine suitability on site-specific basis | Easy digging. Moderate to unstable | Determine on site-specific basis | Determine on site-specific basis | ||
| * Based on BS 5930, 1999. † Based on Pettifer and Fookes, 1994. ‡ Based on Manual, 1991. | |||||||
