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Geology of the Mevagissey district — a brief explanation of the geological map Sheet 353 Mevagissey

B E Leveridge, with contributions by R J Merriman, M T Styles, and M E Lewis

Bibliographic reference: Leveridge, B E. 2008. Geology of the Mevagissey district — a brief explanation of the geological map. Sheet explanation of the British Geological Survey. 1:50 000 Sheet 353 Mevagissey (England and Wales).

Keyworth, Nottingham: British Geological Survey. Printed in The UK for the British Geological Survey By Halstan Ltd, Amersham.

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(Front cover) Cover Photograph Portloe looking east from Puckey's Hill [SW 9398 3942] showing the Roseland Breccia Formation in the cove (Photograph B E Leveridge; P662661).

(Rear cover)

Notes

The area covered by the geological Sheet 353 is referred to as 'the district'. National Grid references are given in square brackets: they lie within 100 km squares SW and SX. Abbreviations in brackets refer to symbols used on the geological map. P numbers associated with plate captions and in the text refer to the British Geological Survey photographic archive.

Acknowledgements

This Sheet Explanation is based upon data held for this district at the BGS exeter regional Centre. The accompanying map was surveyed by M T Holder, A J J Goode and B E Leveridge. R J Merriman provided metamorphic data and information, and M t Styles the analysis of geochemical data. M E Lewis provided the hydrogeology text. Cartography was done by I A Longhurst: page-setting by A Hill: A A Jackson is the series editor.

Thanks are due to many landowners, farm ers and householders for property access and helpful assistance during the survey. Karl Williams (sculptor) is thanked particularly for providing the photograph for (Plate 11) (© Karl Williams).

The grid, where it is used on figures, is the National Grid from the Ordnance Survey. © Crown copyright. All rights reserved Ordnance Survey licence no. 100017897/2007.

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

An explanation of sheet 353 (England and Wales) 1:50 000 Series map.

(Rear cover)

Some of the most remarkable rocks in Britain are displayed along this stretch of the Cornish coast. The unusual nature of the rocks between Gerrans Bay and Gorran Haven was recognised by geologists early in the 19th century. A few shelly fossils were retrieved from prominent quartzites and these indicated an Ordovician age. However, in the 1930s, Eileen Hendriks discovered that most of the rocks contained Devonian plant remains, and she suggested that the older rocks had been introduced by thrusting.

The modern interpretation of the geology of the district has played a key role in understanding the evolution of the whole of the Variscan province of south-west England. When Avalonian continental crust began rifting in the earliest Devonian, the Gramscatho Basin was the first basin that formed, and ongoing extension led to the formation of oceanic crust. In the Mid Devonian, the process was reversed with the closure of the basin (part of the Rhenohercynian Ocean Basin) and continental collision in late Devonian to early Carboniferous times.

During this process thick flysch sandstone sequences and olistostromes accumulated at the southern active basin margin. Early Palaeozoic sedimentary, igneous and metamorphic rocks together with contemporaneous Devonian strata and igneous debris are included in the breccias. Ocean floor basalts and sediments were transported northwards in nappes that were thrust onto the northern margin of the basin. During these dramatic events the rocks of the district were deformed and metamorphosed, the process migrating northwards throughout the province during the Carboniferous.

In the late Carboniferous to early Permian a regime of regional extension was established, and granites with their quartz-porphyry offshoots (including the Pentewan elvan) were intruded. Ores—tin and gold—were generated with the granite or concentrated by associated hydrothermal activity.

The topography seen today was sculpted during the Cenozoic era. It records a series of 'still-stands' in marine retreat; the Quaternary was a period of increased erosion, and deposition that included alluvial gold and tin. Head mantles the lower retreat platforms, and alluvium flanks the main rivers.

Chapter 1 Introduction

This Sheet Explanation provides a summary of the geology of the district represented on the geological 1:50 000 Series Sheet 353 Mevagissey, published as a solid and drift edition in 2000. Reference is also made to the Sheet 354 Eddystone, the broad geology of which is shown on small-scale inset maps. The onshore area is underlain by rocks of Devonian age, but post-Palaeozoic events have left their imprint: Cenozoic retreat stillstand featuring is present throughout the area; Pleistocene head occurs on the lowest 'raised' platforms with Holocene deposits in the valleys and forming the beaches.

The district, on the English Channel coast of south Cornwall, is within the Cornwall County Council districts of restormel, to north and east, and Carrick, the latter area being part of Roseland. It is predominantly rural and agricultural. Tregony in the west is the largest village community and was a thriving port until the river Fal silted up in the 16th century. It is now the 'gateway to the Roseland peninsula', tregony Bridge providing the lowest vehicular crossing point on the river. Mevagissey, in the east of the district, is the only town. Formerly a busy fishing port it still supports smaller inshore fishing activities, but its narrow streets and picturesque harbour make it a centre of attraction for tourists. Tourism, albeit on a smaller scale than that of west Cornwall, is an important element of the district's economy, but apart from the holiday accommodation backing the fine beach at Pentewan, little concession to populism is made. The main attraction for visitors is the coastal walking with vistas of Gerrans, Veryan and Mevagissey bays and the dramatic headlands of Nare Head and Dodman Point. The nestling coastal villages of Portloe and Gorran Haven, the unexpected beach haven of Porthluney, a myriad of quiet coves, the sheltered wooded valleys accommodating the 'Lost Gardens of Helligan', Caerhays Castle and gardens, and the fine village of Veryan, all have their devotees.

History of research

The perceptive observation by S J Trist (1818), in the first volume of the transactions of the royal Geological Society of Cornwall, stated that 'the quartzite of Carne has no connexion with our primitive hills'. This led him to question its introduction to the province 'by what mighty operation'. The answer to that question is at the heart of geology in the Mevagissey district.

Rocks of the district, assigned to the 'Grauwacke Group' by De la Beche (1839), attracted the attention of the major geological figures of the 19th century with finds of early Palaeozoic fossils by Peach (1841). Murchison (1846) considered that brachiopods retrieved from quartzites exemplified his 'Lower Silurian' (Ordovician) to which he accordingly designated the rocks of this area, whereas Sedgwick (1852) regarded them as typical of his Cambrian. Sedgwick also described the succession as largely Devonian, Cambrian rocks having been introduced by a 'great fault'. Identification of Silurian ('upper Silurian') fossils in limestones at Porthluney by Green (1904) led Reid (1907) to amend the then accepted Ordovician age attribution to 'Lower Palaeozoic'. Conglomerates and breccias extending between Gerrans and Mevagissey bays were, however, assigned to unconformable Grampound Grit of Devonian age. Hendriks (1931, 1937), who found Devonian (Lang, 1929) plants through much of the succession, amalgamated most former lithostratigraphical divisions into one unit, the Gramscatho Beds. She considered the Lower Palaeozoic rocks to have been introduced by major thrusting into an essentially upper Devonian sequence (her Gidley Well Beds). This work prompted a re-interpretation of the nearby Lizard (Sheet 359), and Flett (1933) termed rocks in the Meneage equivalent to those of this district with early Palaeozoic rocks and breccias as the 'Crush Zone' beneath the overthrust Lizard Complex. In Roseland, Sadler (1973), having provided conodont evidence supporting the Devonian age of bedded successions, invoked tectonic disruption of overthrust nappe slices comprising Ordovician quartzite and overlying unconformable early to Mid Devonian volcanic rocks.

The early Palaeozoic rocks of south Cornwall were recognised as clasts within a major sedimentary breccia by Lambert (1965) in Meneage, Leveridge (1974) in Roseland, and confirmed by the work of Barnes (1981, 1983). Termed 'wildflysch' or 'olistostrome,' the deposit represents climactic sedimentation in a progradational flysch sequence disposed in major thrust nappes (Leveridge et al., 1984). Holder and Leveridge (1986a) and Leveridge et al. (1990) linked new lithostratigraphical divisions to a dynamic stratigraphy that developed at the southern active margin of the Gramscatho Basin (Figure 1) when it closed in the late Devonian. The basin constituted part of the rhenohercynian oceanised basin (Holder and Leveridge, 1986b; Franke, 1989), and this stratigraphy occupies most of the district. Along the northern margin of the district it is juxtaposed by thrusting with rocks of the Looe Basin (Leveridge et al., 2002), which formed part of the passive margin of the rhenohercynian Ocean Basin.

Chapter 2 Geological description

Devonian and basement

Onshore rocks represent two distinct Variscan tectonic provinces. To the north, early to Mid Devonian sediments accumulated on a thinned continental margin within the Looe Basin and the northern parts of the Gramscatho Basin (Figure 1), whereas in southern parts of the Gramscatho Basin Mid to Late Devonian deep-water flysch sequences were deposited in an oceanic environment. Juxtaposition of the differing successions occurred during closure of the Gramscatho Basin and collision late in the Devonian when relatively far-travelled (allochthonous) nappes of flysch sediments were thrust onto the northern continental margin of the basin. They form the main part of a nappe stack that includes crystalline basement located offshore in the English Channel. Sediments of the northern margin were, in turn, folded and thrust by the same northerly migrating deformation that accompanied the closure, but transport was relatively minor and they form parautochthonous successions ((Figure 2); Holder and Leveridge, 1986a). The solid rocks are described in terms of successions characterising these two regimes (Figure 3).

Looe Basin (Parautochthon)

At the time of the resurvey the relationship of the fault-bound crops of the Polglaze and Hallane formations to each other, to the Grampound Formation and the other formations of the Gramscatho Basin was unclear. Recent work by Shail and Leveridge (2005) shows that the Polglaze Formation succeeds the Hallane Formation, which passes into a Meadfoot Group (Bovisand Formation) facies in the Bodmin area (Sheet 347) to the north. The relationship of deposits of the Looe and Gramscatho basins is indicated in (Figure 8).

Hallane Formation (Hln)

The Hallane Formation constitutes a small coastal area between Hallane Beach and Black Head on the northern margin of the district, where it lies in the overturned limb ofamajorfirstphase(D1) southward-verging antiform with associated slaty cleavage that dips gently northwards. These rocks were assigned to the Meadfoot Group by Reid (1907) and described by ussher et al. (1909) as typifying an upper sand-poor mudstone facies of the group. The formation comprises largely grey mudstone, dark grey mudstone and paler silty mudstone, interlaminated and locally thinly interbedded. Within the district particularly, there are also thick laminae and thin beds of brown-weathering calcareous siltstone characterised by load casting and low-angle cross-lamination. The lithology also forms beds of nodules, distinct from the disrupted beds present within mesoscopic folding. Sporadic larger lenses and nodules with ochreous weathering are carbonate-rich concretions centred on siltstone nodules. Sparsely present are thin beds of grey graded sandstone, about Hallane Beach platform. In the cliffs above there are irregular thin to medium beds of limestone largely composed of crinoid and brachiopod bioclasts.

Until recently no age definitive fauna had been identified in the formation. Macrofossils recorded by ussher et al. (1909) including crinoid columns and bryozoan (Protovirgula dichotoma M'Coy?) debris, and conodonts reported upon by Dean (2005) are referable only to the Devonian. Molyneux (2006) has reported that palynomorphs retrieved from the formation at Hallane Beach include Emphanisporites annulatus and E. schultzii that indicate an emsian to early Eifelian age. A possible Icriodus huddlei conodont (Dean, 2006) from limestone at Hallane Beach would be compatible with an Emsian attribution. To the north, in the core of the major antiform, recorded above, early emsian palynomorph assemblages have also been identified (Molyneux, 2006).

The Hallane and Polglaze formations (see below) thus appear to correlate with the more familiar classic sequences of the Meadfoot Group to the north, and to be, as proposed by Ussher et al. (1909), their fine-grained distal equivalents.

Polglaze Formation (Polg)

Differentiated on the map as a separate formation, the integrity of the Polglaze Formation as a coherent lithostratigraphical division is uncertain, because the nature of original lithologies and their interrelationships have been largely obscured. This is due to strongly developed shearing of the first and second (D2) phases and extensional fabrics of the third (D3) phase of the regional deformation (see below). This division between the 'Grampound Grit' to the south and the Meadfoot Group (ss), was recognised by Ussher et al. (1909) in their mapping to the west, as 'quartz-eyed slate'. In this district at the transitional base that closely corresponds with a north-west–south-east fault at Polgwyn Beach [SX 0300 4795], Hallane lithologies are replaced by grey silty mudstone with diffuse laminae of paler siltstone and sporadic thin to medium beds of grey graded sandstone. However, at the top of the formation at Polrudden Cove [SX 0260 4742] the sandstone occurs as tectonically disrupted lenses, commonly in a sheared silty mudstone matrix with irregular nodules, small lenses, and segments of milky quartz veins (Plate 1a). The way up is seen at Polgwyn Beach where it is stratigraphically above the inverted Hallane Formation. Facing on early (S1) cleavage in small tectonically isolated early (F1) fold hinges is also to the south.

The formation has yielded palynomorph microfossils (Molyneux, 2006) despite the intense deformation. Assemblages are similar to that recorded from near the top of the Hallane Formation and indicate an emsian to early Eifelian age.

Gramscatho Basin (parautochthon)

Grampound Formation (Gmp)

The Grampound Grit as defined by Reid (1907) included rocks that are now attributed tothe Portscatho, Carne and Roseland Breccia formations — sandstones and breccioconglomerates envisaged as representing the basal Devonian resting unconformably upon early Palaeozoic rocks. Reprinted versions of the Mevagissey Sheet (1957, 1975) incorporated the work of Hendriks (e.g. 1937), and gave a probable Devonian age for most of the rocks in the district, but without amending the relationship of the 'unconformable' Grampound Grit. The new Grampound Formation terminology is restricted to a lithostratigraphical division extending from Pentewan westwards along the northern margin of the sheet.

The formation comprises laminae and subordinate very thin beds of pale grey fine-grained quartzo-feldspathic sandstone and coarse-grained siltstone with grey slaty mudstone. Inland the sandstone weathers to a characteristic pale yellow and the mudstone to a yellowish green colour. The sandstone shows graded bedding and, in places, low-angle cross-lamination (Plate 1b).

The Treworgans Sandstone Member (Tns) occurs low in the formation, and is repeated by large-scale folding. The sandstone is interbedded with the main lithofacies, and brown-weathering concretions of calcium carbonate are present locally in both the sandstone and the finer-grained facies. It is exposed in Treworgans Quarry [SW 899 495] near the north-west margin of the district. Formerly described as a basal member of the parautochthonous Porthtowan Formation (Holder and Leveridge, 1986a; Leveridge et al., 1990) it is here included as a member of the Grampound Formation, which is overlain locally and apparently conformably by the Porthtowan Formation.

The member comprises thick (up to 6 m) beds of sandstone that amalgamate locally (Plate 1c), but elsewhere are separated by thin to thick units of the main laminated facies. The sandstone beds are grey and dark greenish grey where fresh, weathering brownish or yellowish buff, and are fine- to very coarse-grained. Component grains, rounded to angular and poorly sorted, have similar modes and are in similar proportions to the Portscatho turbidites (see p.8), but in contrast grains are fresh and matrix is minimal. Some beds show simple grading, a few with quartz-pebble rich bases. Other beds, including some of the thickest, are not graded. Clasts of mudstone are common, either concentrated at bases or dispersed thoughout the ungraded beds. Araft of thinly interbedded sandstone and mudstone, in excess of 4 m in length, is present within one of the thicker sandstone units at Gamas Point [SX 0231 4718]. Flame structures at the bases of beds and disturbed subjacent strata are not uncommon.

Leveridge et al. (1990) proposed that the sedimentary features of many of the thicker sandstone units of the Treworgans Sandstone Member were consistent with their rapid emplacement as grain flows. The thickness of some beds, paucity of matrix, size and freshness of the grains point to a more proximal source than the allochthonous sandstones of the district. That source was manifestly different from that which supplied the fine-grained sediment forming much of the formation. It was probably the high separating the Looe and Gramscatho basins (Figure 8).

The age of the formation is not known. Sparse conodonts yielded by carbonate rich lenses at Pentewan have an early Devonian aspect (M Dean, personal communication 2005) but are worn and probably reworked. In the Mevagissey district the Grampound Formation forms the footwall of the Carrick thrust but to the west it forms the basal division of the parautochthonous succession of the Gramscatho Basin (Leveridge et al., 1990) that accumulated on the thinned northern continental margin of the basin (Leveridge and Hartley, 2006).

Carrick Nappe (allochthon)

Portscatho Formation (Pto)

The Portscatho Formation is dominant within the Mevagissey district. Although exposure inland is poor the coast of Mevagissey Bay provides an almost continuous section through most of the formation. It constitutes the Carrick nappe (Figure 2); (Figure 3), with the Carrick thrust defining its lower boundary and the Veryan thrust its top. The nappe was derived from the deeper more southerly part of the Gramscatho Basin (Leveridge et al., 1990), cutting up through the parautochthonous succession of the basin on the Carrick thrust, and is juxtaposed in the north with the Grampound Formation. Essentially the right way up, the formation has a structural thickness onshore of some 4000 m, but, to the west in the adjoining Falmouth district (Sheet 352), it extends to 5400 m.

The formation comprises alternating fine- to medium-grained bluish grey sandstone beds, which weather greenish buff, and dark grey slaty mudstone with sporadic thin beds of siltstone and chert. The sandstone varies from thinly to thickly bedded, with individual beds up to a maximum 2 m thick. Scour, groove and flute moulds are apparent on some bases, and internal structures are referable to the Bouma (1962) sequence for classic turbidites. That simple pattern is disturbed by delayed and multiple grading, coarse traction carpets and wavy laminated bedding. Grain-flow sandstones are common towards the top of the sequence. The formation contains fining-up and coarsening-up sandstone, and mudstone-dominant subfacies (Floyd and Leveridge, 1987), the last being prominent around and north of Mevagissey.

The sandstones are greywackes composed of approximately equal amounts of quartz, feldspar (plagioclase and potassic) and lithic grains within a very fine-grained clastic matrix that constitutes an average 25 per cent of the rock. The lithic grains are of three categories, volcanic, sedimentary and metamorphic, the latter deriving from both igneous and sedimentary parents. Changing lithic grain proportions through the sequence show a marked and progressive decrease of volcanic grains and complementary increase in metamorphic grains. This is reflected in a corresponding change in geochemistry, the lower half of the sequence being characterised by high Zr content and the upper half by a high and variable Cr content. Petrographical and geochemical discriminant analyses indicate (Floyd and Leveridge, 1987) that the provenance of the Portscatho Formation was a continental margin magmatic arc undergoing progressively deeper dissection. Mid-Proterozoic model Nd ages (TDM) for the Portscatho Formation led Floyd et al. (1991) to propose that the igneous source rocks (plutonic and volcanic) were not contemporaneous with sedimentation but part of an earlier Proterozoic to early Palaeozoic accreted arcs terrain. Sedimentological characteristics typify a deep-water fan environment (e.g. Walker, 1978) with a general progradation from outer- to mid-fan regimes through the formation. The lack of shallow-water sedimentary structures in the sandstones, and mudstones yielding sapropelic organic residues throughout the formation also indicate the deep-water origin of the formation.

Fossil wood fragments, assigned to Araucarioxylon hendriksi, formerly Dadoxylon hendriksi (Lang, 1929), are dispersed through the upper part of the sequence (Hendriks, 1931, 1937), and are attributed to the Frasnian by Creber (1984, in Leveridge et al., 1990). Micropalaentological determinations on miospores and acritarchs have indicated a Frasnian age for much of the formation (Le Gall et al., 1985; Leveridge et al., 1990), and Wilkinson and Knight (1989) have indicated that, in Meneage, the formation may extend into the Famennian. The lower part of the formation has not yielded determinable fossils and proposals of a Mid Devonian age (e.g. Holder and Leveridge, 1986a) remain speculative.

Veryan Nappe (allochthon)

Pendower Formation (Pdr)

The Pendower Formation crops out on the northern coast of Gerrans Bay, on the foreshore and cliffs of Pendower and Carne beaches. The Veryan thrust forms the base at Pendower [SW 8971 3802] to the west, and the top occurs some 150 m south-east of Gidley Well [SW 9079 3824]. This is the type section (Leveridge et al., 1990), the uppermost part of which forms the foreshore at the western margin of the district to the south-west of Carne. Striking north-eastwards inland, it forms only minor exposures in valleys, streams and road cuttings, with one quarry section in pillowed basalt at Tubb's Mill [SW 9620 4325]. It does not form a mappable crop along the coast of Mevagissey Bay or Gorran Haven, being essentially occluded by late steep north-east–south-west and east–west faulting, but limestone beds recorded by Sadler (1973) at Pabyer Point [SX 025 426] may represent a slice of the formation within an east–west fault zone.

The sequence is up to 430 m thick (structural thickness), comprising slaty mudstone, sandstone, limestone and chert. Mudstone is dominant, and is variously coloured — beige, green, brown, grey or black. It is locally manganiferous, with speckled oxide-covered surfaces, and also contains concentrations of metals such as copper, nickel, vanadium and zinc (Shail, 1992). The interbedded sandstone, limestone and chert form cycles of differing orders up to 100 m thick (Leveridge, 1974). Within those cycles each of the lithologies may predominate, with chert being prominent in the basal part of the formation and limestone dominating the upper major cycle ('Veryan Limestone' of Sadler, 1973) exposed to the east of Gidley Well.

The sandstone, grey-green weathering buff and yellow, is fine- to very coarse-grained lithic greywacke. Acid volcanic rocks and schists largely constitute the lithic grains. It forms graded laminae to medium beds, lenses and small channel fills. Bases and lenses are composed locally of fine breccia with pebble clasts, including chert and limestone. Soft sediment deformation structures, folding, slumping and top truncation are common.

Limestone, which is grey weathering buff, forms laminae, thin discontinuous beds, single beds up to 1 m thick, and composite units up to 2 m (Plate 1d). They are turbidites, normally graded, commonly with massive bases, parallel laminated and cross-laminated sections, and some beds have a fine breccia traction carpet. Component carbonate grains are micrite, shell and crinoid fragments and algal calcispheres (Leveridge et al., 1990). Basal breccias comprise fragments of limestone with subordinate sandstone, chert, quartzite, schists and acid volcanic rocks.

Chert bedding units, up to 0.3 m thick, consist of thin to thick laminae showing colour grading up from black to grey. Comprising microcrystalline quartz, sericite and carbonaceous radiolarian tests, the grading reflects an upward decrease in the clay mineral and test concentration. The thickest development of chert is near the base of the formation. At this level within the district pillow basalts interdigitate with the sedimentary rocks about Tubbs Mill. These have the geochemical characteristics of ocean floor basalt (large-ion-lithophile enriched mid-ocean-ridge basalt (MORB), Floyd, 1984; Barreiro, 1996; (Figure 6)).

The formation has not yielded any diagnostic macrofossils. Microfossils in the form of conodonts, predominantly polygnathids, are present within the limestone beds throughout the formation. Although transported by turbidity flow, their consistent younging upwards through the sequence, and state of preservation, indicate repeated penecontemporaneous derivation rather than the reworking of older consolidated deposits. Conodonts indicate a mid to late Eifelian age (Sadler, 1973; Leveridge, 1974) and thus deposits of the Pendower Formation constitute the oldest biostratigraphically dated bedded allochthonous rocks of the Gramscatho Basin.

The Pendower Formation is interpreted as a hemipelagic shielded deep basin sequence. It consists of grey, green, and brown, metalliferous mudstones and interbedded radiolarian chert, with turbiditic limestone of pelagic platform provenance and subordinate thin beds of coarse lithic greywacke sandstone of continental basement provenance (Holder and Leveridge, 1986b). The pillowed mid-ocean-ridge basalts, chert and metalliferous mudstone association has been attributed to proximity to a spreading centre, and the clastic rocks have been interpreted as syn-rift deposits (Shail, 1992). The calciclastic and siliciclastic rocks were derived by erosion of a tectonically controlled block of continental basement within the basin. Pelagic carbonate sediments capped this remnant block, and the cyclic sedimentation is thought to reflect periodic syn-rift tectonic activity. Deposition occurred within a northerly sub-basin of the asymmetrical Gramscatho Basin, with the incipient formation of oceanic crust (eifelian). The Lizard ophiolite floors part of an earlier southern sub-basin (Emsian, Clark et al., 1998a).

Carne Formation (Cne)

The Carne Formation rests conformably on the Pendower Formation in Gerrans Bay [SW 9092 3814] to the south-west of Carne (Holder and Leveridge, 1986a). Its upper boundary with the Roseland Breccia Formation, also conformable, lies within the rock promontory bounding Pennarin Cove on its north-west side [SW 9122 3783]. Fault-bound sections of the formation also form the Veryan Bay coastal section, over 1 km eastwards from West Portholland. The formation is largely occluded by faulting farther eastwards, but does form a thin fault-bound section above Great Perhaver Beach near Gorran Haven. The structural thickness of the formation is up to 240 m.

In the Gerrans Bay section, the predominant fine- to medium-grained sandstone forms thinning- and fining-up sequences. Channel fill is evident in the lower part of the formation; slump folding and blocks and lenses of disrupted sandstone beds are prominent in its central part, and thinly to thickly bedded sandstone turbidites form its upper part. The coast between West Portholland and Porthluney in the Veryan Bay section is largely within the central part of the formation (repeated by faulting) and its upper part. Here the upper part is also dominated by lenticular and channel bedded sandstone, some sandstone fills locally being up to 5 m thick. The sandstone is lithic greywacke, similar in composition to that of the Portscatho Formation.

Grey slaty mudstone is interbedded with the sandstone throughout the formation, and forms units up to a few metres thick in its central part in the Gerrans Bay section. The mudstone contains laminae of siltstone and fine-grained sandstone. Sporadic laminae and thin beds of limestone and chert are also present.

Breccia, some 14 m thick, succeeds the basal sandstone unit of the Carne Formation (Figure 4)a. This occurence, similar to the succeeding formations was the basis of the Gidley Well Beds terminology for the latter by Hendriks (1937). It has normal sedimentary lower and upper contacts. The lower 3 m of the breccia comprises pale grey slaty mudstone with dispersed blocks and rafts of limestone (up to 8 × 1.5 m in section) of Pendower Formation type. This is overlain by dark grey and black mudstone with sporadic, more rounded lenticles of brownish yellow limestone up to 1.5 m in length (Plate 2), lenticles of sandstone, small blocks and fragments of quartzite and a variety of schists, with interbedded lenses of polymict microbreccia.

Apart from a Machaerocanthus fish spine, collected by Hendriks (1937) from a clast near the base of the breccia, no macrofossils have been identified from the formation. Conodonts from the limestones within the breccia unit have indicated a late Eifelian age for the large raft of Pendower type limestone (Sadler, 1973). A Lower Devonian (Emsian) age for the yellow-weathering limestone clasts was given by Leveridge (1974), who also reported polygnathid conodonts of Givetian aspect from a bed or elongate lens of coarse-grained limestone near the top of the breccia unit.

The Carne Formation is a sequence of greywacke sandstone turbidites, olistostrome, channel-fill sandstone, slumped sandstone, and interlaminated mudstone, siltstone and sandstone. It is interpreted as an association of middle and upper fan distributary and interchannel deposits, slope deposits and proximal turbidites (Barnes, 1983; Holder and Leveridge, 1986a).

Roseland Breccia Formation (Rbr)

The Roseland Breccia Formation is characterised by a lack of a well-defined internal stratigraphy, although at least two non-exclusive associations are present. The main component of the formation is dark grey silty mudstone. It is present mainly as the matrix of a variety of matrix-supported breccias and is prominent in slumped sandstone sequences, and forms a background sediment. Framework breccia and conglomerate and igneous rocks (predominantly basic but also intermediate and acidic) occur as both interbeds and separate large clasts, and are the other common association. The formation constitutes extensive coastal sections in Gerrans and Veryan bays and about Gorran Haven.

Gerrans Bay

With a structural thickness of 1 km or more, a significant part of the formation forms a dip section in the Gerrans Bay coast between Pennarin Cove and nare Head. Constituting the lower part of the formation in Pennarin Cove are interlaminated mudstone and siltstone with interdigitated 'pebbly mudstone' breccia. The breccia comprises largely angular intrabasinal mudstone, siltstone and sandstone clasts, with dispersed small phyllite, schist and quartzite extrabasinal fragments. A fault cuts these lithologies; it is gently inclined towards the south and fault-wall evidence indicates largely late down-dip extensional movement. The similar lithologies of footwall and hangingwall strongly suggest that it is not a major fault. This was however the 'Great thrust' of Hendriks (1937), thought to be responsible for inserting the 'far travelled' large Ordovician quartzite masses of the cliffs at the centre of the cove into the Devonian succession. The white quartzite crags above, striking up to Carne village (Plate 3), are part of a continuous raft, 0.5 km in length. One of the large blocks of the cove is, in part, disrupted along cleavage with mudstone tectonically inserted along fractures and the original bedding, but in general a variety of clasts are juxtaposed with the quartzite along cleavage.

Succeeding and interbedded with the mudstone breccia at its base is a sequence of trachytic lapilli tuffs and thin vesicular lavas forming Pennarin Point [SW 9133 3766]. These volcanic rocks, topped by blocky lava breccia with interdigitating black mudstone and carbonate segregations, pass into black and dark grey mudstone with dispersed small and large (>1 m) angular to rounded clasts in Paradoe Cove [SW 9144 3767] and Tregagle's Hole cove [SW 9152 3763]. Lenses of blue-green tuff and an irregular sharply defined mass of pale buff weathering polymict conglomerate in the lower part of this sequence crop out in the cliff on the western side of Paradoe Cove (Plate 4). In the centre of the cove, and along cleavage from the conglomerate, there are lenticular and blunt-ended clasts of quartzite, limestone, and indurated sandstone. Up dip, black mudstone contains lenses of siltstone, graded coarse sandstone, sporadic igneous rock fragments and blocks of quartzite. Similarly in Tregagle's Hole cove within the black slaty mudstone there are dispersed grains and fragments of sedimentary, igneous and metamorphic rocks, lenses of tuff and phyllite/quartzite breccia, several metres in length, irregular blocks of quartzite, phyllite breccia, and discontinuous beds and lenses of fine- to coarse-grained sandstone. In the spur between Tregagle's and Mallet's coves there are lenses of chamosite oolite siltstone. In Mallet's [SW 9150 3760] and the adjoining coves to the south, these lithologies are present with large blocks, up to 10 m across, of metabasaltic breccia (Plate 5), and polymict breccia containing phyllite, mudstone, and bedded chert.

The base of the overlying volcanic rocks of Nare Head is exposed in the shore between Malmanare and Shannick points. Intruded along the junction is a talcose rock containing lenticular nodule remnants of bastite serpentinite. The nare Head igneous mass is in excess of 100 m thick, and contains pillow lavas at its base at Shannick Point [SW 9153 3725] that show it is the right way up. Above to the headland, grey-green variolitic basalt lava and local pillow breccias are interleaved with microgabbroic sheets inclined moderately south-eastwards.

Veryan Bay, west coast

The several coves and headlands of the western coast of the bay display the great variety of lithologies along strike within the formation. The igneous mass of nare Head is seen on its eastern coast, as in rosen Cliff [SW 922 374], to comprise units of massive basalt, pillow lava (Plate 6) and volcanic breccia. Basal lava rests on indurated folded sandstone and siltstone towards Kiberick Cove and black coarsely crystalline gabbro is intruded above the basal pillow lava on the northern margin of the mass west of Kiberick Cove.

In both Kiberick Cove [SW 925 380] and the Straythe [SW 930 387] (Figure 4)b, dark grey silty mudstone, with variably dispersed lithic grains and blocks of limestone, quartzite, schist, sandstone and volcanic rocks (Plate 7), hosts lenses and sequences of framework breccias including basic volcanic rocks and mica and graphitic schists, and polymict breccia and conglomerate.

At Jacka Point [SW 9385 3925], Portloe, polymict pebble conglomerate and coarse conglomerobreccia are interbedded with 'greenstone' (altered basic volcanic) units that were variously fragmented and metamorphosed prior to incorporation. The upstanding igneous mass of Hartriza Point [SW 942 397], which rests above schist breccias within dark grey mudstone, is a breccia of small fragments to large blocks of fine-grained foliated rock comprising prehnite, actinolite, and epidote assemblages indicative of a higher metamorphic grade than the host slaty mudstone. Caragloose Point [SW 947 399] comprises predominantly dark green to black framework breccia of hornblende schist clasts, interfingering with which is a series of thin (to 2 m) interconnecting white granitoid sills (Plate 8) that is also brecciated.

Veryan Bay, north coast

The variety of lithologies along the coastal section is similar to that elsewhere but the proportions are different. There are fewer basic volcanic rocks and schists and more penecontemporaneous acidic igneous rocks, as at Greeb Point [SW 9820 4055] (Plate 9) and near Cadythew rock [SW 992 405]. At Greeb Point, an apparently conformable unit displays volcaniclastic (tuff) and subordinate magmatic (keratophyric lava) textures. At Cadythew rock, quartz-feldspar porphyry forms magmatic pods (some showing intrusive relationships), lenses and a massive unit, and is associated with bedded tuff.

Gorran Haven and Great Perhaver Beach

Between Gorran Haven [SX 013 416] and Great Perhaver Point [SX 015 420]acid igneous rocks including tuffs and more massive discontinuous units are present within dark grey slaty mudstone containing quartzite clasts at Little Perhaver Point, and pillow lavas with massive porphyritic units and basaltic tuffs form the Great Perhaver Point promontory [SX 015 419]. The thick lava sequence has no mappable continuity along strike. Forming cliff-top crags above Great Perhaver Beach are the quartzites of Carn rocks [SX 016 425] that provided some of the early fossil finds such as the brachiopod Orthis calligramma (Dalman), and trilobites Calymene pulchella (Dalman) and C. cf. blumenbachii Brongniart establishing their Ordovician (i.e. Lower Silurian of Murchison, 1846) age.

The age of the formation is however still not established. To date there is no biostratigraphical evidence from unequivocally penecontemporaneous sedimentary rock. Quartzites (Ordovician, e.g. Murchison, 1846; Llandeilian: Sadler, 1973) and limestones (Silurian, e.g. Wenlockian: Bather, 1907), now known to be clasts within the breccias, yielded a variety of macrofossils particularly to early workers (e.g. Peach, 1841; Collins, 1879; Green, 1904). Limestone clasts within the breccias have since yielded conodonts that predominantly represent the three stages of the Lower Devonian (Hendriks et al., 1971; Sadler, 1973) but also the late Silurian (Ludlovian: Leveridge, 1974) and possibly eifelian (Sadler, 1973). Conodonts retrieved from limestone considered to be a thin interbed in Mallet's Cove indicated a probable late Givetian age (Leveridge, 1974). Hendriks et al. (1971) reported the conodont Ancyrodella buckeyensis of early Frasnian age from limestone between lava pillows at Mullion Island in Mounts Bay to the south-east of the district in rocks thought to be part of the breccia formation. The subsequent revision of the Middle–Upper Devonian boundary by the International Subcommission on Devonian Stratigraphy (see Ziegler and Klapper, 1985) did not affect the age attribution as the fossil remains diagnostic of the Frasnian. Le Gall et al. (1985) suggested a Famennian age for the 'Meneage Formation', which encompassed the Carne Formation and the Roseland Breccia Formation above, but that was on the basis of a Frasnian age for the Portscatho Formation, then thought to underlie the Roseland Breccia Formation stratigraphically. A late Mid to Late Devonian age for the formation is probable.

The Roseland Breccia Formation is a major olistostrome (e.g. Barnes, 1983), composed of grey silty mudstone with dispersed clasts of sedimentary, metamorphic, volcanic and magmatic rocks, with interbedded sedimentary rocks including breccias and conglomerates, and acid and basic volcanic rocks. Extrabasinal clasts, introduced by slumping, slides and sediment gravity flows, include shelf sediments of Ordovician, Silurian and early Devonian age, a variety of schists and granitoids, and blocks of volcanic and metamorphic breccia. Barnes (1981) interpreted the acid igneous rocks near Cadythew rock and at Little Perhaver as distal tuffs into which blocks of massive proximal lavas had slumped. Contemporaneous basic and acidic lavas, volcanic breccias and minor intrusions have mid-ocean-ridge and within-plate, calc-alkaline affinities (see below; Barreiro 1996), and some larger clasts are of ocean-floor basalt (e.g. Nare Head, Roseland: Barnes 1983; Floyd 1984).

Within this nappe sequence there is thus a dramatic progradation from shielded basin sedimentation to climactic slope deposition.

Dodman Nappe (allochthon)

Dodman Formation (Dm)

The overlying Dodman nappe (Figure 2); (Figure 3) comprises the Dodman Formation (formerly the Dodman Phyllites), an undated succession of interbedded sandstone and mudstone of flysch-like appearance (Barnes 1981, 1983). It is progressively more sandstone rich up through the sequence. The sandstone beds are thin to very thick (over 3 m), and sedimentary characteristics similar to those of the Portscatho Formation are apparent in the lowest few hundred metres of the sequence. However these are progressively obscured by recrystallisaion of the sandstone and mudstone towards Dodman Point. There is an apparent reverse metamorphic gradient present onshore in the Dodman Formation. From epizone it increases to an incipient mica schist of higher greenschist grade, early deformation (D1) transposition taking the form of (S1) cleavage-parallel segregation of micaceous and quartzose layers with pyrite porphyroblasts, and, in the cliffs above Vault Beach, sparse very small garnets are developed.

A Mid Devonian metamorphic age for the Dodman Formation onshore (Clark et al., 1998b) indicates an earlier stratigraphical age, as does its in-sequence position within the nappe stack. A late early Devonian age was assigned to the formation by Leveridge et al. (1990) in their modelling of the tectonostratigraphy of this and adjacent districts.

Offshore data in Plymouth Bay suggest that the Start Schists may be part of the same nappe. Hornblendic schists associated with mica schist at Start, and locally flysch-like in appearance, have normal mid-ocean-ridge basalt (N–MORB) signatures (Floyd et al., 1993b; Merriman et al., 2000). Comparability with some schists of the lower tectonic unit beneath the Lizard Ophiolite (Old Lizard Head Series and Landewednack Horneblende Schists; Floyd et al., 1993a) cannot yet be excluded, although some authors (nutman et al., 2001; Cook et al., 2002) propose an earlier history for the Old Lizard Head Series.

Normannian Nappe (ZEG, O) (allochthon)

A further nappe of crystalline rocks, the normannian nappe (Holder and Leveridge, 1986b), is mapped offshore (SWAT 9: BIRPS & ECORS, 1986; Edwards et al., 1989) within the area of Sheet 353. It has surface expression to the east in the garnetiferous gneiss of Eddystone Rock in Plymouth Bay, and to the south-west probably in the Man of War Gneiss (Ordovician: Sandeman et al., 1997) off Old Lizard Head and possibly the schists of the Old Lizard Head Series (Nutman et al., 2001). Locally interposed between the Dodman and normannian nappes to the south-west are the tectonic units of the Lizard Ophiolite (late early Devonian). Occupying a similar structural position offshore within Gerrans Bay, on the eastern margin of the district, is a basic/ultrabasic sheet (Edwards et al., 1989; J W F Edwards personal communication, 1998).

This nappe has been interpreted as a core remnant of the southern continental margin of the Gramscatho Basin, which became an active plate margin during the Devonian and supplied sediment to the basin as it was closed (Holder and Leveridge, 1986a). The nature of those sediments, including the clasts of the Roseland Breccia Formation, indicate the character of that former margin, an igneous and metamorphic complex of Proterozoic and early Palaeozoic age (e.g. Floyd et al., 1991) capped by shallow shelf deposits of the Ordovician, Silurian and early Devonian.

Igneous rocks

Devonian–Carboniferous

Floyd (1984) differentiated two distinct basaltic suites in south and west Cornwall, one of which comprises within-plate tholeiitic basalts, and the other mid-ocean-ridge type basalts. Subsequently the former were recognised as characterising the parautochonous area and the latter confined to the allocthonous area (Holder and Leveridge, 1986a). Within this district the former is sparsely represented by the dolerite intrusions of Drennick and Black Head, which are the southerly parts of a discontinuous body extending over some 7 km to the north-west. The latter suite is represented by the volcanic rocks about Tubbs Mill and some of the igneous rocks of the Roseland Breccia Formation.

Igneous rocks of the Roseland Breccia Formation range from basic to acidic (Figure 6)." data-name="images/P1004238.jpg">(Figure 5). They include both those accumulating in situ and derived rocks (Figure 6). The former have undergone autobrecciation in some cases, possibly some brecciation in situ by seismic pumping, and varying degrees of tectonic brecciation, and some of the latter have undergone varying degrees of metamorphism, brecciation, and cementation prior to incorporation into the olistostrome. The mineralogy and chemistry of these rocks are complex (see Barnes, 1981; Barreiro, 1996) but are discussed briefly below because they provide an important key to the processes operating about the southern active plate boundary of the Gramscatho Basin, and thus the collision that had a dominant effect on the geology of south-west England.

Dolerite (parautochthon)

Dolerite forms large bodies at Black Head and Drennick and smaller intrusions in the cliffs above Hallane Beach, cropping out on the beach just north of the district. The small intrusions are fine grained and grey-green in colour, weathering yellowish green. They are generally intruded parallel to the early cleavage, but locally bifurcate with irregular contacts, and are themselves cleaved. This suggests that intrusion occurred contemporaneously with the first major (D1) deformation.

The larger masses have transgressive and faulted side margins, and a gently inclined upper contact with host rock in the case of the Black Head body. That intrusion is dark greenish grey, coarse grained and gabbroic in parts, with euhedral phenocrysts of feldspar and augite altered to carbonate and chlorite, in a matrix rich in feldspar with microlitic amphibole. Notable are irregular 'masses' of quartz enclosing feldspar phenocrysts, regarded as primary by Ussher et al. (1909).

Basic rocks (allochthon)

The volcanic rocks about Tubbs Mill are a localised series of basaltic lavas and subordinate associated sills within the Pendower Formation near the base of the Veryan nappe. The larger bodies, as at Tubb's Mill quarry [SW 9620 4325], include pillow lavas (vesicular and feldspar porphyritc in parts), and a fine-grained non-porphyritic possibly intrusive facies (see Reid, 1907). Geochemically, these rocks are ocean floor tholeiitic basalt enriched in large-ion-lithophile elements typical of a mid-ocean-ridge setting (Floyd, 1984; Barreiro, 1996).

Basaltic rocks very similar in appearance and geochemistry (Figure 6)." data-name="images/P1004238.jpg">(Figure 5); (Figure 6) to the Tubbs Mill rocks are also present towards the top of the Veryan nappe as clasts, up to 1.5 km across, within the Roseland Breccia Formation. Prominent are the Nare Head body [SW 916 370] and the pillow lavas of Great Perhaver Point [SX 015 419] but there are several minor occurrences. They are also tholeiitic ocean floor basalts (Barnes, 1981) but enriched, rather than depleted, in light rare-earth elements (LREE).

Bedded units at Jacka Point [SW 939 395], and at Hartriza Point [SW 9425 3965] (samples 10–13) are mid-ocean-ridge basaltic rocks. They are also brecciated. The brecciation in at least one sample (13) is largely contemporaneous with the first major deformation (D1) and occurred after emplacement, but in others (samples 10–12) it probably occurred largely before emplacement. Within the latter group, the epidotisation is typical of hydrothermal alteration close to a spreading centre. The amphibolite metamorphism of sample 12 (Figure 6) is indicative of derivation from a metamorphic parent at sedimentary source. Mid-ocean-ridge basalt also constitutes epidotised breccia fragments recemented and included as angular blocks, as at Mallet's Cove (sample 3; (Plate 5)).

Acidic rocks (allochthon)

The extrusive bedded felsic tuff units and associated massive magmatic rocks of Greeb Point, Hemmick Beach and Gorran Haven (samples 15–18), apart from xenocrystic quartz have alteration assemblages of quartz, carbonate and chlorite, but volcaniclastic and magmatic textures are still apparent. They have been interpreted as distal deposits and slumped proximal deposits of penecontemporaneous acid volcanicity (Barnes, 1981, 1983), but some also exhibit intrusive relationships. They are calc-alkaline rocks, typical of syn-collisional granites.

Brecciated granitoid intrusives are seen at Caragloose Point (samples 19 and 20; (Plate 8)), where they interfinger with hornblende schist breccia, and nearby at Percolan Beach [SW 944 399], where they intrude mica schist and basic volcanic rock breccia. Phenocrysts of orthoclase, albite, quartz, muscovite, spinel and garnet constitute fragments in a variably sheared fine-grained felsic matrix. The fabric suggests pretectonic brecciation, autobrecciation or possibly brecciation by fluid flow due to seismic pumping. Geochemical characteristics are similar to the extrusive acid volcanic rocks and a similar origin is thereby indicated.

On the bases of Nd and Sr proportions, and LREE concentrations (BGS data) it is apparent (see also Barnes, 1981; Barreiro, 1996) that the acidic rocks are not derived from the basic magmas. They are andesites and dacites typical of the calc-alkali suite, and were probably derived from Late Proterozoic–early Cambrian continental crust (Barreiro, 1996) by melting of continental crust during the collision process. At the same time lithospheric crust generated mid-ocean-ridge basalts were being obducted, eroded and incorporated into the host Roseland Breccia Formation.

Permian

Quartz porphyry

The one minor intrusion of quartz porphyry or elvan in the district occurs north of Pentewan where it intrudes the Polglaze Formation. Trending east-south-eastwards and steeply inclined in a cliff-top quarry near Polrudden, the intrusion splits in the cliffs below, with a major branch trending north-eastwards that is inclined moderately to steeply north-westwards and strikes north-eastwards over 500 m along the cliff. The porphyry, up to 8 m thick, is off-white where fresh but weathers to a rich golden yellow, locally with pinkish hues. It comprises phenocrysts of quartz, kaolinised feldspar and sparse muscovite in a quartzo-felspathic and muscovite matrix. Locally coarse grained at its centre, it is fine grained towards its margins, with flow foliation and folding common in the marginal 0.5 m, particularly within any distinct 'upper' margin. Grey slaty mudstone xenoliths, including folded slate fragments, are common at margins where they may appear matrix or framework supported. The intrusion cuts the main tectonic fabrics of host rocks in places and elsewhere it is coplanar with the main cleavage, as in parts of the cliff section.

The elvans of the province postdate the main deformations but are polyphase, and linked to particular granite intrusions (e.g. Leveridge et al., 2002). Geochemical data on the Brannel elvan [SW 955 518], which is along strike to the west-north-west of Pentewan, shows no great similarity with that of the St Austell Granite although its Rb/Sr radiometric age of 270 ± 9 Ma overlaps with that of the granite at c 274 Ma (Darbyshire and Shepherd, 1985, 1994). No structural, geochemical or temporal ties between the Pentewan elvan and the nearby St Austell Granite are established.

Permo–Triassic (PT)

Permian and Triassic rocks are not mapped onshore but are extensive offshore (see inset on Sheet 353) where they are unconformable on Devonian and basement rocks. The northern boundary of the deposits, which thicken to a maximum of 9000 m to the south at the centre of the Plymouth Bay Basin (evans, 1990), is everywhere on or close to the projected crops of major onshore thrusts. In interpretations of offshore seismic lines, the northern boundary of the main areas of thick deposits is controlled by extensional faulting that roots down into the former thrusts (e.g. SWAT 9, Section 3, Sheet 353). The rocks are undivided on the map but grab samples and sparse borehole data indicate that they are reddish brown sandstone and breccia.

Cenozoic

Remnant cliff features are a product of stillstands in marine retreat during the Cenozoic, and are present throughout the district. Although much degraded in parts, they are mappable at intervals of a few metres to a few tens of metres. A major stillstand erosion surface is apparent at about 100 m on the headlands of nare Head and Dodman Point (Plate 3); (Plate 9). It is part of a major platform present around the south Cornwall coast, notably on the Lizard where it slopes gently coastwards from about 100 m to 75 m and is termed the '90 metre platform'. Deposits have been recognised generally only on the lowest 'raised' platforms below the prominent platform at about 35 m OD, namely at 20 to 25 m, 10 to 15 m and 2.5 to 5 m (cf. Mottershead, 1977). These platforms are now regarded as having been cut during marine retreat in the Pleistocene.

Head

Head is composed of poorly stratified and unstratified clay, silt, sand, gravel and angular lithic clasts, up to block-size, of local origin. It is largely the product of downslope solifluction under Pleistocene periglacial conditions. Head occupies valley bottoms, minor basinal depressions, and generally mantles the remnant Pleistocene coastal platforms below about 35 m OD. In the Fal valley, residual head flanks more recent river alluvium at Creed, and in the Portluney valley head passes downstream to accumulating alluvial deposits at Tubbs Mill. Coastal head is present at Chapel Point [SX 026 433], Vault Beach [SX 011 409] and below Carne [SW 910 381]. At the last locality, 12 m of head overlies up to 2 m of raised beach deposits that rest on the 2 to 5 m raised platform (Plate 10). These head deposits have been ascribed to the onset phase of the Devensian in the neighbouring Falmouth district (Leveridge et al., 1990).

Buried channel deposits

Colenso (1829: see De la Beche, 1839) recorded a sequence, about 19 m thick, of alluvial deposits on bedrock at the Happy union Stream-tin works in the St Austell river valley at Pentewan. At the top of a basal layer of stream-tin (up to 3 m thick; see below) oyster shells were attached to surface cobbles and intergrown tree roots. A coating (0.3 m thick) of organic silt included leaves, nuts and in situ moss. Above brown and grey silt (about 3 m thick) included similar plant debris and layers of shells. This was overlain by 0.1 m of shelly marine sand and 0.6 m of clay, which, in turn, was succeeded by 12 m of sand. The lower half of the sand sequence included tree trunks, red deer remains, whalebones and human skulls; the upper half, to surface, consisted of fluvial sand and gravel interleaved with marine sand and silt. The sediments were laid down during a period of sea-level 'recovery' that followed the Devensian cold phase.

Fluvial deposits

Recent river alluvium forms surface deposits in the main river valleys and commonly comprises up to 2 m of clay and silt overlying coarse gravel. Part of the alluvium in the Fal valley comprises pale grey sand and off-white clay — redistributed detritus from tinning and the china clay mining operations upstream at St Austell.

Blown sand

Blown sand accumulates at the mouths of the main rivers largely separating beach deposits and the fluvial deposits of the river valleys. In the case of the St Austell river at Pentewan it also rests upon head, and in the Porthluney river at Caerhays, blown sand directly overlies bedrock.

Landslip

Landslips, which are not extensive, are present along the coastal cliff sections of the allocthonous lithostratigraphical divisions, rather than the parautochthonous units that strike at a high angle to the coast. This is because the slips are discontinuity controlled, principally by cleavage where dipping towards the sea, but also by strike-parallel bedding, and oblique jointing and faulting. Most of the landslip mapped is contemporary, but the major area of landslip above Kiberick Cove reputedly records an historical event. Measuring some 400 m by 300 m, it has a steep rocky back-slip feature, steep partly fault-controlled margins to south and north-east, and an associated rotational antiformal fold within the Roseland Breccia Formation rocks of its toe on the foreshore.

Structure and tectonics

The cross-sections on Sheet 353 are deceptively simple showing the differing successions of the area disposed within a stack of three major nappes thrust over the Grampound Formation. Structures present within the major lithostratigraphical and tectonic units are summarised in (Figure 7).

Although early workers recognised the ubiquitous presence of a slaty cleavage and secondary cleavages, folding within the bedded rocks, and minor thrusting, it was the breccias of the Roseland Breccia Formation with early Palaeozoic macrofossils that prompted concepts of major mylonitisation (e.g. Reid, 1907) or major thrusting (e.g. Hendriks, 1937, 1939) in the district. The recognition of the sedimentary nature of the breccias and their position within sequence, an improved biostratigraphy for the major lithostratigraphical divisions of south Cornwall, and new offshore seismic data, permitted identification of the major thrust nappes during the detailed remapping of the Falmouth district (Sheet 352) by the Geological Survey (Leveridge et al., 1984; Leveridge et al., 1990). In association with metamorphic data, the dynamic nature of the stratigraphy of the nappes was explained in terms of northerly progradation of flysch sedimentation, deformation and forward thrust nappe propagation and metamorphism accompanying the closure of the Gramscatho Basin from the south (Holder and Leveridge, 1986a; Leveridge et al., 1990 fig. 3).

Early deformation structures (D1) in the deposits of the Gramscatho Basin are the indicators of that northerly transport. Generally folds that are tight to isoclinal in the Carrick and Veryan nappes and isoclinal in the Dodman nappe appear to be cylindrical in outcrop but may face westwards, northwards or eastwards on the axial plane slaty cleavage. On the larger mesoscopic scale they comprise elements of sheath folds, structures that typify thrust terrains, their sheath- or tongue-like form developing from early formed, approximately east–west folds by ongoing flattening and simple shear extension. That extension direction is north-north-west–south-south-east in the district, and is shown by grain lineation on cleavage in slaty mudstones and sandstones and the preferred orientation and extension of clasts in the breccias. The northerly overriding transport direction is indicated by the asymmetry of the sheath folds in that extension direction. The reorganisation of the rock bodies during deformation and development of slaty cleavage in mudstones, pressure solution and recrystallisation in the sandstones largely constituted the regional metamorphism. Available radiometric dating (see below) of that metamorphism indicates that deformation of basin sediments was operating at least from the late Mid Devonian in the south and migrated northwards successively through the sediments shortly after their deposition and as the nappes propagated northwards. It affected the northerly Carrick nappe in the latest Devonian–earliest Carboniferous. This accompanied its emergence on to the penecontemporanous sedimentation surface of the Mylor Slate Formation (Leveridge et al., 1990; (Figure 2)) of the parautochthonous part of the basin in the latest Devonian. This expulsion of the nappes of deep-water deposits on to the northern margin of the basin marked basin closure and collision in south Cornwall.

Deformation however did not then terminate but continued to migrate during the tournaisian–Visean and early namurian through the deposits of the passive margin rift basins to the north (Leveridge et al., 2002), and the first to be affected were those of the Looe Basin. Early folding in the small area of deposits of that basin in the district, and in a substantial part of the Meadfoot Group of the St Austell Bay coastal section to the north also shows incipient sheathing but, in contrast, facing and transport are to the south. This has been interpreted (Shail and Leveridge, 2005) to be a result of the inversion of the deposits of a broadly symmetrical half-graben that was the southerly sub-basin of the Looe Basin (see (Figure 8)).

With the closure of passive margin basins to the north contraction continued, resulting in a province-wide deformation in the late Silesian. In the district, structures of this phase are sporadically developed. These are northwesterly vergent folds and cleavage, with variable crenulation to transposition fabrics that are moderately to steeply inclined south-eastwards. This deformation (D2) in Devonian, Tournaisian–Visean and early Namurian rocks south of the Culm Basin was the first deformation of the post early namurian rocks of that basin — a complex graben system that was fully inverted in the late Westphalian (Leveridge and Hartley, 2006).

In the latest Carboniferous rebound, extension was accompanied in its early stages on the southern side of the Culm Basin by southerly directed thrusting and southerly vergent (D3) folding, and later by extension on low-angle faulting followed by steep normal faulting (alexander and Shail, 1996). The folding is very sparsely present within the district. One of its main manifestations, to the south of Pentewan, is in the steepening and overturning southwards of the Portscatho Formation rocks and their D1 and D2 structures. Late extensional faults are numerous and present throughout the district, some reactivate earlier thrusts and others cause significant displacement of lithostratigraphical units.

Metamorphism

Metamorphism of the Devonian rocks in the district is essentially part of the regional metamorphism, and its nature and timing in the area are key to an understanding of the Variscan geology of the province. It is the product of the mechanical disruption and reorganisation of grains, pressure solution and recrystallisation, particularly of the micaceous and siliceous minerals in the mudstones and fine matrix of the sandstones. It is reflected in the preferred crystal orientation fabric of the slaty cleavage that is an axial plane cleavage to D1 folds, and thus it relates to the dynamothermal effects of the deformation. Elsewhere in the province the role of the depth of burial in influencing or determining grade has also been emphasised (e.g. Warr et al., 1991; Merriman and Kemp, 2001).

The remapping programme included a survey of the metamorphic grade of rocks of the allochthonous sequences of the district, employing the illite 'crystallinity' method that charts reaction progress in the mudrocks. The distribution of the Kübler Index, the measure of crystallinity, of twenty samples in the district points to grade being controlled by the tectonostratigraphy. Highest grades, in the greenschist facies, are developed in the phyllites and incipient schists of the Dodman Formation, emplaced as the uppermost nappe in the allochthonous succession. Mudrocks within the Veryan nappe are mostly epizonal slates, eqivalent to the low-greenschist facies. The uppermost part of the nappe beneath the Dodman thrust does show a slightly lower high-anchizone grade near Hemmick Beach. However, the petrography of metabasic clasts within the Roseland Breccia Formation reveals a wide range of grades, from prehnite-pumpellyite to amphibolite facies, confirming that many were metamorphosed prior to incorporation in the olistostrome. Within the Carrick nappe mudrocks are high anchizonal grade slates, showing the lowest grades in the district.

The grade of metamorphism of the parautochthon to the north of the Carrick thrust was determined by Warr et al. (1991) to be high anchizone passing northwards to epizone, the latter interpreted to be a product of later thermal metamorphism within the aureole of the St Austell Granite. A slight revision of zonal boundary values has placed all within the epizone (see inset on Sheet 353).

The general trend of grade increasing south-eastwards into the uppermost nappe of the allochthonous succession could suggest that the metamorphic pattern was largely developed prior to tectonic transport. Before nappe emplacement the oldest strata, represented by the Dodman Formation, could have been more deeply buried and recrystallised to higher grade mineral assemblages than the less deeply buried rocks now found in the Veryan and Carrick nappes. Imbrication and nappe emplacement could then have inverted the original pattern of burial metamorphism, so that the lowest grade strata in the Carrick nappe are now found at the base of the thrust stack that comprises the allochthonous succession. The role of such burial metamorphism has been clearly defined by Merriman (2005).

However, such is not fully compatible with the stratigraphy of the nappes, and the consequent interpreted assemblage of the allocthonous successions (Leveridge et al., 1990) of the district. The sequences are not separate parts of a simple succession inverted by thrusting — rather, they are different but in part correlative facies juxtaposed by thrusting. Each will have been subject to early burial metamorphism, but as described above, metamorphism was largely imposed during deformation, directional translation and tectonic loading. Leveridge and Holder (1985) have described the situation in this dynamic stratigraphy where sedimentary rocks deformed nearer tectonic source to the south are incorporated into slightly younger sediments. Those are then in turn deformed and metamorphosed during the same migrating deformation and metamorphism. Thus, whilst the lower grade at the top of the Veryan nappe may be a reflection of burial metamorphism, the general decrease of grade north-westwards may relate in part to distance from the driving source of the deformation and metamorphism. Warr et al. (1991), who recognised that the sub-province grade in the Gramscatho Basin was not related to stratigraphical age and therefore not determined solely by burial, proposed that the deformation process and continental thickening by thrusting also played a role.

Metamorphism within the Dodman nappe provides some evidence of this process. Onshore a reverse metamorphic gradient can be observed over 1.8 km, from epizone grade slates to low-grade schists of a higher greenschist metamorphic zone; this represents a temperature gradient of some 300°C to 500°C. Offshore, the distance to the thrust junction with the normannian nappe is similar, about 2.0 km, and at that junction amphibolite grade metamorphism is developed to the south-west near Lizard Point. This is evident in the tectonised margin of the Man of War Gneiss (about 374 Ma; Sandeman et al., 1997), in part of the normannian nappe, in the recrystallisation of the Landewednack amphibolite Schist (about 380–390 Ma; Clark et al., 1998b; Nutman et al., 2001), and possibly the local staurolite-sillimanite-kyanite assemblages of the Old Lizard Head Schists of the Lizard Complex. Such metamorphism may represent temperatures up to 750°C. This approaches compatibility with the syntectonic intrusion of the comingled granite and enriched mid-ocean-ridge basalt — the Kennack Gneiss — with its amphibolite grade metamorphism (about 376 Ma; Sandeman et al., 2000) at that suture boundary, and also the generation of the calc-alkaline granitoid intrusive sills of this district at Caragloose Point. Thus the reverse metamorphic gradient of the Dodman nappe would appear to be related not simply to a burial mechanism but to the pressure and temperature increases generated and conducted about the boundary of the overriding southerly continental margin of the Gramscatho Basin.

Geochronology

Within the district and immediate vicinity limited radiometric data (K/ar; Dodson and rex, 1971, as amended by Warr et al., 1991) indicate that metamorphism was a late Devonian to early Carboniferous event. More recent data (Clark et al., 1998b) extends that event back to the late Mid Devonian (40ar–39ar 385 ± 2 Ma) in the Dodman Formation. Radiometric determinations show uplift metamorphic ages range from late Mid Devonian to early Carboniferous (385–355 Ma), and the uplift ages young from the south-south-east to north-north-west through the nappe pile. Such dates indicate the key feature of the allochthonous sequences — the close temporal interrelationship of sedimentation, migrating deformation, forward propagating thrusting and metamorphism.

Chapter 3 Applied geology

Mineral resources

Stream tin was in the past obtained from the river Fal, below Creed, the Porthluney valley and the St Austell river valley west of Pentewan. The Happy union Stream-works in St Austell river valley were famous in Cornwall being well below sea level at 20 m below OD (Reid, 1907). The tin-ground layer there, comprising sand to boulders of stanniferous ore, was 1–3 m thick, and rested directly on the solid rock base of the overdeepened valley. It was overlain by a sequence of freshwater and marine alluvial deposits (Colenso, 1829) accumulated during the post-Pleistocene drowning and silting-up of the valley.

At all three stream-tin sites quantities of gold were retrieved, with Borlase (1758, in Reid, 1907) recording considerable quantities from the parish of Creed that included a nugget of three quarters of an ounce.

The Ordovician quartzites vary from very dark grey and fine grained with an argillaceous component to white, clean and coarse grained. The latter commonly comprises rounded medium to coarse quartz sand grains, with interlocking quartz crystals grown in optical continuity with grains. The resulting rock is extremely indurate and in the past this was quarried at Carne in Roseland for road metal, and reportedly (Bristow, 1996) was a probable source of high purity silica for electronic equipment in the Second World War.

Building stone

Pentewan Stone

The elvan near Pentewan, known as Pentewan Stone, was an important building stone resource from medieval times to the early 20th century. This was because of its colour and also because it is a fine-grained freestone that is readily wrought or carvable (Plate 11). It thus adorns many churches and the finest buildings in Cornwall (see Bristow, 1996).

Quarrying

Disused small quarries, present in most of the main lithostratigraphical divisions, were formerly sources of local building stone. The Grampound and Portscatho formations, particularly, provided sandstone blocks for farm buildings and walls. Tredinnick Quarry [SW 931 491], just off the district at Grampound in the Grampound Formation, still provides building stone, slate and 'gritstone'.

Hydrogeology

The mean annual rainfall varies from 1000 mm over nare Head to about 1150 mm over the north-west part of the district (Meteorological Office, 1977). The mean annual potential evapotranspiration is 620 mm and the mean annual actual evapotranspiration 540 mm (thompson et al., 1981).

Reid (1907) commented that although the 'rainfall is copious and there are plenty of small springs, the water supply is scarcely satisfactory. Nearly everywhere a well will yield enough water for a house or farm, but it is not easy to find uncontaminated springs large enough to supply a village.'

Yields are low in the bedrock, due to the degree of induration and closed discontinuities at depth. Primary permeability is low with water occurring mainly in fractures, joints and cleavages that are open only near surface. Thus productive boreholes are generally less than 60 m deep. The water table reflects the topography, with water generally rising to within 10 m of the ground surface. However, some boreholes do not strike water above 30 m from surface and a few boreholes are dry. Small quantities of groundwater have been obtained in the past from most of the formations, as the topographic maps indicate the presence of wells and springs throughout the district.

Information on yields is available from records in the national Well record archive for sources in the Carne, Roseland Breccia and Portscatho formations, and the environment agency have licensed abstractions for these plus the Pendower Formation, with the total volume of groundwater licensed from the district being 79 305 m3/annum. Pumping tests are generally of one-hour duration and do not represent long term yields.

Two sources are recorded in the Carne Formation, a 30.5 m-deep borehole at Portholland [SW 9544 4166] and one 45.7 m deep at Veryan [SW 9368 4109], both yielding less than 0.1 l/s during one-hour baler tests for unspecified drawdowns.

In the Roseland Breccia Formation sources vary from 0.06 to 1.3 l/s, this highest yield being obtained from a 19.8 m-deep borehole at Gorran Haven [SX 0091 4229] during a one-hour test. Water was struck at a depth of 10.6 m, but then rose to 3.6 m below ground surface. A 30.5 m-deep borehole at Boswinger [SW 9913 4122] struck water at 28.9 m that rose to 6.1 m below ground level. The yield was 0.6 l/s during a one-hour test.

Yields from the Portscatho Formation are all 0.3 l/s or less; both dry and artesian boreholes have been encountered in this formation. The yield of 0.3 l/s was obtained from a 66 m-deep borehole at trevilvas Farm, Grampound [SW 925 474] during a six-hour test, producing a drawdown of 30 m. Analysis of the pumping test gave a transmissivity value of less than 1 m2/d. A 61 m-deep, dry borehole was drilled at Mevagissey [SX 0058 4696], but a 5.7 m-deep borehole at the Waterworks [SX 0176 4373] in the same village was artesian in 1922.

Where the rocks have undergone faulting, the degree of fracturing may be greater and borehole yields may be increased. A 30.5 m-deep borehole at ruan High Lanes [SW 9277 4190] located near the Veryan thrust, yielded 0.8 l/s. The water rose 25 m above where struck. A spring that supplied the village of Grampound and part of Creed parish near the Carrick thrust at Bossillion [SW 9428 4794], probably issues from fractured sandstone in the Grampound Formation.

Water quality information is available from one source, at tregenna Farm [SW 9457 4065] in the Roseland Breccia Formation, where water has an alkalinity of 114 mg/l and a total dissolved solids concentration of around 300 mg/l. Water is generally harder than would be expected, with water from some igneous rocks depositing tufa (Reid, 1907). Sodium and chloride ion concentrations are slightly elevated due to the influence of the sea on rainfall composition.

Superficial deposits are relatively thin and probable low yield, although larger quantities may be obtained from valley alluvium, where there is likely to be hydraulic continuity with surface watercourses. Water in the shoreface and beach deposits and the permeable sand and gravel of the marine deposits is liable to saline intrusion.

Groundwater protection

The groundwater sources in the district obtain water from shallow minor aquifers that are highly vulnerable to contamination from both diffuse (e.g. nitrates and pesticides) and point source (e.g. storage tanks) pollutants due to the thinness of the unsaturated zone. Successful aquifer remediation in this ground is problematic, prolonged and expensive, and therefore the prevention of pollution is important.

Information sources

Sources of further geological information held by the British Geological Survey relevant to the Mevagissey district and adja cent areas are listed here. Information on BGS publications is given in the current BGS Catalogue of Geological Maps and Books, available on request and at the BGS website (www.bgs.ac.uk). BGS maps, memoirs, books, and reports relevant to the district may be consulted at BGS and some other libraries. They may be purchased from the BGS Sales Desk, or via the bookshop on the BGS website. This website also provides details of BGS activities and services, and information on a wide range of environmental, resource and hazard issues. Searches of indexes to some of the materials and documentary records collections can be made on the BGS website. Geological enquiries, including requests for geological reports on specific sites, should be addressed to the BGS enquiry Service at Keyworth. The addresses of the BGS offices are given on the back cover and at the end of this section.

Maps

Books

Documentary records collections

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

Borehole and trial pit records

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

Hydrogeological data

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

Geophysical data

These data are held digitally in the national Gravity Databank and the national aeromagnetic Databank at BGS Keyworth. Indexes can be consulted on the BGS website.

BGS Lexicon of named rock units

Definitions of the stratigraphic units shown on BGS maps, including those named on Sheet 353 (Mevagissey), are held in the BGS Stratigraphic Lexicon database, which can be consulted on the BGS website. Further information on this database can be obtained from the Lexicon Manager at BGS Keyworth.

BGS photographs

The photographs used in this Sheet explanation are part of the national archive of Geological Photographs, held at BGS in Keyworth and edinburgh. Part of the collection can be viewed on the BGS website. Copies of the photographs can be purchased from BGS.

Materials collections

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

References

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

Alexander, A C, and Shail, R K. 1996. Late- to post-Variscan structures on the coast between Penzance and Pentewan, south Cornwall. Proceedings of the Ussher Society, Vol. 9, 398–404.

Barnes, R P. 1981. The geology of south Cornish mélanges. Unpublished PhD thesis, University of Southampton.

Barnes, R P. 1983. The stratigraphy of the sedimentary melange and associated deposits in south Cornwall, England. Proceedings of the Geologists' Association, Vol. 94, 217–229.

Barreiro, B. 1996. Sr and Nd isotopic compositions of igneous rock fragments from the Roseland Breccia and other related units, Cornwall. NERC Isotope Geosciences Laboratory Report Series, No. 84.

Bather, F A. 1907. Discovery in west Cornwall of a Silurian crinoid characteristic of Bohemia. Transactions of the Royal Geological Society of Cornwall, Vol. 13 (1914), 191–197.

BIRPS, and ECORS. 1986. Deep seismic profiling between England, France and Ireland. Journal of the Geological Society of London, Vol. 148, 45–52.

Bouma, A H. 1962. Sedimentology of some flysch deposits. (Amsterdam, London, New York: Elsevier.)

Bristow, C M. 1996. Cornwall's geology and scenery: an introduction. (St Austell, Cornwall: Cornish Hillside Publications.) I SB N 900147 00 9

Clark, A H, Scott, D J, Sandeman, H A, Broml ey, A V, and Farrar, E. 1998a. Siegenian generation of the Lizard ophiolite: U-Pb zircon age data for plagiogranite, Porthkerris, Cornwall. Journal of the Geological Society of London, Vol. 155, 595–598.

Clark, A H, Sandeman, H A, Liu, C, Scott, D J, Farrar, E, Archibald, D A, Bromley, A V, Jones, K A, and Warr, L N. 1998b. A n emerging geochronological record of the construction and emplacement of the Lizard ophiolite, south-west Cornwall (Extended Abstract). Proceedings of the Ussher Society, Vol. 9, 276–277.

Colenso, J W. 1829. A description of Happy-Union Tin Stream-works at Pentuan. Transactions of the Royal Geological Society of Cornwall, Vol. 4 (1838), 29–39.

Collins, J H. 1879. On the geological structure of the northern part of the Meneage peninsula. Transactions of the Royal Geological Society of Cornwall, Vol. 11, 458–473.

Cook, C A, Holdsworth, R E, and Styles, M T. 2002. The emplacement of peridotites and associated oceanic rocks from the Lizard Complex, south-west England. Geological Magazine, Vol. 139, 27–45.

Darbyshire, D P F, and Shepherd, T J. 1985. Chronology of granite magmatism and associated mineralization, south-west England. Journal of the Geological Society of London, Vol. 142, 1159–1178.

Darbyshire, D P F, and Shepherd, T J. 1994. Nd and Sr isotope constraints on the origin of the Cornubian batholith, south-west England. Journal of the Geological Society of London, Vol. 151, 797–802.

Dean, M T. 2005. Conodont biostratigraphical control for Devonian rocks cropping in the vicinity of Newquay and Mevagissey. British Geological Survey Internal Report, IR/05/014R.

Dean, M T. 2006. Conodont biostratigraphical control for Devonian rocks cropping in the Bodmin and Mevagissey districts of Cornwall. British Geological Survey Internal Report, IR/06/044R.

De la Beche, H T. 1839. Report on the geology of Devon, Cornwall and west Somerset. Memoir of the Geological Survey of Great Britain. (London: HMSO.)

Dodson, M H, and Rex, D C. 1971. Potassium-argon ages of slates and phyllites from southwest England. Quarterly Journal of the Geological Society of London, Vol. 141, 315–326.

Edwards, J W F, Day, G A, and Leveridge, B E. 1989. T hrusts under Mount's Bay and Plymouth Bay. Proceedings of the Ussher Society, Vol. 7, 131–135.

Evans, C D R. 1990. United Kingdom offshore regional report: the geology of the western English Channel and its western approaches. (London: HMSO for the British Geological Survey.)

Flett, J S. 1933. The geology of the Meneage. Memoir of the Geological Survey of Great Britain, Summary of Progress for 1932, part 2, 1–14.

Floyd, P A. 1984. Geochemical characteristics and comparison of the basic rocks of the Lizard Complex and the basaltic lavas within the Hercynian troughs of south-west England. Journal of the Geological Society of London, Vol. 141, 61–70.

Floyd, P A, and Leveridge, B E. 1987. Tectonic environment of the Devonian Gramscatho Basin, south Cornwall: framework mode and geochemical evidence from turbiditic sandstones. Journal of the Geological Society of London, Vol. 144, 531–542.

Floyd, P A, Shail, R, Leveridge, B E, and Franke, W. 1991. Geochemistry and provenance of Rhenohercynian synorogenic sandstones: implications for tectonic environment discrimination. 173–188 in Developments in sedimentary provenance studies. Morton, A C, Todd, S P, and Haughton, P D (editors). Geological Society of London Special Publication, No. 57.

Floyd, P A, Exley, C S, and Styles, M T. 1993a. Igneous rocks of south-west England. Geological Conservation Review Series. (London: Chapman and Hall.) ISBN 0 412 48850 7

Floyd, P A, Holdsworth, R E, and Steele, S. 1993b. Geochemistry of the Start Complex greenschists: Rhenohercynian MORB? Geological Magazine, Vol. 130, 345–352.

Franke, W. 1989. Tectonostratigraphic units in the Variscan belt of central Europe. Geological Society of America, Special Paper, No. 230, 67–90.

Green, U. 1904. The discovery of Silurian fossils of Ludlow age in Cornwall. Geological Magazine, Vol. 1, 289–290.

Hendriks, E M L. 1931. The stratigraphy of south Cornwall. Report of the British Association for 1930, 332.

Hendriks, E M L. 1937. Rock succession and structure in south Cornwall, a revision. With notes on the Central European facies and Variscan folding there present. Quarterly Journal of the Geological Society of London, Vol. 93, 322–360.

Hendriks, E M L. 1939. The Start-Dodman–Lizard Boundary Zone in relation to the Alpine structure of Cornwall. Geological Magazine, Vol. 76, 385–402.

Hendriks, E M L, House, M R, and Rhodes, F H T. 1971. Evidence bearing on the stratigraphical successions in south Cornwall. Proceedings of the Ussher Society, Vol. 2, 270–275.

Holder, M T, and Leveridge, B E. 1986a. A model for the tectonic evolution of south Cornwall. Journal of the Geological Society of London, Vol. 143, 125–134.

Holder, M T, and Leveridge, B E. 1986b. Correlation of the Rhenohercynian Variscides. Journal of the Geological Society of London, Vol. 143, 141–147.

Lambert, J L M. 1965. A reinterpretation of the breccias in the Meneage crush zone of the Lizard boundary, south-west England. Quarterly Journal of the Geological Society of London, Vol. 121, 339–357.

Lang, W H. 1929. On fossil wood (Dadoxylon hendriksi, n.sp) and other plant remains from the clay slates of south Cornwall. Annals of Botany, Vol. 43. 663–683.

Le Gall , B, Le Herisse, A, and Deunff, J. 1985. New palynological data from the Gramscatho Group at the Lizard front (Cornwall): palaeogeographical and geodynamical implications. Proceedings of the Geologists' Association, Vol. 96, 237–253.

Leveridge, B E. 1974. The tectonics of the Roseland coastal section, south Cornwall. Unpublished PhD thesis, University of Newcastle-upon-Tyne.

Leveridge, B E, and Hartley, A J. 2006. The Variscan Orogeny: the development and deformation of Devonian/Carboniferous basins in south-west England and South Wales. 225–255 in The geology of England and Wales. Brenchley, P J, and Rawson, P F (editors). (London: The Geological Society of London.)

Leveridge, B E, and Holder, M T. 1985. Olistostromic breccias at Mylor/Gramscatho boundary, south Cornwall. Proceedings of the Ussher Society, Vol. 6, 147–154.

Leveridge, B E, Holder, M T, and Day, G A. 1984. Thrust nappe tectonics in the Devonian of south Cornwall and the western English Channel. 103–112 in Variscan tectonics of the North Atlantic region. Geological Society of London Special Publication, No. 14.

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, Holder, M T, Goode, A J J, Scrivener, 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).

Merriman, R J. 2005. Clay minerals and sedimentary basin history. European Journal of Mineralogy, Vol. 17, 7–20.

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. Geoscience in south-west England, Vol. 10, 77–85.

Merriman, R J, and Kemp, S J. 2001. Metamorphism of the Palaeozoic rocks of the Torquay district, Devon, 1:50k Sheet 350. British Geological Survey Internal Report, IR/01/184.

Meteorological Office. 1977. Average Annual rainfall (millimetres), International Standard Period 1941–1970, Southern Britain, 1:625 000.

Molyneux, S G. 2006. A palynological investigation of samples from the Devonian of S W England. British Geological Survey Internal Report, IR/06/107.

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.

Murchison, R I. 1846. A brief review of the classification of the sedimentary rocks of Cornwall. Transactions of the Royal Geological Society of Cornwall, Vol. 6, 317–326.

Nutman, A P, Green, D H, Cook, C A, Styles, M T, and Holdsworth, R E. 2001. SHRIMP U-Pb zircon dating of the exhumation of the Lizard Peridotite and its emplacement over crustal rocks: constraints for tectonic models. Journal of the Geological Society of London, Vol. 158, 809–820.

Peach, C W. 1841. An account of the fossil organic remains found on the south-east coast of Cornwall, and in other parts of that county. Transactions of the Royal Geological Society of Cornwall, Vol. 6 (1864), 12–23.

Reid, C. 1907. The geology of the country around Mevagissey. Memoir of the Geological Survey of Great Britain, Sheet 353 (England and Wales).

Sadler, P M. 1973. An interpretation of new stratigraphic evidence from South Cornwall. Proceedings of the Ussher Society, Vol. 3, 535–550.

Sandeman, H A, Clark, H A, Scott, D J, and Malpas, J. 2000. The Kennack Gneiss of the Lizard peninsula, Cornwall, S W England: co-mingling and mixing of mafic and felsic magmas accompanying Givetian continental incorporation of the Lizard ophiolite. Journal of the Geological Society of London, Vol. 157, 1227–1242.

Sandeman, H A, Clark, H A, Styles, M T, Scott, D J, Malpas, J G, and Farrar, E. 1997. Geochemistry and U-Pb and 40Ar–39Ar geochronology of the Man of War Gneiss, Lizard Complex, south-west England: pre-Hercynian arc-type crust with a Sudetan–Iberian connection. Journal of the Geological Society of London, Vol. 154, 403–417.

Sedgwick, A. 1852. On the slate rocks of Devon and Cornwall. Quarterly Journal of the Geological Society of London, Vol. 8, 1–19.

Shail, R K. 1992. Provenance of the Gramscatho Group, south Cornwall. Unpublished PhD thesis, University of Keele.

Shail, R K, and Leveridge, B E. 2005. Large-scale southward facing structures in the Looe Basin (Lower Devonian), south-west England (Abstract). Geoscience in south-west England, Vol. 11, 164.

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Trist, S J. 1818. Notes on the limestone rocks in the Parish of Veryan. Transactions of the Royal Geological Society of Cornwall, Vol. 1, 107–113.

Ussher, W A E, Barrow, G, and MacAlister, D A. 1909. The geology of the country around Bodmin and St Austell. Memoir of the Geological Survey of Great Britain, Sheet 347 (England and Wales).

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Warr, L N, Primmer, T J, and Robinson, D. 1991. Variscan very low-grade metamorphism in south-west England; a diastathermal and thrust related origin. Journal of Metamorphic Geology, Vol. 9, 751–764.

Wilkinson, J J, and Knight, R R W. 1989. Palynological evidence from the Porthleven area, south Cornwall: implications for Devonian stratigraphy and Hercynian structural evolution. Journal of the Geological Society of London, Vol. 146, 739–742.

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

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

(Index map)

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

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

Figures and plates

Figures

(Figure 1) Mevagissey district and sedimentary basins of south-west England.

(Figure 2) Parautochthonous and allochthonous terrains in south Cornwall.

(Figure 3) Successions of the Mevagissey (353) Sheet.

(Figure 4) Maps showing breccias near Gidley Well and the Straythe. A Detailed map showing the relationships of the Pendower and Carne formations, with olistostrome in the latter, at Gidley Well. b Map showing disposition of igneous rocks and breccias in the Roseland Breccia Formation about the Straythe.

(Figure 6)." data-name="images/P1004238.jpg">(Figure 5) Major elements plot of igneous rock types from the Pendower and Roseland Breccia formations. Numbering as for (Figure 6).

(Figure 6) Basic and acidic igneous rocks of the Pendower Formation (1) and Roseland Breccia Formation (2–20). Chemical characteristics, metamorphic grade and tectonic discrimination is based on major, trace and rare-earth element data held by BGS.

(Figure 7) Summary of Variscan deformation events in the Mevagissey district. Regional deformation phases as defined by Alexander and Shail (1995, 1996). D1a and D2a designated D2 and D4 respectively on adjacent Sheet 353 Falmouth (Leveridge et al., 1990)

(Figure 8) Diagram showing the interpreted sequence of D1 events producing the relationships of lithostratigraphical and tectonic units juxtoposed about Pentewan. The Gramscatho Basin marginal Pentewan thrust is occluded in the district. The region was subjected to further contraction during D2 and extension during D3. Not to scale.

(Front cover) Cover Photograph Portloe looking east from Puckey's Hill [SW 9398 3942] showing the Roseland Breccia Formation in the cove (Photograph B E Leveridge; P662661).

(Rear cover)

(Geological succession) Geological succession in the Mevagissey district

Plates

(Plate 1a) Lithologies found in the district. Mudstone, siltstone and sparse sandstone of the Polglaze Formation, with quartz veining, strongly sheared along D1 and narrowly oblique D2 cleavages in the Polrudden 'Shear Zone' [SX 0256 4752] (P662662).

(Plate 1b) Lithologies found in the district. Interlaminated grey silty mudstone and graded fine-grained sandstone of the Grampound Formation [SX 02478 4725] (P662663).

(Plate 1c) Lithologies found in the district. Medium to very thick beds of sandstone of the treworgans Sandstone Member, Gamas Point; looking south-west [SX 0231 4718] (P662664).

(Plate 1d) Lithologies found in the district. Amalgamated thick to thin beds of limestone turbidite of the Pendower Formation, near Gidley Well; looking north [SW 9081 3815] (P662665).

(Plate 2) Sedimentary breccia, olistostrome, clasts, including this phacoid of Emsian limestone, in pale grey and very dark grey mudstone matrix, near the base of the Carne Formation, Gerrans Bay, looking east [SW 9088 3810] (P662666).

(Plate 3) Ordovician quartzite crags at Carne within the Roseland Breccia Formation. Cliffs, middle right, are within the MORB volcanic rocks of nare Head; looking south-east [SW 9124 3816] (P662667).

(Plate 4) Roseland Breccia Formation at Paradoe Cove: the predominant facies is a mudstone matrix-supported breccia with clasts up to block size of quartzite (bottom centre and right), limestone (lower centre), lenticular tuff bed (bottom left), and lens of polymict conglomerate (centre left). Looking north-north-east [SW 9144 3767] (P662668).

(Plate 5) Blocks of metabasalt (MORB) breccia (centre and right) and conglomerate lens (centre left) in the Roseland Breccia Formation, Malmanare Point south of Mallet's Cove; looking south [SW 9149 3754] (P662669).

(Plate 6) Ocean-floor pillow basalt (MORB), the right way up, near the base of the nare Head volcanic mass, within the Roseland Breccia Formation; looking north-north-west [SW 9180 3803]. Photograph R K Shail (P662670).

(Plate 7) Lenticles of indurate sandstone, with fragments and blocks of quartzite and limestone, showing crude down-dip alignment in the matrix-supported Roseland Breccia Formation, Parc Caragloose Cove; looking east [SW 9286 3837] (P662671).

(Plate 8) Granitoid sills, off-white, interfinger with greenish black hornblende schist breccia in the Roseland Breccia Formation at Caragloose Point; looking east [SW 9465 3991] (P662672).

(Plate 9) Greeb Point and the associated prominent feature (centre ground) formed predominantly of acid ash flow tuff in the Roseland Breccia Formation, and Dodman Point (top left) in the metasandstones and the metamudstones of the Dodman Formation; looking south-east [SW 9800 4095] (P662673).

(Plate 10) Head comprising crudely stratified unconsolidated clay, silt, sand and locally derived rock fragments, up to 12 m thick, resting on the 2.5–5 m platform near Carne; looking north [SW 9094 3804] (P662674).

(Plate 11) Sculptures in Pentewan Stone — altered quartz porphyry (sculptures and photograph © Karl Williams; P662675).

Figures

(Geological succession) Geological succession in the Mevagissey district

QUATERNARY Onshore Offshore
Made ground Mud, sand and gravel
Landslip
Blown sand
alluvium
Shoreface and beach deposits
Head
PERMO-TRIASSIC undivided strata
DEVONIAN PARAUTOCHTHON ALLOCHTHON
GRAMSCATHO BASIN
Gramscatho Group
Carrick Nappe
Portscatho Formation
(Mid Devonian? To Frasnian/?Famennian) Devonian undivided, Middle and upper Devonian undivided, Lower Devonian undivided. Includes Parautochthon, Carrick nappe and Dodman nappe. The latter locally comprises mica/amphibole/ dolomite schists. Basic and ultrabasic rocks (probably Devonian)
LOOE BASIN
Veryan Nappe
Meadfoot Group (early To Mid Devonian) Roseland Breccia Formation
Polglaze Formation (Giventian? To Frasnian)
Hallane Formation
Carne Formation (Givetian?)
GRAMSCATHO BASIN
Gramscatho Group
Grampound Formation (early To Mid Devonian?) Pendower Formation (Mid To Late Eifelian)
Treworgans Sandstone Member
Dodman Nappe
Dodman Formation (early Devonian?)

(Figure 6) Basic and acidic igneous rocks of the Pendower Formation (1) and Roseland Breccia Formation (2–20)

Chemical characteristics, metamorphic grade and tectonic discrimination is based on major, trace and rare-earth element data held by BGS

Sample number Location Grid Reference Rock type Grade/ alteration Nb/Y Vs Zr/ TiO2 Nb-Zr-Y Zr Vs Zr/Y REE Plots
1 Tubbs Mill* [SW 9626 4223] basalt lava pillowed prehnite–pumpellyite subalkaline WPthB + VAB WPB LREE enriched
2 Pennarin Point* [SW 9134 3767] basic tuff non-diagnostic subalkaline WPthB + VAB WPB
3 Mallet's Cove† [SW 9155 3752] mafic breccia epidotised subalkaline MORB + VAB MOrB LREE depleted/flat
4 Nare Head† [SW 9205 3728] basalt lava prehnite–pumpellyite subalkaline WPthB + VAB WPB LREE depleted/flat
5 Kiberick† (NH) [SW 9241 3785] basaltic pillow lava non-diagnostic subalkaline MORB + VAB MORB LREE depleted/flat
6 Kiberick† (NH) [SW 9228 3789] gabbro greenschist? subalkaline MORB + VAB MORB

off ridge

 LREE depleted/flat
7 Perhaver† SX 0145 4189] basalt lava, pillowed greenschist subalkaline WPthB + VAB WPB LREE depleted/flat
8 The Straythe* [SW 9297 3871] mafic lava, pillowed, brecciated epidotised basanite/ nephelinite WPalkB no field
9 Caragloose rock [SW 9298 3835] mafic intrusive, brecciated epidotised (prehnite-pumpellyite) subalkaline P-MORB MORB LREE enriched
10 Jacka Point* [SW 9388 3832] mafic, brecciated epidotised subalkaline MORB + VAB MORB LREE enriched
11 Jacka Point* [SW 9387 3931] mafic, brecciated epidotised subalkaline MORB + VAB MORB LREE depleted/flat
12 Jacka Point* [SW 9387 3929] mafic breccia amphibolite subalkaline MORB + VAB MORB LREE depleted/flat
13 Hartriza Point* [SW 9423 3967] basalt lava, brecciated prehnite-actinolite subalkaline MORB + VAB MORB LREE depleted/flat
14 Caragloose Point [SW 9467 3993] amphibolite breccia amphibolite subalkaline WPalkB WPB LREE enriched
Sample Number Location Grid reference Rock type Nb/Y vs Zr/TiO2 Suite Y + Nb vs Rb Nb vs Y
15 Gorran Haven [SX 0134 4169] felsic tuff andesite calc-alkaline volcanic arc granite syn-collision granite + VAG
16 Hemmick Beach [SW 9924 4054] felsic tuff/ lava trachyandesite calc-alkaline volcanic arc granite syn-collision granite + VAG
17 Percunning Cove [SW 9914 4056] felsic tuff/ lava trachyandesite calc-alkaline volcanic arc granite syn-collision granite + VAG
18 Greeb Point [SW 9825 4063] felsic tuff trachyandesite calc-alkaline volcanic arc granite syn-collision granite + VAG
19 Caragloose Point [SW 9465 3992] granite sill (margin) trachyte calc-alkaline volcanic arc granite syn-collision granite + VAG
20 Caragloose Point [SW 9465 3992] granite sill (centre) trachyandesite calc-alkaline volcanic arc granite syn-collision granite + VAG
* in sequence unit

† clast in breccia

LREE Light rare-earth element

WPB Within-plate basalt

WPthB Within-plate tholeiitic basalt

WPalkB Within-plate alkali basalt

VAB Volcanic-arc basalt

VAG Volcanic-arc granite

MORB Mid-ocean-ridge basalt (P: plume)

(Figure 7) Summary of Variscan deformation events in the Mevagissey district

Deformation Structure Characteristics Related
D1 (regional) S1 cleavage Ubiquitous; strike NE moderately inclined south-eastwards in the south, E–W in the north moderately to steeply inclined southwards south of Carrick Thrust, gently to moderately inclined northwards to north of thrust; slaty in argillaceous rocks, pressure solution in arenaceous rocks, stronger to south; mineral grain and preferred orientation extension lineation NNW–SSE Anchizonal, epizonal and low greenschist metamorphic grades
F1 Folds Minor to large mesoscopic sheath folds; asymmetrical tight to isoclinal; predominant southerly trend with facing eastwards or westwards and northerly translation about extension lineation in Gramscatho Basin rocks; incipient sheathing and southerly translation in Looe Basin rocks
Thrust faults Minor to major; within sequence bounding successions, verge northwards in Gramscatho Basin
D1a (local) S1a cleavage Sporadic only; strike slightly clockwise of S1 and inclination slightly steeper; crenulation to transposition cleavage
F1a folds Minor couplets; trend ESE; close to tight; verge NNE
D2 (regional) S2 cleavage Sporadic; NE strike in south, E–W in north; inclined moderately to steeply southwards; spaced crenulation to transposition, dislocation to penetrative shear; common quartz association
F2 folds Minor to large mesoscopic; trend NE to E–W; moderately to steeply inclined southwards; close to tight; verge north-westwards
Thrust faults Minor, associated with cleavage and folds
D2a (local) S2a cleavage Sporadic; NE trend, steeply inclined SE; spaced crenulation
D3 (regional) F3 folds Minor to large mesoscopic couplets developed locally; southerly vergence Local southward steepening and rotation of D1 and D2 structures
S3 cleavage Sporadic; gently inclined northwards; spaced crenulation
Latest Variscan Faults Extensional movements on earlier thrust faults; steep normal faults, predominantly E–W
Regional deformation phases as defined by Alexander and Shail (1995, 1996). D1a and D2a designated D2 and D4 respectively on adjacent Sheet 353 Falmouth (Leveridge et al., 1990)