Geology of the Torquay district — a brief explanation of the geological map Sheet 350 Torquay
B E Leveridge, R C Scrivener, A J J Goode, and R J Merriman abridged by A A Jackson from the Sheet Description
Bibliographic reference: Leveridge, B E, Scrivener, R C, Goode, A J J, and Merriman, R J. 2003. Geology of the Torquay district — a brief explanation of the geological map. Sheet Explanation of the British Geological Survey. 1:50 000 Sheet 350 Torquay (England and Wales).
Keyworth, Nottingham: British Geological Survey, 2003.
©NERC 2003 All rights reserved
(Front cover) Front cover Thatcher Rock viewed from Ilsham Marine Drive [SX 940 633] looking south-eastwards. Foreground cliffs are in the Meadfoot Group (undivided) and the island comprises rocks of the Daddyhole Member, Torquay Limestone Formation. (Photographer B E Leveridge; GS1284.)
The area covered by the geological Sheet 350 Torquay is referred to as 'the district'. National grid references are given in the form [890 125], all lie within the 100 sq km grid SX. Abbreviations in brackets refer to the symbols used on the geological map. The number given with the plate captions is the registration number in the British Geological Survey photograph collection.
This Sheet Explanation has been abridged by A A Jackson from the Sheet Description, which was written by B E Leveridge, R C Scrivener, A J J Goode, and R J Merriman. M E Lewis contributed script on the hydrogeology of the district M E Drummond mapped and researched part of the area during tenure of a University of Newcastle upon Tyne/BGS CASE studentship (1979–1982). Christine Castle also kindly gave permission to use data from her PhD thesis (1982) submitted to the University of Hull, regarding the ages of members of the Torquay Limestone Formation. Figures were drawn by R J Demaine, P Lappage and G Tuggey, BGS Cartography, Keyworth.
The grid, where it is used on figures, is the National Grid taken from Ordnance Survey mapping.
© Crown copyright reserved Ordnance Survey licence number GD272191 /2003.
Geology of the Torquay district (summary from the reart cover)
The Torquay district has a long association with geology as a science, fossils from its limestones playing a key part in establishment of the Devonian system M 1840. Today it has a high concentration of geological Sites of Special Scientific Interest. The attraction of the Riviera coastline' owes much to its contrasting cliffs, the red Permian breccias about Paignton and the sun-reflecting Devonian limestone cliffs flanking Torbay at Torquay and Brycham. Traversing inland the ever-changing topography and scenery is closely linked to the variety of hard and softer rocks, formed in a remarkable geological setting the like of which is rarely preserved.
Devonian sedimentary and volcanic rocks were deposited in an extending continental plate margin, a passive margin, in a series of rift basins and complementary highs. Those of the Torquay district form successions of the southernmost basin, the Looe Basin, and the adjacent South Devon Basin. These successions record the change from terrestrial to marine sedimentation that occurred in the Early Devonian and then prevailed throughout the Devonian Period. The Middle and Upper Devonian basins are characterised by grey and purplryh red mudstone, and the highs by platform and reef limestones and volcanic rocks. Subsequent Variscan orogenic deformation, migrating from the south during the Carboniferous, folded and cleaved the Devonian rocks and closed the extensional basins. Basin deposits were pushed out northwards on to the adjacent highs, and the high deposits on to adjacent basin sequences, in major thrust slices. Relaxation of the compressional stress at the and of the Carboniferous generated extensional basins in the uplifted terrestrial topography about Torquay. Rapid erosion produced torrential deposits which filled those basins during the Permian.
Gold and it ore deposits were emplaced within the limestones by hydrothermal fluids in the Triassic period. Later, during the Cainozoic, erosion sculpted the solid rocks and largely formed the topography that we see today. Erosion and deposition during the Quaternary has produced the superficial deposits, head, fluvial and marine alluvium, which locally blanket the solid rocks.
The influence of geology is largely benign, but attention is drawn to potential hazards, such as concealed peat within alluvium, cavities within the limestones, and landslip; these all bear upon future development within the district.
Chapter 1 Introduction
This Sheet Explanation presents an account of the geological map Sheet 350 Torquay. The district is underlain by Upper Palaeozoic rocks of Devonian age that are, in part, overlain unconformably by Permian deposits, mainly in the vicinity of Torquay and Paignton. The cliffs along the coast show dramatic colour contrasts between red-brown Permian rocks and pale grey of the Devonian strata, and were studied by Sir Henry T De la Beche (1829) in the early part of the 19th century when he published an article 'On the Geology of Tor and Babbacombe Bays, Devon'. This predated by several years his pioneering mapping of Devon for the fledgling Geological Survey (De la Beche, 1839). Contemporaneous collections of fossils from the Torquay limestones played a key part in the establishment of the Devonian System by Sedgwick and Murchison (1839). Ussher mapped the district at a scale of 1:10 560, and the geological description given in his memoir (1903, rewritten by Lloyd, 1933) remains an important source of general geological information. Other geological studies in the 19th and early 20th centuries included detailed investigations of the Devonian with fossil collections that still provide a valuable research resource (e.g. Champernowne, 1881), and the cave deposits of Kent's Cavern (e.g. Pengelly, 1866). The Kent's Cavern complex is of national importance owing to its wealth of faunal remains and record of human habitation extending back some 450 000 years. Flint axes, attributed to the time of Homo heidelbergensis, make this the oldest scheduled ancient monument in Britain. At present, there are 11 geologically related Sites of Special Scientific Interest (SSSI) in the Torbay area, which remains an important centre for geological teaching and research.
Devonian sedimentary successions similar to those of the Torquay district occur across north-west Europe (Holder and Leveridge, 1986). They define a pattern of east-west-trending basins that formed the northern, passive margin of the extensive Rhenohercynian ocean basin (Franke, 1989). In this district two major basins are recognized: the Looe Basin in the south, separated by the Brixham High from the South Devon Basin. The Rhenohercynian ocean basin dosed during the Devonian, and associated deformation migrated northwards through the margin during the early Carboniferous (Dinantian) resulting in folding, and thrusting with successive basin inversions from south to north. When the rift basins were closed, stress was transmitted synchronously through the province producing the late Carboniferous orogeny. The collapse of that orogenic edifice, regional north-south extension and granite intrusion followed at the end of the Carboniferous and in the early Permian. It was in this dynamic early post-orogenic phase of the history of the region that the Permian basins of the district were formed.
Within the district, there is little evidence of the geological history of the Mesozoic era, a period of 171 million years. Marine retreat features in the region record a falling sea level from early Cainozoic times as a 'staircase of wave-cut platforms; those platforms at elevations lower than 40 m above OD are related to Quaternary events. Low sea level during the Pleistocene glaciations caused the river systems to cut down to nearly 40 m below OD. Cave systems occur at various levels and can be related to phases of marine regression. Animal remains and artefacts are a feature of the cave 'earth' deposits. Periglacial head deposits of late Pleistocene age commonly infill the smaller valley bottoms or have been exposed on the cliffs by marine erosion.
Chapter 2 Geological description
The Devonian sedimentary and volcanic rocks of this district lie eastwards along strike from those of the Plymouth district (Sheet 348; Leveridge et al., 2002). Those rocks were recognised as having been deposited in fault-bounded basins and intervening highs, which formed part of a continental passive margin sequence. Crustal extension and faulting during the late Silurian and Devonian lead to the formation of graben and half graben. This process of continental rifting developed sequentially in a northerly direction; the southern basins filled with sediment while those to the north were still in the process of formation (Figure 1). Thus the age of the sediment-fill varied from basin to basin; the stratigraphy is essentially local and there is no single regional succession. Rift faults provided pathways for basaltic magma and volcanic rocks enhanced the submarine highs; under suitable conditions carbonate platforms, including limestone reefs, developed on these highs.
From south to north, the successions in the Torbay district are: Looe Basin Succession, Brixham High Succession, South Devon Basin Succession (Southern sub-basin), Torquay High Succession, and South Devon Basin Succession (Northern sub-basin) (Figure 2). The successions are fault bounded, but largely para-autochthonous; they represent the relative location of original basins and highs, but parts have become detached from their root zones and are allocthonous. The highs were generally maintained at shallow water depths allowing carbonate platform and reef development, these sediments being reworked from time to time as limestone debris was eroded and deposited into the adjacent basins. The formations are described in approximate chronological order from oldest to youngest; a fuller description of the successions within the individual basins together with biostratigraphical and structural details is given in Leveridge et al. (2003) and a summary of the regional tectonic development in Leveridge et al. (2002). Biostratigraphy of the limestones is sumarised in (Figure 3).
Dartmouth Group (Dm)
The Dartmouth Group occupies the southern part of the district and is part of the Looe Basin Succession (Figure 1); (Figure 2), and is disposed in three major northward-verging thrust nappes. Bedding is generally inclined southwards at moderate to steep angles, and is the right way up, so that the oldest rocks are in the hanging wall of the bounding thrust and the youngest rocks are to the south. Neither the base nor the top is seen in the Torquay district. The group is early Devonian (Pragian) in age.
Mudstone dominates the group. It is red, purple and pinkish grey in colour, bioturbated in part, and is interlaminated with silt-stone and fine-grained sandstone. Siltstone and sandstone also occur in units, commonly up to 5 m thick, and are generally grey-green in colour, but red in places; bedding is thin to thick and the sandstone is fine to medium grained. Sedimentary breccia occurs sporadically; clasts consisting of mudstone and sandstone are set in a silty mudstone matrix. Pale quartzitic sandstone also occurs, with beds showing low-angle cross-lamination and basal scour structures. Basaltic lava, tuff and associated hyperbyssal microgabbro intrusions crop out along the coast around the mouth of the Dart.
These sedimentary rocks were deposited in lacustrine conditions, or as subaerial sheet-flood sands in a rapidly subsiding basin. The sporadic breccias are the deposits of mass flow and indicate intrabasinal instability.
Meadfoot Group (Mdt)
The Meadfoot Group is early Devonian (Pragian to Emsian) in age and occurs in the Looe Basin, Brixham High and Torquay High successions (Figure 1); (Figure 2). In the Looe Basin Succession, the group is divided into two formations, the Bovisand and Staddon formations. It is not subdivided in the thrust sheets derived from the Brixham High and the Torquay High successions, where its deposition predated the development of the South Devon Basin and both sub-basin highs.
The undivided group at Torquay includes the type section of the antecedent 'Meadfoot Beds'. It comprises grey silty mudstone, laminae and beds of sandstone, and thin beds of bioclastic limestone. The mudstone is bioturbated, with trace fossils such as Chondrites and Spirophyton common. Sandstone is cross-bedded and shows scour and slump structures, isolated ball and pillow stuctures, and lenticular units (up to 8 m thick) with channel forms. The depositional setting was a shallow marine shelf, with a southerly palaeoslope indicated by slumping and orientation of the main channels; the shelf was subject to tidal influence, suggested by subordinate east–west channeling (compare with Richter, 1967). The supply of coarser fluvial sediment to the Looe Basin via the main channels seems probable.
The Bovisand Formation (Bov) (PragnianErnsian) derives its name from Bovisand Bay in Plymouth Sound. In this district, the base is not exposed and the upper contact with the Staddon Formation is faulted. The formation consists predominantly of grey mudstone with laminae and beds of siltstone, sandstone and limestone; it is locally rich in macrofossils. Sandstone units, up to several tens of metres thick, comprise thin to thick beds of fine- to coarse-grained sandstone and also contain thin interbeds of grey mudstone. Sandstone that crops out on the coast at Southdown Cliff [SX 928 541], where the sequence is inverted, and at Long Sands [SX 920 525], where it is the right way up, are probably part of the same emit exposed on opposite limbs of the Man Sands Antiform. Sandstone that crops out south-east of Croftland [SX 898 525] represents a second lower unit. The mudstone represents deposition from suspension in a marine, offshore-shelf setting, whereas the sandstones are storm generated turbidites and near-shore deposits. The mapped sandstone units are probable correlatives of the Long Sands Sandstone members of Leveridge et al. (2002), which comprise repeated cycles of offshore mud and silt with wave- and storm-dominated shoreline sands in a rapidly subsiding basin. The Staddon Formation (StG) (Emsian), strikes east-west, and is bound to the south by a thrust or reverse fault and to the north by a low-angle southerly inclined thrust. Much of the sequence is inverted. The formation consists predominantly of yellowish grey sandstone, which is fine to coarse grained and occurs in thin to thick, massive beds, commonly with parallel and low-angle cross-lamination. Interbeds of mudstone occur in the lower part of the sequence, and wispy interlaminated siltstone and fine-grained sandstone in the higher part; these are grey, yellow, green or red in colour.
The formation represents an upward transition from a shallow marine environment to deposition in standing water on a coastal plain or shallow marine embayment, and finally in a fluvial regime with shallow channels and overbank deposits.
- Meadfoot Group (undivided), at 'Meadfoot Beds' type locality, in cliffs at Meadfoot Beach [SX 933 633] and Thatcher Point [SX 942 631]
- Bovisand Formation, both mudstone and sandstone are well exposed in the cliffs of Man Sands [SX 923 536], Long Sands [SX 920 524] and Scabbacombe Sands [SX 919 520]
- Staddon Formation, largely occluded from the coast by faulting, crops out along the banks of the River Dart, about Lower Kilngate [SX 876 535] on the west bank, and north of Higher Noss Point [SX 880 531] on the east bank
Nordon Formation (No)
The Nordon Formation is present in all the successions; it represents the background marine basin sedimentation, and ranges in age from Early to Late Devonian (latest Emsian to Famennian; Figure 1); (Figure 2). It is best developed in the southern sub-basin of the South Devon Basin where four limestone members are distinguished and there is some interdigitation of basic volcanic rocks. The formation is bound to the north and south by faults.
The formation consists largely of mudstone, which is grey to bluish grey, locally weathering buff and green, and with purple and red staining near the outcrop of Permian rocks. It becomes calcareous where it is associated with limestones and tuffaceous where associated with volcanic rocks. A fine-grained siltstone interlamination occurs in places, but otherwise the mudstone appears to be massive. Apart from the named members, limestone also occurs as laminae, thin beds and lenses. Dispersed corals and shells are common, especially in the lower parts of the sequence. Volcanic rocks occur around Dartington and Uphempston, but are elsewhere poorly exposed. They consist of fine- to coarse-grained, bedded, crystal tuff, which is green where fresh and weathers to an ochre brown colour. The limestone members are described in ascending strati-graphical order (Drummond, 1982) determined on the basis of their conodonts.
Bourton Limestone Member (BL) (Lower Eifelian) consists of dark grey, thin to medium bedded, crinoid wackestone and micrite; some beds show normal grading.
Marldon Limestone Member (ML) (Upper Eifelian-Lower Givetian) comprises thin to thick bedded pelloidal wackestone and packstone with thin grey mudstone partings, which pass laterally to more massive stromatoporoid floatstone or pelletoidal packstone. Corals and gastropods are locally common.
Dartington Limestone Member (DrtL) (Upper Givetian) is dark to pale grey in colour, thinly to thickly bedded and massive. It is muddy towards the top and base. Beds of micrite, wackestone, pack-stone, floatstone and rudstone are sharply defined; normal grading and basal scour structures are common.
Penny's Wood Limestone Member (PWL) (Frasnian) comprises thinly interbedded grey to dark grey limestone and grey mudstone.
Parts of the Marldon and Dartington members may represent shoaling and local colonization of deposited carbonate banks. Otherwise the members are interpreted as largely derived carbonate mud and bioclastic debris that was transported by storm currents, turbidite and debris flow. The stratigraphical age of the members, and their relationship to breaks in limestone deposition on the highs to north and south, are illustrated in (Figure 3).
In the northern sub-basin derived limestones within the formation are mapped about Torquay. At Babbacombe, in the cliff section, an inverted Lower Frasnian (Castle, 1982) part of the formation, comprising dark grey pyritous mudstone with thin limestone interbeds and basaltic lava, has been referred to previously as the Babbacombe Slates.
In the Looe Basin succession (Figure 2), the Nordon Formation is represented by the Dittisham Member (Dit) (latest EmsianEifelian) an inverted sequence that crops out around Dittisham [SX 865 549]. The member comprises dark bluish grey mudstone with subordinate grey mudstone, laminae and thin beds of pale grey limestone, and sporadic shell coquinas. Towards the top, lenses and thin beds of basic tuff are present.
St Mary's Bay Member (SMB) (Mid-late Efelian) of the Nordon Formation occurs within the Brixham High Succession, where it splits the lower two members of the Brixham Limestone Formation. It outcrops around St Mary's Bay, where it is generally overturned to the north, and is mapped inland in a series of fault-bound blocks as far as Stoke Gabriel [SX 845 475]. The member consists largely of grey to dark grey slaty mud-stone, weathering greenish grey and grey-buff. Limestone occurs as laminae, thin beds and lenticles, and the proportions in the lower part suggest a transition from the Sharkham Point Member of the Brixham Limestone Formation. Laminae and thin beds of tuffaceous siltstone and fine-grained sandstone are dispersed in the lower part of the member. Limestone beds towards the top are coarsely bioclastic in part; some show normal grading. Macrofossils are present locally, and there is some bioturbation. The evidence suggests that the carbonate platform on which the Brixham Limestone accumulated was drowned as water depth increased and a supply of fine clastic sediment became available. Limestone beds towards the top of the member are interpreted as storm-derived sediments.
- St Mary's Bay, shore platform [SX 933 549] to [SX 932 554]
Ashprington Volcanic Formation (AV)
The Ashprington Volcanic Formation is Mid to Late Devonian in age (Figure 2). It is developed mainly on the Brixham High, but forms the topmost part of the Looe Basin Succession (Greenway Tuff Member) and distal parts of the flows extend into parts of the adjacent South Devon Basin Succession. It passes laterally to, and interdigitates with, the Brixham Limestone Formation, forming a thrust nappe within which a formation top and base are not present.
The formation, where associated with the Brixham High, consists predominantly of basalt lava with subordinate volcaniclastic rocks. Lava flows are up to several tens of metres thick, and generally show a well developed cleavage. The basalt is dark grey-green, weathering to purple, red and ochre yellow. The volcaniclastic rocks show a similar range of colours on weathering; hyaloclastite and bedded tuff form interbeds with the lava, and occur as thicker sequences in places, particularly to the east. The tuffs are crystal or lapilli rich, locally agglomeratic, and accretionary lapilli are developed sporadically. Beds are thin (laminae) to thick, and commonly show normal or reversed grading.
Interbedded with the volcanic rocks, particularly to the east, is limestone and grey or purplish red mudstone. The formation was erupted from a volcanic centre that was in partly emerged.
Greenway Tuff Member (GyT) forms the uppermost part of the Looe Basin Succession. The member comprises basic tuff and basalt lava; the tuffs are dominant in the vicinity of Greenway, and the lavas are more prominent westwards in the lower half of the member. The tuff is grey to blue-green in colour, weathering ochre-brown, and is otherwise similar to the main crop.
- Well exposed in River Dart cliff sections to north-east and south of Greenway Quay [SX 870 548]
Brixham Limestone Formation (BxL)
The Brixham Limestone Formation accumulated on the Brixham High that formed with the development of the South Devon Basin to the north of the Looe Basin (Figure 1). It is subdivided into members in the coastal section around Brixham, and at Goodrington (Figure 3), but these members cannot be traced inland owing, in part, to the paucity of exposure. St Mary's Bay Member of the Nordon Formation separates the two lower members in the coastal section.
Sharkham Point Member (SP) (early to mid Eifelian) is the lowest member of the formation and exposed along the coast, running westwards from Sharkham Point [SX 937 946], where the sequence is inverted, younging northwards; north of Sharkham Point, it is folded into open to close upright folds. The lowest rocks exposed consist of interbedded reddened grey slaty mudstone and thin graded beds of limestone that is shelly crinoidal packstone and wackestone. These are succeeded by blue-green, thinly bedded, lapilli tuff with interbeds of limestone and mudstone, which in turn passes up into limestone. The limestone comprises grainstone, packstone and wackestone, locally with abundant stromatoporoids, and is thinly bedded, but bedding becomes thicker towards the top of the sequence. The member was deposited in conditions that became shallower allowing shoals to develop and eventually a carbonate platform with patch reefs was established. Berry Head Member (BHL) (late Eifelian-late Givetian) forms the headland at Berry Head and probably constitutes a significant part of the undivided formation that extends a further 6 km westwards to north of Stoke Gabriel. The base is conformable with the St Mary' Bay Member of the Nordon Formation, but the top is not seen, being thrust over the succeeding Churston Member to the south of Churston Cove [SX 9195 9695]. The member comprises beds of rudstone and nodular limestone in the lower part, which pass up into thinly bedded grey wackestone and packstone with interbedded grey calcareous mudstone. A transition from thin graded beds to massive laminar-bedded crinoidal and stromatoporoid grainstone and floatstone occurs in the central part of the member. The upper part comprises massive pale grey bind-stone and floatstone. The member is interpreted as part of a biogenic bank complex. Goodrington Member (CD) (Givetian) crops out about Goodrington and is exposed inland in roadside sections and quarries. It interdigitates with the volcanic rocks of the Ashprington Volcanic Formation and purplish red mudstone of the Saltern Cove Formation. The member is bound in the Goodrington area by steep faults to the south and a thrust to the north. The lower part of the member comprises medium- to thick-bedded rudstone and floatstone with stromatoporoids and shelly fauna, and the upper part is thin- to thick-bedded micrite, with stromatoporoids (Amphipora); it is dolomitic in part. The depositional setting is interpreted as a back-reef restricted lagoonal environment (Drummond, 1982).
Churston Member (ChL) (Frasnian) forms the coastal section between Fishcombe Cove [SX 920 569] and Broad Sands [SX 898 574]. Both southern and northern boundaries are thrust faults, overthrust by the Berry Head Member to the south and overthusting the Saltem Cove Formation to the north. Thinly interbedded slaty mud-stone, crinoid wackestone, and lapilli tuff, low in the member are overlain by thin- to medium-bedded framestone and floatstone that pass up into massive laminar stromatoporoid bindstone and floatstone, which is dolomitised in part. The member was deposited on a shoaling, high-energy platform that was possibly emergent at times.
Torquay Limestone Formation (TqyL)
Torquay Limestone Formation is essentially the sequence of the Torquay High Succession. It is equivalent to the East Ogwell Limestone Formation to the west of the Sticklepath Fault (Selwood et al., 1984), and largely coeval with the Brixham Limestone Formation, the Asprington Volcanic Formation and part of the Nordon Formation (Figure 2). The Torquay Limestone Formation crops out on the Torquay headland, where it occurs within thrust sheets that also include parts of the Nordon Formation and the Meadfoot Group. A diagrammatic representation of the reef development and subsequent faulting of the Fast Ogwell Limestone Formation is shown in (Figure 4).
In places the Torquay Limestone Formation can be subdivided into members (Scrutton, 1977), but in the large fault-bound outcrops around Torquay it has not been subdivided owing to the paucity of definitive fossils and variation in fades.
Daddyhole Member (DhL) (early Eifelian) consists of dark grey to grey limestone, with sporadic beds of calcareous mud-stone and thin beds of tuff. Bedding in the limestone varies from thin to thick (Plate 1); the thinner beds are largely detrital, the thicker beds consist of stromatoporoids and corals, commonly in growth position, or are micritic. In the Hope's Nose area, massive lenticular units pass laterally and upwards (with disconformity) into thin beds. Sedimentary features and macrofossils indicate that deposition was in quiet shallow-water conditions that allowed carbonate bank and patch reefs to be established,
Walls Hill Member (WI-IL) (Givetian) consists, typically, of pale grey, limestone; beds are medium to thick, micritic or coarsely bioclastic, and in situ stromatoporoids form massive beds. Matrix, where present, consists of corals and crinoid debris. The lower boundary of the member is taken at the base of lowest massive white limestone overlying dark grey thinly bedded limestone of the Daddyhole Member seen at the base of the cliff at the northern end of Redgate Beach [SX 935 649]. Between the basal limestone and the typical fades there are several metres of thinly bedded red muddy limestone. The limestone facies of this member represents major stromatoporoid reef development (Scrutton, 1977).
Barton Limestone Member (BaL) (Mid Giventian-Early Frasnian) consists of thick, massive beds of crinoidal and biodastic grey limestone; small stromatoporoids and large tabular corals are present in places. Thinly bedded micrite is subordinate. In Lummaton Quarry [SX 915 665], lenticular beds with abundant brachiopods, gastropods and trilobites occur at the base of the member, overlying the Walls Hill Member. This is known as the 'Lummaton Shell Bed' (Ussher, 1907), and has yielded macrofossils and microfossils that indicate a mid Givetian age, Maenioceras terrebratum Zone ((Figure 3); House, 1963). On the coast, the limestone forms the cliffs at Babbacombe, but there the member is thin to medium bedded and, in part, dark grey, crinoidal and graded. The main body of the member appears to represent a patchwork of bioherms within worked criinoidal sand. A peripheral fades may be represented at Babbacombe.
Torquay Limestone Formation (TqyL) (undivided) is shown on the map in several large fault-bound outcrops around Torquay. The transition between Eifelian and Givetian occurs in the northerly dipping sequence of Windmill Hill [SX 909 658] and Daison Hill [SX 913 657] (Ussher, 1907; Castle, 1982). It corresponds with the transition from grey, thinly bedded limestone (with interbedded calcareous mudstone) to pale grey, thickly bedded (beds to 20 m), massive limestone towards the southern end of those hills.
In Walden Hill [SX 915 638] and Braddon Hill [SX 920 639], thickly bedded, massive pale limestone with corals, stromatoporoids and crinoid matrix, indicate a Givetian age, middle Polygnathus varcus Subzone (Castle, 1982). A break comparable with that between the Walls Hill and Barton Limestone members has not been recorded.
East Ogwell Limestone Formation (EOL)
The East Ogwell Limestone Formation crops out mainly in the adjacent district to the north (Sheet 339 Newton Abbot; Selwood et al., 1984). It comprises pale to medium grey and greyish pink, thick-bedded, massive limestone. In places it is coarsely bioclastic with corals, brachiopods and subordinate stromatoporoids, but elsewhere it is micritic with few fossils. The formation ranges in age from the Givetian to early Frasnian (varcus Zone to the Polygnatus asymmetricus Zone; Selwood et al., 1984). Two members are differentiated locally at the base, the Pulsford Member and the Blair Hill Tuff Member. Blair Hill Tuff Member (BHT) consists of bedded chloritic and calcareous tuff, green where fresh weathering brown and purple, fine to coarse grained, and locally lapilli rich. The Pulsford Member (Pu) comprises grey to dark grey biomicritic limestone in thin lenticular beds with thin interbeds of grey calcareous mudstone. The limestone is pelletal in places and elsewhere crinoidal debris is common. It rests conformably on a Nordon Formation sequence.
Saltem Cove Formation (SC)
The Saltern Cove Formation (FrasnianUpper Famennian) occurs in the South Devon Basin, in both the Northern and Southern sub-basins. It is primarily a deposit of the basin, but to the south it interdigitates with peripheral deposits of the Brixham High, limestone from the Brixham Limestone Formation and tuffs of the Ashprington Volcanic Formation. Some breccia, possibly derived from farther afield, also occurs. The red lithologies of the Saltern Cove Formation are coeval with the grey lithologies of the
Nordon Formation elsewhere in the basin (Figure 2); it is equivalent to the Whiteway Slate of Selwood et al. (1984), but that division included substantial tracts of grey and greenish grey slaty mudstone that would now be assigned to the Nordon Formation.
The formation consists largely of mud-stone and fine-grained siltstone, which is red and reddish purple with local green reduction spots and some grey-green beds. Bedding varies from very thin (laminae) to thick; normal grading is seen locally and some pale laminae of graded fine-grained limestone are developed sporadically. Breccias, up to 2 m thick, occur in the upper part of the coastal sequence at Saltem Cove and Waterside Cove. The breccias are both matrix supported and framework supported. Clasts consist largely of micritic limestone, coralline limestone and fine-grained volcanic rock, and in the coarser beds limestone nodules and rounded blocks are present. The coarse framework breccias show crude grading. A limestone raft, 3.8 m in length, is exposed in one of the thicker matrix-supported breccia beds (Plate 2).
Similar red lithologies in the Plymouth district to the west, have been interpreted as terrestrial detritus introduced by turbid flow into the basin environment in which the grey lithologies represent the marine background settlement deposits. The breccia beds are mass flow and slurry deposits. Derived limestone clasts yield fossils of Frasnian and Famennian age (Van Straaten and Tucker, 1972). The younger of these may indicate shallowing over the Brixham and Torquay highs and renewed carbonate sedimentation. However, it is more likely that the limestone clasts were derived from an active high on the north side of the South Devon Basin (Figure 1); (Figure 2) related to the development of the Tavy Basin farther north, after the drowning of the South Devon Basin highs and infilling of the basin (Leveridge et al., 2002)
- Sattern Cove [SX 895 584] to Waterside Cove [SX 896 587] coastal type section
- coastal section at Anstey's Cove [SX 936 647] It is bound to north, south and east by steep faults, overthrust by Torquay Limestone Formation, and intruded by the Black Head microgabbro
- Petit Tor Beach [SX 927 624] south-westwards towards St Mary Church. It is bound by steep north-west-trending faults, and overthrust by the Petit Tor Downs limestone to the south; to the north it is overlain unconformably by Permian breccia.
Intrusive igneous rocks
Microgabbros (dolerite; D) show similar composition to the lavas, and in places are closely associated with basic tuffs (V), for example at Black Head on the Torquay promontory the Black Head microgabbro is intruded largely into the Upper Devonian Saltern Cove Formation.
The extrusive and intrusive basic igneous rocks of the district compare with those elsewhere in the passive margin sequences of south Devon and central Cornwall (Leveridge et al., 2003). They are predominantly within-plate alkaline basalts, with subordinate sub-alkaline basalt with geochemistry indicative of a mantle source. The magma is an Ocean Island Basalt type, which in a continental plate setting implies strong extension (Merriman et al., 2000).
The Permian outcrops in two sedimentary basins or cuvettes and also as a number of small outliers. The rocks include breccia and red sandstone, which rest with marked unconformity on the underlying Devonian rocks (Plate 3), and represent some of the older parts of the Permian system. An unconformity occurs at the base of the Watcombe Formation (Plate 5), which forms the lowest part of a younger suite of Permian rocks at Torbay.
Torbay Breccia Formation (TB)
Breccia and conglomerate make up most of this formation. Typically, they occur as fining upwards units, from a coarse base that may show evidence of erosive channeling into an underlying bed. The extent to which bedding is evident varies considerably and a crude cross-bedding is common in places. Clasts are commonly rounded and consist of Devonian limestone, sandstone and slaty mudstone with subordinate vein quartz, homfels and chert. Quartz-porphyry clasts are also present in several localities, and are locally abundant, for example at Livermead Head. The matrix is generally red-brown in colour consisting predominantly of sand with variable amounts of silt and clay Where the matrix is cemented with iron oxide the rock is friable and soft-weathering, but where there is a carbonate cement the beds are more indurated and have locally been worked for building stone. Substantial red sandstone units are included within the Torbay Breccia Formation; locally these show large-scale cross-bedding, indicative of an aeolian origin. At the coast, such sandstones may be seen at Goodrington [SX 8962 5802], where they exceed 35 m in thickness, and at Oddicombe Beach [SX 9256 6592]. The base of the Torbay Breccia Formation is seen at the northern end of Saltem Cove, where coarse breccias, dominated by limestone and sandstone clasts (Plate 4a), rest on Lower Devonian mudstone and sandstone of the Meadfoot Group. At several localites to the south of the main Permian crops, Devonian limestone contains palaeokarst features infilled with red-bed material. Of these, the most striking is at Shoalstone Beach, where 'dykes' within the Devonian limestone are infilled with sandstone, siltstone and comb-layered carbonates. In the St Mary Church area [SX 918 659], evidence of a strong pre-Permian topography has been exposed where remnants of the Torbay Breccia Formation was preserved before deposition of the overlying Watcombe Formation. At Oddicombe Beach and near Edginswell steeper dips towards the top of the Torbay Breccia Formation, indicate tilting before the deposition of the Watcombe Formation.
The breccia and sandstone of the formation are interpreted as debris flow or flash flood deposits within braided river systems, interleaved with fluvial and aeolian dune sands.
At the coast, it is possible to identify a number of lithological units, but inland the detailed stratigraphy is obscured by the urban development of Torquay and Paignton.
The Corbyn's Head Member (CH) is some 15.0 m thick around the headland. Medium- to coarse-grained bedded sandstone that varies in colour from purplish and reddish brown to buff, pale grey and greenish grey occurs in beds that are generally less than 0.5 m thick (Plate 4b). It passes up into beds of clast-supported conglomerate. The sandstone beds commonly show cross-bedding and channeling with lags of pebbles or very coarse sand. Locally, the sandstone contains irregular lenses of pebbles and cobbles of sandstone, vein quartz, limestone, chert and quartz-porphyry, and intraformational clasts of red-brown mudstone. Thin beds of red-brown or grey-green mudstone and siltstone, with desiccation cracks, are present in places. The clast-supported conglomerate beds comprise fairly well-rounded lasts, up to cobble size, of red siltstone and sandstone, chert, limestone and red-stained quartz-porphyry in a polymict sand matrix. The section at Corbyn's Head is interpreted as fluvial or delta plain sediments passing upwards into coarser alluvial fan deposits.
Watcombe Formation (Wat)
The Watcombe Formation is present in the northern part of the district, where it rests, locally with marked unconformity, on the Torbay Breccia Formation. The formation is of variable lithology: much of the material inland is coarse breccia, with clasts of sandstone, slate, limestone and rare quartz porphyry in a poorly sorted matrix of sand, silt and clay. 'Shale-paste breccias', decayed red slaty mudstone lasts in a clay matrix, are present locally. Brown mudstone and siltstone is interbedded with the breccia in places. For example, at Petit Tor Beach, bedding units up to 2 m thick comprise muddy siltstone with coarse sand or fine breccia at the base and interlaminated mudstone and siltstone at the top. Iron rich concretions and reduction spots are common in these units. Impersistent interbeds of pale medium-grained sandstone showing cross-lamination and basal flute moulds are present in the coastal sequence. The breccias to the west of the formation are interpreted as debris flows; the siltstone and mudstone units suggest deposition from turbid flows.
The Petit Tor Member (PTB) was formerly regarded as part of the Devonian limestone sequence, equivalent to the nearby Torquay Limestone Formation because it contains a similar range of fossils (House, 1963). However, it is a breccia at the base of the Watcombe Formation, resting unconformably on the Saltern Cove Formation (Plate 5), and the fossils occur within blocks and clasts. The breccia consists of locally derived limestone blocks, up to several metres across, and rafts and lenses of cleaved red silty mudstone of the Saltern Cove Formation. Locally, clasts form a framework, but elsewhere the breccia is matrix supported, with blocks supported in a sand and pebble matrix that shows a crude bedding. Where the long axes of the clasts lie parallel with the depositional bedding in the coarse framework-breccias, this has been interpreted in the past as primary bedding. Elsewhere, the haphazard orientation of bedding and cleavage in some, or a majority, of clasts, in addition to the sand and silt matrix indicates that these are derived from earlier sedimentary rocks. The breccia is interpreted as a local talus deposit possibly adjacent to a contemporaneous fault scarp.
Oddicombe Breccia Formation (Obr)
The Oddicombe Breccia is restricted to a small area at the margin of the district and, northwards, it passes laterally into the Teignmouth Breccia. It rests conformably on the Watcombe Formation. The breccia is dominated by rounded clasts of Devonian limestone with some clasts of sandstone and quartz-porphyry in a matrix of hematite-stained silty sand. Beds commonly fine upwards and show imbrication of the coarser basal clasts; bedding is mostly planar.
No deposits of Palaeogene or Neogene age have been recognised within the district, although the substantial deposits of the Bovey Basin are developed about the Sticklepath Fault complex a few kilometres to the north-west. However, marine retreat, still-stand features that developed during these periods are ubiquitous, and can be mapped at intervals of a few metres from the highest levels down to the coast or valley bottoms. The lowest four features have been identified as of probable early Pleistocene age (Mottershead, 1977). These are solid rock platforms that are gently inclined seawards and backed by rock cliffs (commonly degraded in part) at about 35, 20 to 25, 10 to 15 and 2.5 to 5 m above OD. Caves within the limestones that occur above the lowest level of unconsolidated late Pliocene deposits in the province (at about 40 m above OD) are likely to be Neogene in age, or older. Sandstone dykes and crudely bedded cavity fills (e.g. Saltem Cove) provide some evidence for the presence of rifts and caves during, or before, the Permian. Recorded maxima in the height distribution pattern of Devon caves can be correlated with pronounced sea level still-stands within the Palaeogene and Neogene at 130 m OD and 70 m above OD, and indicate their active formation during these periods (Leveridge et al., 2002). Cavities and phreatic tubes can be seen in the district above 40 m OD. They are developed along faults, joints and bedding in quarries around Babbacombe, in the Walls Hill Limestone Member, and are also commonly encountered in borehole core (BGS records).
Quaternary (Pleistocene and Holocene)
Several substantial cave systems have been recorded within the district; some have subsequently been removed by quarrying (Lloyd, 1933). Kent's Cavern complex is of national importance because of its wealth of faunal remains and record of human habitation extending back some 450 000 years; flint axes are attributed to the time of Homo Heidelbergensis. It is the oldest scheduled ancient monument in Britain. The entrances are at about 55 m and 58 m above OD. The system is now thought to have been initiated 2 million years BP, in the Neogene period. It comprises a complex of caverns, rifts along faults and phreatic tubes with wall and ceiling decoration, multiphase roof breakdown and stalagmite formation, and flow-stone floors. The crude stratigraphy present in the system is essentially of Quaternary age, and comprises interdigitated lithic breccia, cave earths, and stalagmite floors. The oldest animal remains within the breccia are of the cave bear (Ursus deningeri) dating back some 500 000 years. Other remains include mammoth, rhinoceros, lion, hyena, horse, wolf and fox. Neanderthals and Homo sapiens occupied the caves in the Ipswichian interglacial and early Devensian stages (125 000 to 72 000 years BP).
Clastic deposits long recognised as raised-beach deposits are present at several localities (Lloyd, 1933) but are sufficiently extensive only at Hopes Nose (Plate 6) to be depicted on the 1:50000 scale map. The base of the Hopes Nose deposit lies at about 9 m above OD; this is the second raised shore platform of the district, as there are nearby remnants of the lower platform most of which has been removed by erosion. The deposit comprises thin to medium beds of carbonate-cemented sand with pebbles, dispersed and as layers. This overlies a basal bed of limestone boulders and cobbles with slate and quartz vein fragments in a coarse sand matrix. Shell debris is locally abundant in the sandstone, Ostrea being common. This sequence is about 4 m thick. It is overlain by unconsolidated fine-grained sand, possibly blown sand, up to 1.2 m thick. The succeeding 2 m of silty sand with rock fragments is a remnant of an overlying head deposit. This sequence is typical of the lower raised shore platforms of the province. Between Shoalstone Point and Berry Head, small remnants of raised beach are present at about 7 m above OD, overlain by blocky limestone head.
Head comprises poorly stratified or unstratified deposits of clay, silt, sand, gravel and locally derived angular lithic clasts, up to block size, transported downslope by solifluction under Pleistocene periglacial conditions. Only the thicker deposits are shown on the 1:50 000 scale map. This occupies valley bottoms or mantles the coastal platforms below about 35 m OD. Significant head deposits are present as residual valley fill at Paignton [SX 891 619] and Torre Abbey [SX 908 638], and as raised platform deposits near Black Head [SX 937 645] and Kingswear [SX 893 503]. Elsewhere, these head deposits have been ascribed to the onset phase of the Devensian glacial period (Leveridge et al. 1990).
Head or regolith of local rock fragments in clay matrix also forms a mantling deposit in much of the district, developed mainly in early postglacial times. It is generally less than 1 m thick, but exceptionally may be up to 2 m on lower hill slopes.
The river valleys were overdeepened during the Pleistocene when sea level retreated, and fell to more than 120 m below OD (Evans, 1990). With regression, after the late Pleistocene Devensian glacial stage, river, alluvium was deposited in the valleys, interfingering with marine sediments in the lower reaches. Boreholes at Maypool, near Dittisham, record solid rock at depths of about 35 m below OD, and downstream, at Kingswear, about 1.5 m of clay with boulders is succeeded by some 16 m of silt. There are records of submerged forest, and tree stumps rooted in grey clay can be seen during exceptionally low tides along the beaches of Torbay (Lloyd, 1933). Radiocarbon dating of wood, from similarly located deposits in Cornwall, yields an age of 4278 ± 50 years BP (Goode and Taylor, 1988). Within the alluvial spreads at Paignton, Goodrington and Torre Abbey, up to 6.5 m of peat is recorded in boreholes, within sequences up to 21 m thick. The alluvium comprises silt with lithic clasts, and towards the top there is interbedded silt and mud with marine shells.
Extensive sand and gravel terracing is present in the valleys of the River Dart and contributary streams above Totnes. Gravels comprise variable proportions of slaty mudstone, limestone, basic volcanic rock and granite. Minor featuring suggests that the lower terrace deposits extend some 2 km north of Totnes and probably represent a separate terrace level lower than the main terrace to the west near Staverton. The superficial deposits rest upon a platform, or platforms, of solid rock above the level of the modern (Holocene) river alluvium. The solid rock is exposed in small 'cliffs' between terrace deposits above and Holocene alluvium below. The age of the terrace deposits and solid terracing is uncertain, but the general height of the latter suggests correlation with the coastal Pleistocene raised shore platforms.
Alluvium, the deposit of modem river floodplains, is present along the valley floors of the rivers and larger streams in the district. The main tracts are above tidal reach on the River Dart and its tributaries; south of Totnes alluvial deposits are subject to reworking by marine processes. Generally, the alluvium can be divided into a lower unit of coarse gravel and an upper unit of clay and silt, which may contain thin gravel beds or lenses of peat. The alluvium of the River Dart and its tributaries consists of about 3.0 m of silt and clay resting on up to 2.0 m of coarse gravel, which in turn rests on bedrock. Small areas of Alluvial Fan Deposits are also recorded.
Tidal river and creek deposits flank the River Dart occupying adjacent creeks. Mud with rock debris derived from the solid and regolith of the riverside cliffs constitute the major part of these deposits, with sand and shell banks sparsely developed in the lower reaches.
Saltmarsh deposits occur in small areas along the River Dart in its higher tidal reaches between Dittisham and Totnes. They comprise brown and grey silty clay, supplied by regular tidal inundation and fixed by plant growth.
Shoreface deposits comprise sand and gravel, the proportion of the former generally being larger to the south. The sand is predominantly quartz, with subordinate shell fragments and local lithic grains. A pale reddish brown hue is imparted by iron oxide staining inherited from the local Permian rocks.
The low alluvial tracts behind modern sea defences are shown as Marine Coastal Zone Deposits, although in many cases this is thin, and consists of a capping of silty clay and blown sand on river alluvium.
Offshore the sea bed sediments (see inset figure on Sheet 350 Torquay) consist predominatly of sand. North of Berry Head, gravel rests directly upon solid rock and forms a significant spread extending northeast from the mouth of Tor Bay. Mud occurs in some eastern parts of the bay. The thickest deposits (over 15 m) lie at the foot of a still-stand cliffline more than 25 m below OD, which runs north-south on the seaward side of the bay.
Cut and fill deposits are associated with the main railway line in the Littlehempston and Totnes areas, the coast line between Torquay and Kingswear, and sections of the dual carriageway of the A3022 and A380 Torbay by-pass.
Quarrying operations in the district have generally left little visible spoil. The variety of uses of the limestone, building stone, aggregate and lime burning consumed most of the rock quarried, and coastal quarries disposed of waste in the sea. Those areas in the Hele district of Torquay, where the Watcombe Formation was formerly exploited for brick making and pottery have mostly been reclaimed and used as building sites. Mining spoil associated with the Sharkham Point ironstone workings has been dispersed or used to infill small excavations in the limestone.
Domestic and construction waste makes up the substantial disused waste tip [SX 902 564] to the north-west of Hele. Locally it is up to 12 m thick but it is possibly thicker in places as it infills part of a head basin and former brick workings. There are several other disused tips in the district, as on the alluvium at Goodrington [SX 888 594] or in disused quarries as in the Walls Hill Member at Babbacombe [SX 933 654]. Licenced tipping is currently active on the Nordon Formation on the lower slopes of the River Hems valley, 1 km north of Totnes [SX 808 617].
The cross-sections shown on Sheet 350 illustrate something of the structural complexity of this district. Early workers recorded cleavage, folding and faulting and inferred the presence of major thrusts and overfolding to explain how older rocks came to overlie younger strata (Ussher, 1903; Lloyd, 1933). Richter (1969) described cleavage, folds and faults, and recognised four deformation phases in the rocks of the region. These are part of the Variscan structural sequence (Figure 5).
A major compressive deformation migrated northwards into the Culm Basin of north Cornwall and central Devon during the early Carboniferous (Dinantian) forming northward-verging folds, cleavage, thrusts and north-west-trending, dextral, strike-slip faulting. The inversion of the Looe Basin within the district produced the Man Sands antiformal structure at the out-thrust front. The capping deposits on the Brixham High, were then thrust off their basement by 'shortcut' thrusts over deposits of the southern sub-basin of the South Devon Basin (Figure 1). A similar process subsequently affected the Torquay High, with platform and reef carbonates thrust northwards over deposits of the northern sub-basin. Later in the Carboniferous (Silesian), as the whole extensional rift system closed, continuing stress from the south produced the second northerly directed overriding deformation, which in the Torquay district was close to co-axial with the first. It is probable that the early thrusts were reactivated, and further flat-lying thrusts also developed, transporting some of the deposits of the highs still farther to the north. The full inversion of the CuIm Basin was associated with southerly out-thrusting and gravitational collapse southwards. The third phase structures in the district represent part of this process during which some of the earlier faults probably also became slides accommodating southerly transport.
Following the late Carboniferous compression, the north-south extensional rebound of the province was accompanied by intrusion of the granites, in the latest Carboniferous and early Permian times, with backslip on earlier thrusts and steep east-west faulting. It is probable that the east-west faults bounding the main Lower Permian outcrops around Paignton and to west of Torquay developed at this time. There appears to have been rotation within these fault-bound blocks, which produced the angular unconformity between Lower and Upper Permian rocks. Further details of the tectonic relationship of strata are given in Leveridge et al. (2003).
Variscan events are summarised in (Figure 5). Major folding is evident to the north of Totnes, where limestone members show a faulted synform and antiform with overturning in the common limb. The presence elsewhere of large-scale folding is indicated, for example, by the inversion of the Barton Limestone Member at Babbacombe, and the normal relationship of Sr deavage and bedding in the Saltem Cove Formation at Waterside Cove. The largest structure affects the Meadfoot Group and Tamar Group of the Looe Basin Succession, south of the thrust contact with successions to the north. The hinge zone of the north-vergent antiform is between Man Sands [SX 923 534] and Long Sands [SX 922 527] within the Bovisand Formation. Part of the overturned northern limb forms Southdown Cliff (Plate 7), which displays parasitic minor Ft folding. The inverted limb includes much of the Staddon Formation, the Dittisham Member and the Greenway Tuff Member. Dr structures are ubiquitous, and the sporadic structures of D2 and D3 are observed to interfere with them on a a mesoscopic scale (Plate 8a).
The Sticklepath Fault passes through Torquay, and is mapped as a zone, 1 km wide, within which there are several faults. The trace of the main strand of the fault is beneath Torre railway station and Shiphay Hospital where post-Variscan movements have produced a fault zone some 10 m wide, recorded in excavations. The Sticklepath Fault is one member of a set of regional north-west-Mending faults that are attributed to syn-D1 deformation (Leveridge et al., 2002). The relative positions of the Torquay and East Ogwell limestone formations indicate a cumulative residual minimum displacement of 5 km on the Sticklepath Fault; displacement is dextral and strike-slip.
D1 and D2 Thrust faults
Mesoscopic thrust faults are associated with F1 and F2 folds (Plate 8b), but the thrusts that bound major nappes are not well exposed. The presence of the major thrust faults is generally indicated by stratigraphical and structural geometrical relationships, but faults with low to moderate dip have also been observed in borehole cores.
The Looe Basin Succession is thrust northwards on to deposits of the Brixham High and adjacent South Devon Basin (Southern Sub-basin). Between Dittisham and Brixham the uppermost deposits of the Looe Basin are preserved locally, but elsewhere these are overridden and occluded by overthrust Staddon Formation. Thrusting between successions in this belt is considered to be a combination of Dr and ID2 structures. Within the Looe Basin Succession, the boundary of the older Dartmouth Group with the underlying younger Bovisand Formation is interpreted as a thrust, both divisions in the vicinity of the boundary are the right way up, and inclination of the boundary is commonly different from that of the bedding. The Brixham High limestone and volcanic rock divisions are similarly overthrust northwards. There are also significant thrusts within these sequences, as between the Middle Devonian Berry Head Member and Upper Devonian Churston Member of the main platform sequence, and between the main body of Brixham Limestone Formation and the back reef Goodrington Member. To the west, the lower boundary of the Ashprington Volcanic Formation is generally gently inclined, whereas bedding within the formation and in the underlying rocks dips moderately to steeply. The thrust sheet is interpreted as having been derived from the Brixham High, now overthrust by the Staddon Formation, with evidence displayed of its roots in the Dittisham area to the south. The thrust front of the nappe passes west to east through Totnes and Longcombe [SX 836 600] where it is displaced by steep faulting. It is possible that the main thrust nappe, including the undivided Brixham Formation, continues south-eastwards. The flat-lying thrust-bound Meadfoot Group about Paignton, thought by Coward and McClay (1983) to be derived from the main Meadfoot Group to the south, is probably also part of the thrust nappe family derived from the Brixham High. This is because the Staddon Formation, which is essentially a deposit of the Looe Basin, remains in sequence, and the lower metamorphic grade of these Meadfoot Group rocks is incompatible with derivation from the southern part of the district.
The overthrust nature of the Torquay High rocks in relation to basinal deposits of the South Devon Basin (Northern Sub- basin) is evident west of the Sticklepath Fault (Figure 4) where the limestone sheet boundaries are planar, and the juxtaposed rock sequences differ in age. East of the fault in the Torquay area, Meadfoot Group rocks are present within at least two thrust slices; they are again of lower grade than those to the south, and are included within the nappe family derived from the Torquay High.
Late Variscan and post-Variscan structures
The deformation history of the district following the main compressive Variscan deformation is shown essentially by the faults. The faults fall into three main trends, are moderately to steeply inclined and there is evidence of multiple phases of movement.
E–W faults control the distribution of Permian rocks, indicating significant movement during or after deposition of these rocks. In the Babbacombe area, the moderate northerly dip of the Lower Permian and the gentle northerly dip of the unconformable Upper Permian in an area constrained by east-west faulting suggests syndepositional movement on these faults. Significant also in this regard is the east-west fault between Babbacombe and Shiphay, which bounds limestone in the Hele area within which bedding and cleavage have been rotated from a southerly to northerly dip.
N–S faults are not extensive structures but are locally numerous (e.g. Goodrington, and eastern Torquay), and terminate against or displace east-west faults. A near-vertical fault of this general trend in the coastal cliffs between Broadsands and Saltern Cove has a fault gouge and brecciation up to 30 m wide in subzones indicative of polyphase movement. Locally, at Crystal Cove [SX 896 580], the whole of the fault zone is replaced by carbonate, reflecting its role as a major fluid pathway (Plate 9).
NW–SE faults were established early in the deformation history of the district and were the focus of subsequent movement during appropriate stress conditions. In this district, post-Variscan movement along these faults was significant as indicated by displacement and disruption of the Permian sequence.
The deformed rocks of the district exhibit low-grade metamorphism (Figure 6), as elsewhere within the Variscan sedimentary basins of the region. The metamorphic grade of pelitic rocks is shown by the Kubler Index of illite crystallinity (Merriman and Kemp, 2001). The distribution of late diagenetic to epizone grades shows a general decreasing trend from south to north. This pattern appears to be modified by displacements on the north-west-trending faults to a degree not reflected in displacement of lithostratigraphical units, at least along the Dart Valley Fault. The regional metamorphism is largely a measure of the reconstitution of the rock body associated with the development of S1 cleavage during the first phase of deformation (D1), but also reflects the initial burial and thickness of sedimentary overburden.
The epizone to the south is largely within the thrust nappes derived from the Looe Basin, but also includes the immediately adjacent, and probably overridden, areas to the north. These epizonal rocks were formed at temperatures of 300 to 350°C under a tectonic overburden of at least 7 km. The limited extent of high anchizonal rocks to the north includes rocks of the Brixham High and South Devon Basin (Southern Sub-basin). The anchizonal grade implies burial under an overburden thickness of at least 5 kin, generated by the Looe Basin inversion or during D2 out of sequence thrusting.
Low anchizone rocks to the north include rocks from the Brixham High and South Devon Basin (Southern and. Northern sub-basins). Isolated within that zone is a significant area of high anchizonal rocks to the north of Totnes, probably a reflection of the burial of older rocks in the deeper part of the Southern Sub-basin of the South Devon Basin. Also within the low anchizone is the Meadfoot Group of the 'Marldon Nappe', an indicator that it was probably not derived from the Looe Basin.
The low anchizone late diagenetic zone rocks of Torquay show the lowest metamorphic grade of the district, and include the oldest rocks of the Torquay High nappe family. The grade indicates that the Meadfoot Group of Torquay was not, in relative terms, deeply buried prior to derivation from the high. The corollary is that these rocks are not an extension of the 'Marldon Nappe', but may reflect dextral transport along the Sticklepath Fault from lower metamorphic grade rocks to the north. Fault reactivation may also have caused localised retrogression in some of these rocks.
The Torquay district lies at the eastern end of the South-west England Metalliferous Province, but beyond the group of tin and base metal deposits that are genetically associated with the Dartmoor Granite. In consequence, the range of deposits and mineral occurrences are restricted, and most derive from the movement of low-temperature hydrothermal fluids in and around bodies of Devonian limestone. The deposits include iron ore deposits and iron-rich ochre. Gold has been recorded from carbonate veins in the Devonian limestone.
Iron ore and ochre
The main iron ore deposits of the district occur at Sharkham Point. They are well developed replacements of the limestone by hematite grading into unaltered rock. Elsewhere there are breccias of limestone, slaty mudstone and tuff, with localised replacements and botryoidal overgrowths of goethite, limonite and hematite. Some joint surfaces show aggregates and coatings of white or pale pink baryte crystals.
At Hope's Nose, gold veins occur as a swarm in which the individual veins are separated by up to 20 m of host limestone, irregular pods and lenses within steeply the mineralisation is restricted to the inclined fracture zones. Within the carbon-massive stromatoporoid reef facies of the ate veins, the occurrence of gold is associDaddyhole Member. Individual veins trend ated with buff- to cream-coloured calcite roughly east-west and are developed as and dolomite, usually of saccharoidal texture. Cavities in the gold-bearing carbonate may be filled with yellow-brown iron oxide minerals, together with traces of goethite and hematite.
High levels of palladium are associated with the veins (Clark and Criddle, 1982; Scrivener et al., 1982), and rare palladiumarsenic-antimony minerals, isomertieite and mertieite-II occurs as inclusions in certain of the gold specimens. Examination by Stanley et al. (1990) led to the identification of a suite of precious and base metal selenide minerals as minor components of a small number of the carbonate veins. Leake et al. (1991) have drawn attention to the complex concentric nature of palladium-rich zones in gold grains from Hope's Nose, which is identical to that shown by gold, palladium and platinum grains collected in drainage geochemical surveys of the adjacent South Hams district of South Devon.
The mineralogy and ore textures of the iron and gold occurrences described above are consistent with mineral precipitation from low-temperature hydrothermal fluids. In the case of the Hope's Nose gold occurrence, this is confirmed by fluid inclusion studies on quartz and carbonate (Scrivener et al., 1982), which indicate that the mineralising fluids were highly saline, calcium chloride-rich brines in the temperature range from 65° to 120°C. In the case of the iron ore deposits of Sharkham Point, there is no evidence from fluid inclusion or geochmnological studies, however, the association of iron oxide minerals with baryte, the limestone host rock and the proximity of red beds, suggest similarities with the Llanharry-Taff Wells mineralisation of South Wales. At Llanharry, the hematite-goethite ores were formed from highly saline fluids, at temperatures not exceeding 100°C, with evidence for the presence of Ca/Mg chloride in the mineralising brines (Rankin and Criddle, 1985). Both the gold and iron oxide mineralisation appear to be genetically linked to basinal brines, which accumulated in superincumbent red beds, and which migrated into the Devonian basement during extensional fracturing, possibly during the Triassic period.
Chapter 3 Applied geology
Geological factors that have a bearing on land use and development are reviewed briefly in this section.
Mineral products from the Torquay district have included building stone, roadstone and aggregate, brick making material, pottery clay, mineral pigments and iron ore. These mining operations are now closed with the exception of the limestone quarry at Yalberton which still produces a small quantity of stone.
Large quantities of Devonian limestone were formerly exported from the coastal quarries for use as building stone, and it has also been used locally in the sea walls and other structures. Some of the coloured limestones were cut and polished for ornamental use as 'Torquay marble'; these were worked mainly from quarries at Ipplepen and Petit Tor. Permian breccias and Devonian sandstones have been used in the construction of walls, and cleaved Devonian mudstone was formerly quarried near Brixham for roofing slates.
Roadstone and aggregate
Devonian micogabbro and the harder volcanic rocks have been worked for roadstone from a number of quarries. Devonian limestone is a source of general aggregate, but present-day production is small and spasmodic.
Brick making material
Brick clay has been worked extensively in the area around Hele from the clay-rich parts of the Permian Watcombe Formation.
Much of this material appears to have been weathered breccia of reddened fissile mud-stone fragments in a clay-rich matrix, 'shale paste breccia'. The Watcombe Formation has also been worked for clay, which was used in the manufacture of terracotta ware; other brick pits exploited weathered mud-stone from the Lower Devonian near Claylands Cross, Paignton.
Iron ores and ochre
Iron ore and ochre were worked commercially during the 19th and early 20th centuries, though details of the occurrence and mineralogy of the deposits are sparse. The most extensive workings were at Sharkham Point, Brixham, worked between 1858 and 1914; the material was used for paint manufacture and the harder iron ore for smelting. At Sharkham Point, the ore was worked from open pits and underground galleries, drained by adits to sea level. The ore was irregularly developed along an east-west zone, some 600 m in length. The open pits in the western part of the district have been backfilled with waste, but in the east examples of the mineralisation can be seen in old mine workings. Iron ore deposits were worked at other localities, which yielded ores of the Sharkham Point type, or iron oxide-rich clay (ochre) or a mixture of both.
Native gold has been collected from narrow carbonate veins that cut through the Middle Devonian limestone at Hope's Nose (Plate 10).
Hydrogeology and aquifer vulnerability
The best yielding aquifer in the district is the gravel of the river terrace deposits, and groundwater is abstracted for public supply from Brixham spring [SX 917 549] and the Littlehempston radial collector wells [SX 800 617] and [SX 805 626]. The aquifer comprises two sand and gravel horizons generally separated by 1 to 3 m of silt. A yield of 45 1/s for 5.2 m of drawdown was recorded during a pumping test. Water from the river gravels is generally soft, with total hardness less than 100 mg/l (as CaCO3) but this may increase to over 200 mg/l where water is fed into the aquifer from adjacent limestones. The chloride ion concentration is generally less than 20 mg/l. As the aquifer is in hydraulic continuity with the rivers, it is liable to surface water pollution and potentially high suspended solids and nitrate concentrations.
A high water table can be expected in some of the other superficial deposits, such as alluvium, colluvial valley infill and the former salt marsh and lagoonal deposits. These do not represent major groundwater resources, but the height of the water table is likely to act as a restraint on development.
The sandstones and breccias of the Exeter Group (Permian) form potential aquifers, and many private supplies are drawn from bore-holes into the Torbay Breccia Formation in the Paignton area. Groundwater from the group is hard, but generally of good quality, with a total hardness over 250 mg/l (as CaCO3) in the breccias and sandstones and over 300 mg/l in the Watcombe Formation. Chloride ion concentrations exceed 50 mg/l near the coast.
The Devonian rocks of the district account for only 4 per cent of the water licensed for use in the area, but most of the formations have been utilised for small supplies. Groundwater occurs in joints and fissures, as the primary permeability of the rock is extremely low. Slates initially tend to have higher yields than grits as they are more competent and fracture more readily. However, yields from the slates are generally only sustainable for short periods of time, and tend to decrease in dry weather. Water from the slates have a low total dissolved solids concentrations, similar in composition to the surface waters.
The water-bearing capacity of the Devonian limestones depend on a well-developed series of fractures and bedding planes that have been enlarged by solution, as they have little primary porosity. No permanent surface streams exist on the limestone plateaux; the ephemeral nature of the streams indicates the presence of a subsurface drainage network within the limestone. The limestones respond rapidly to recharge events as their storage capacity is limited, they also discharge water rapidly to the slates and the sea via submarine springs. Borehole yields vary widely, depending on whether water-bearing fissure systems are encountered. Water from the limestones is hard, with pH values over 7 and bicarbonate ion concentrations of up to 300 mg/l. A large part of the outcrop of the limestone is along the coast where the aquifer is likely to be in hydraulic contact with the sea. Therefore sources could be contaminated by sea water with increased chloride ion concentrations caused by saline intrusion occurring as a result of groundwater abstraction reversing the natural hydraulic gradient. Tidal fluctuations in groundwater levels may occur. The volcanic rocks and microgabbros also yield small supplies where they are unweathered and fractured. The Devonian tuffs and lavas around Tothes were historically important for water supply.
Vulnerability to pollution
There are two main types of groundwater pollution, diffuse and point source. Diffuse pollution is caused by the application of fertilisers and pesticides, and locally there is a nitrate problem in the shallow fractured aquifers of this area. Point sources of pollution such as landfill sites and leaking storage tanks also represent a threat to groundwater quality, particularly those sited on the Devonian limestones.
Limestone solubility affects all of the limestone sequences to varying degrees, which presents significant potential foundation hazards. There is a long history of cavity formation and dissolution by groundwater movement (see Cainozoic). Around the coastal sections active cave formation and dissolution along fractures is evident, and there is reported evidence that site investigation boreholes encountered tidal movement of water onshore at depth within the Brixham Limestone Formation. Such movement of water is particularly relevant to any potential tunneling operations in Torquay, where interthrust sequences, which include limestone that does not crop out at surface, are cut and bounded by faults some of which are connected with the coast.
Compressible ground within the superficial sequences means that no significant constructional development can proceed without very thorough site investigations. Peat, up to 6.5 m thick, occurs at depth in the alluvial tracts of Torre Abbey, Paignton and the Clennon valley at Goodrington. In the Clennon valley, the peat occupies a buried channel cut in alluvial clay, which runs north-south rather than east-west along the modern valley.
Slope stability within the Devonian rocks is dependent upon the depth of the excavation and its orientation with respect to bedding, cleavage, joints and faults within the rock. Within the upper weathered zone, shallow trenching is commonly unstable in the argillaceous rocks, particularly where the cutting is subparallel to the cleavage allowing the opening of cleavage and cross-cutting joints. Because of the generally high concentration of structural discontinuities within the rocks, a detailed structure analysis is essential in all the deeper excavations and in all lithologies in order to identify the potential roles of plane failure, wedge failure and toppling failure.
Landslips are recorded in all of the major lithofacies that crop out along the coast. Geological control on instability is a combination of bedding and cleavage plane discontinuities, joints and faults. The hazard potential of landslip in this popular holiday area is very significant.
Redgate Beach [SX 935 648] has been closed due to active landslipping of the Torquay Limestone Formation. In the upper parts of the cliff above the beach large limestone blocks that are tens of metres across have become detached along faults parallel to the cliff and related steeply inclined joints, and are moving eastwards downslope on subjacent clayey weathered slaty mudstone. The current backscarp of the slip is behind and west of the crest of the cliff-top hill, which makes a substantial volume of rock available for future migration and collapse seawards on to Redgate Beach.
At the northern end of Oddicombe Beach, the steep cliffs of Giant Rack [SX 926 660] in the Permian Torbay Breccia Formation are subject to toppling failure (which happened in 2001) on steeply inclined joints. Landslip deposits flanking the promontory impinge upon the popular tourist beach, where they are subject to erosion and remobilisation.
At St Mary's Bay, the cleaved mudstone of the Nordon Formation strikes east–west through the beach section that trends north–south. The cliffs are degraded having been subject to a series of rotational slips in which both strike and dip are very variable, and there has been collapse on rotated and opened joints striking subparallel to the coast. The area of designated landslip shown on Sheet 350 Torquay is active. Other coastal slips are also active in the sandstone and finer grained facies of the Bovisand Formation at Southdown Cliff e.g. [SX 540 926] and the mudstone of the Dartmouth Group e.g. Compass Cove, [SX 884 494].
Inland there are some active landslips on steeper slopes, for example within the Permian along the Clennon valley west of Paignton [SX 865 612].
Geological advice for this area should be sought from the District Geologist, The Exeter Business Centre, Forde House, Harrier Way, Sowton, Exeter, EX2 7HU. Enquiries concerning geological data for the district should be addressed to the Manager, National Geological Records Centre, BGS, Keyworth. BGS catalogue of geological maps and books is available on request, and gives a full list of the geochemistry, geophysics, hygrogeology and mineral maps that are currently available. Other data may be accessed on the BGS web site including a Lexicon of named rock units, information on borehole records and core, and data on other BGS collections, including access to the photographic collection (addresses on the back cover). Further information relevant to the Torquay district is listed below.
1:10 000/1:10 560
The 1:10 000 scale maps covering the 1:50 000 Series Sheet 350 are not published but are available for public reference in the libraries of the British Geological Survey at Keyworth, Edinburgh and Exeter, and the BGS London Information Office in the Natural History Museum, South Kensington, London. Uncoloured dyeline sheets or photographic copies are available for purchase from the BGS Sales Desk. Copies of the fair-drawn maps of the earlier surveys may be consulted at the BGS Library, Keyworth.
The Torquay memoir (Sheet 350), (1903, second edition 1933) is out of print but a facsimile copy may be purchased from BGS Library at a tariff that is set to cover the cost of copying.
Technical Reports relevant to the district are not widely available but may be consulted at BGS and other libraries or purchased from BGS. These include Biostratigraphical reports that are held as internal open file reports. Readers are recommended to contact The Chief Curator, BGS, Keyworth for access to these reports and to the palaeontological collections.
Metamorphism of the Palaeozoic rocks of the Torquay district, Devon, 1:50k sheet 350. Internal Report, IR/01/184.
Borehole data for the district are catalogued in the BGS archives and samples and core from a small number of boreholes in the district are held in the National Geological Records Centre, BGS Keyworth. For further information contact: The Manager, National Geological Records Centre, BGS, Keyworth.
Macrofossil and micropalaeontological samples collected from the district are held at BGS Keyworth. Enquiries concerning all the fossil material should be directed to the Curator, Biostratigraphy Collections, BGS, Keyworth.
Hand specimens and thin sections of rocks from the district are held in the England and Wales Sliced Rock Collection at BGS, Keyworth.
Other relevant collections
Information on licensed water abstraction sites, for groundwater, springs and reservoirs, Catchment Management Plans with surface water quality maps, details of aquifer protection policy and extent of Washlands, and licensed landfill sites are held by the Environment Agency.
Information on Sites of Special Scientific Interest in the Torquay district is held by English Nature, Renslade House, Bonhay Road, Exeter, EX4 3AW.
A full bibliography for this district is given in Leveridge et al. (2003). Most of the references listed below are held in the library of the British Geological Survey at Keyworth, Nottingham. Copies of the references may be purchased from the library subject to the current copyright legislation. BGS Library catalogue can be searched online at: http://geolib.bgs.ac.uk
Castle, C. 1982. Middle and Upper Devonian conodont biostratigraphy of the Torquay area, South Devon. Unpublished PhD Thesis, University of Hull.
Chameernowye, A. 1881. Notes on a find of Homalonotus in the Red Beds at Torquay. Geological Magazine, Vol. 8, 487–488.
Clark, A M, and Griddle, A J. 1982. Palladium minerals from Hope's Nose, Torquay, Devon. Mineralogical Magazine, Vol. 46, 371–377.
Coward, M P, and mcClay, K R. 1983. Thrust tectonics of south Devon. Journal of the Geological Society of London, Vol. 140, 215–228.
De La Beche, H T. 1829. On the geology of Tor and Babbacombe bays. Transactions of the Geological Society of London, Vol. 2, 633.
De La Beche, H T. 1839. Report on the geology of Cornwall, Devon and west Somerset. Memoirs of the Geological Survey of Great Britain, London.
Drummond, M E. 1982. The geology of Devonian limestones of the Brixham-Darlington area, South Devon. Unpublished PhD Thesis, University of Newcastle.
Evans, C R D. 1990. United Kingdom offshore regional report: geology of the western English Channel and its western approaches. (London. HMSO for British Geological Survey.)
Franke, W. 1989. Tectonostratigraphic units in the Variscan belt of central Europe. Geological Society of America, Special Publication, No. 230, 67–90.
Goode, A J J, and Taylor, R T. 1988. Geology of the country around Penzance. Memoir of the British Geological Survey, Sheets 351 and 358 (England and Wales).
Holder, M T, and Leveridge, B E. 1986. Correlation al the Rhenohercynian Variscides. Journal of the Geological Society of London, Vol. 143, 125–134.
House, M R. 1963. Devonian ammonoid successions and facies in Devon and Cornwall. Quarterly Journal of the Geological Society of London, Vol. 119, 315–321.
Leake, R C, Bland, D J, Styles, M T, and Cameron, D G. 1991. Internal structure of south Devon Au-Pd-Pt grains in relation to low temperature transport and deposition. Transactions of the Institution of Alining and Metallurgy. Section B (Applied Earth Science), Vol. 100.
Leveridge, B E, Holder, M T, and Goode, A J J. 1990. Geology of the country around Falmouth. Memoir of the British Geological Survey, Sheet 352 (England and Wales).
Leveridge, B E, B E, Holder, M T, Goode, A J J, Scrivenor, R C, Jones, N S, and Merriman, R J. 2002. Geology of the Plymouth and south-east Cornwall area. Memoir of the British Geological Survey, Sheet 348 (England and Wales).
Leveridge., B E, Scrivener, R C, Goode, A J J, and Merriman, R J. 2003. Geology of the Torquay district. Sheet Description of the British Geological Survey 1:50 000 Sheet 350 Torquay (England and Wales).
Lloyd, W. 1933. The geology of the country around Torquay (second edition). Memoir of the Geological Survey of Great Britain, Sheet 350 (England and Wales).
Merriman, R J, and Kemp, S J. 2001. Metamorphism of the Palaeozoic rocks of the Torquay district, Devon, 1:10 000 Sheet 350. British Geological Survey Internal Report, 112/01/184.
Merriman, R J, Evans, J A, and Leveridge, B E. 2000. Devonian and Carboniferous volcanic rocks associated with the passive margin sequences of south-west England; some geochemical perspectives. Proceedings of the Ussher Society, Vol. 10, 77–85.
Mottershead, D N. 1977. The Quaternary evolution of the south coast of England. 299–320 in The Quaternary History of the Irish Sea. Geological Journal Special Issue, No. 7.
Pengelly, W. 1866. Report of the exploration of Kent's cavern. Proceedings of the Royal Institute, Vol. 7, 309.
Rankin, A H, and Criddle, A J. 1985. Mineralising fluids and metastable low-temperature inclusion brines at Llanharry iron deposit, South Wales. Transactions of the Institution of Mining and the Metallurgy. Section B (Applied Earth Science), Vol. 94. 126–132.
Richter, D. 1967. Sedimentary and facies of the Meadfoot Beds (Lower Devonian) in southeast Devon (England). Geologische Rundschau, Vol. 56, 543–561.
Richter, D. 1969. Structure and metamorphism of the Devonian rocks south of Torquay, southeast Devon (England). Geologische Mitteilungen, Vol. 9, 109–173.
Scrivener, R C, Cooper, B V, George, M C, and Shepherd, T J. 1982. Gold-bearing carbonate veins in the Middle Devonian Limestone of Hope's Nose, Torquay (Abstract). Proceedings of the Ussher Society, Vol. 5,186–188.
Scrutton, C T. 1977. Fades variations in the Devonian limestones of eastern South Devon. Geological Magazine, Vol. 114, 165–193.
Sedgwick, A, and Murchison, R I. 1839. Classification of the older stratified rocks of Devonshire and Cornwall. Philosophical Magazine, Vol. 14, 241–260.
Selwood, E B, Edwards, R A, Simpson, S, Chesher, A, Hamblin, R J O, Henson, M R, Riddolls, B W, and Waters, R A. 1984. Geology of the country around Newton Abbot. Memoir of the British Geological Survey, Sheet 339 (England and Wales).
Stanley, C J, Griddle, A J, and Lloyd, D. 1990. Precious and base metal selenide mineralization at Hope's Nose, Torquay, Devon. Mineralogical Magazine, Vol 54, 483–493.
Ussher, W A E. 1903. The geology of the country around Torquay. Memoir of the Geological Survey of Great Britain, Sheet 350 (England and Wales).
Ussher, W A E. 1907. The geology of the country around Plymouth and Liskeard. Memoir of the Geological Survey of Great Britain, Sheet 348 (England and Wales).
Van Straaten, P, and Tuocca, M E. 1972. The Upper Devonian Saltern Cove gonialite bed as an intraformational slump. Palaentology, Vol. 5, 430–438.
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 Wale, Northern Ireland, and Scotland.The west and east halves of most Scottish 1:50 000 maps are published separately. Almost all BC5 maps are available flat or folded and cased.
The area described in this sheet explanation is indicated by a solid block.
British geological maps can be obtained from sales desks in the Survey's principal offices, through the BGS London Information Office at the Natural History Museum Earth Galleries, and from BGSapproved stockists and agents,
Northern Ireland maps can be obtained from the Geological Survey of Northern Ireland,
Figures and plates
(Figure 1) Devonian rift basins and highs in the region.
(Figure 2) Geological succession of the Torquay district.
(Figure 3) Biostratigraphy of the limestones of the district. Carbonate mud and debris derived from adjacent high; =calcareous mudstone; Br Breccia; ls limestone (age undetermined in the Northern sub-basin); LSB Laminated Shell Beds NB Pre-1985 biozone attribution to stages preceded revision of the Givetian/Frasnian boundary by the International Commission on Stratigraphy. Adjustments are here made accordingly. 1 Current at time of determination; 2 Based on M E Drummond 11982); 3 Based on C Castle (1982); 4 Based on Selwood et al. (1984)
(Figure 4) Diagrammatic sections of the thrust sheets of the Torquay High. a Devonian limestone platform and reef development separating the northern and southern sub-basins of the South Devon Basin, Late Devonian. b Inversion and thrust displacement of the successions within the district and adjacent area west of the Sticklepath Fault, late Variscan. Other structures, barring the late fault separating the Torquay High and Southern Sub-basin successions, are omitted.
(Figure 5) Summary of Variscan deformation events in the district.
(Figure 6) Contoured metamorphic map of white mica crystallinity (Kubler) indices for the Torquay district.
(Plate 1) London Bridge [SX 923 627] viewed from the south-east. Cliffs in the Daddyhole Member, Torquay Limestone Formation, showing a thinly bedded facies and thicker bedded lenticular facies. The steeply inclined beds (about the bridge and at the cliff base on the right) form inverted limbs and the gently inclined beds of the main cliff form the common right way up limb of northward-verging F1 folds (GS1286).
(Plate 2) Large limestone clast (3.8 m in length) in matrix-supported breccia bed in the Saltern Cove Formation, Saltern Cove [SX 8956 5866]. Direction of view towards north (GS1285).
(Plate 3) Lower Permian Torbay Breccia Formation rests unconformably on the Lower Devonian Meadfoot Group at Goodrington [SX 8954 5873]. Direction of view towards the north (GS1287).
(Plate 4a) Torbay Breccia Formation at Goodrington [SX 8957 5878]: thick beds of purple breccia with reddish brown sandstone. Clasts are imbricated, and consist largely of locally derived limestone. Direction of view towards west (GS1290).
(Plate 4b) Corbyn's Head Member of the Torbay Breccia Formation at Corbyn's Head [SX 9075 6322]: parallel lamination and low-angle cross-bedding in purple and green sandstone beds with sporadic granules and pebbles. Direction of view towards the south-west (GS1289).
(Plate 5) Upper Permian Petit Tor Breccia Member (central feature) rests unconformably on the Upper Devonian Saitem Cove Formation (left of centre) at Petit Tor Beach [SX 9365 6648]. The succeeding Watcombe Formation and Oddicombe Breccia Formation (feature top right) dip gently northwards. Direction of view towards the west-north-west (GS1288).
(Plate 6) Pleistocene Raised-beach at Hope's Nose [SX 9475 6330]. Direction of view towards the south-west (GS1291).
(Plate 7) Southdown Cliff [SX 9285 5410]. Sandstone facies of the Bovisand Formation define these mesoscopic folds in the overturned limb of the northward-verging Man Sands Antiform. Direction of view towards the west (GS1292).
(Plate 8a) Gently inclined northward-verging tight D1 fold couplet refolded by steeply inclined D2 folds in limestone of the Berry Head Member, south of Berry Head [SX 9423 5615]. Direction of view towards the west (GS1293).
(Plate 8b) Overturned anticlinal F1 fold verging north-west, displaced along low-angle thrust at Hope's Nose [SX 9471 6332]. Direction of view towards the north-east (G51294).
(Plate 9) Fault zone trending north-south, near Crystal Cove [SX 896 580]. Goodrington Limestone Member on the left is separated from the reddish brown sandstone of the Torbay Breccia Formation on the right by 25 m of crystalline calcium carbonate that occupies the fault zone. Direction of view towards the north (G91295).
(Plate 10) Dendritic native palladian gold from a carbonate vein in Middle Devonian limestone at Hopes Nose Torquay. The delicate gold crystals have been exposed from the calcite host by etching with dilute mineral acid. Specimen is 15 mm in length and is in the collection of Torquay Natural History Society. (Photographer J S Jones; GS1296).
(Front cover) Front cover Thatcher Rock viewed from Ilsham Marine Drive [SX 940 633] looking south-eastwards. Foreground cliffs are in the Meadfoot Group (undivided) and the island comprises rocks of the Daddyhole Member, Torquay Limestone Formation. (Photographer B E Leveridge; GS1284.)
(Index map) Index to the 1:50 000 Series maps of the British Geological Survey
Figure 5 Summary of Variscan deformation events in the district
|ubiquitous; modal strike ENE; generally gently to steeply inclined southwards; axial plane cleavage to F1 folds; slaty in argillaceous beds, pressure solution in arenaceous beds and limestone
|late Diagenetic to High Epizonal metamorphic grade
|minor to major; trend ENE; gentle plunge; asymmetrical close to tight; verge northwards
|minor to major; verge northwards
|NW–SE faults with dextral displacements up to 5 km residual on Sticklepath Fault; subordinate complementary sinistral NNE faults
|sporadic; modal ENE strike; dip south moderate to steep; crenulation cleavage
|minor; trend ENE; moderately to steeply inclined southwards; verge northwards
|minor to major
|sporadic; gently inclined northwards; crenulation cleavage
|mesoscopic couplets developed locally; southerly vergence
|southward steepening of D1 and D2 structures