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Geology of the Newquay district — brief explanation of the geological map Sheet 346 Newquay

L M Hollick, R C Scrivener and C E Burt

Bibliographic reference: Hollick, L M, Scrivener, R C, and Burt, C E. 2014. Geology of the Newquay district — a brief explanation of the geological map. Sheet Explanation of the British Geological Survey. 1:50 000 Sheet 346 Newquay (England and Wales).

Keyworth, Nottingham: British Geological Survey, 2014

(Front cover) Cover photograph Penhale Point looking north-west. The faulted and dipping bedrock sequence comprises interbedded slate and sandstones of the Meadfoot Group. (Photographer: P J Witney, P701733).

(Rear cover)

(Geological succession) Summary of the geological succession in the Newquay district.

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Notes

The word 'district' refers to the area represented on the 1:50 000 geological series sheet 346 Newquay. Specific locations and boreholes are accompanied by National Grid references, given in square brackets, at their first mention in the text, and unless indicated otherwise lie within 100 km square SW. Lithostratigraphical symbols shown in brackets in the text, for example (Wtd), are those shown on the published 1:50 000 geological map. Numbers at the end of photograph descriptions refer to the British Geological Survey official collection numbers.

Acknowledgements

Contributors to this sheet explanation are D Entwisle (foundation conditions), M Lewis (hydrogeology) and C Scheib (radon emissions). Figures were drawn by H Holbrook, pagesetting was by A J Hill and the manuscript was edited by C E Burt, S G Molyneux and J E Thomas. The authors gratefully acknowledge the co-operation of landowners and farmers in allowing access to their lands during the geological survey of the sheet.

The National Grid and other Ordnance Survey data © Crown copyright and database rights 2014. Ordnance Survey Licence No. 100021290.

Geology of the Newquay district (summary from rear cover)

An explanation of sheet 346 (England and Wales) 1:50 000 series map

(Rear cover)

The Newquay district is presently renowned as a tourist destination, with the spectacular landscape and scenery of north Cornwall's Atlantic Coast. However, in previous centuries it was more famous as an industrial area, long associated with the metalliferous mining industry.

Devonian sedimentary rocks crop out through most of the district. These were deposited in a series of rift basins situated on an extending continental plate margin and record differing sedimentary environments, from lacustrine through to deep marine. The Devonian rocks were subsequently deformed in the Variscan Orogeny: a northward propagating mountain-building event that formed the regional east–west strike of the geology across the Cornish peninsula.

The small granite masses at St Agnes and Cligga Head form part of the Cornubian Batholith, a large body of granite, connected at depth, which stretches from Dartmoor south-west through Cornwall to the Isles of Scilly. The batholith has a complex history, with polyphase intrusion and related mineralisation taking place in the Permian. Quartz-porphyry 'elvan' dykes, seen throughout the area, are also thought to be derived from the granites at depth.

The St Agnes–Perranporth mining district shows an extended history of mineralisation and the deposits worked include ores of tin, tungsten, copper, lead, silver, zinc and arsenic. Many of these minerals are related to the intrusion of the granite or to later stage processes, but in the Perranzabuloe district, the 'Great Perran Iron Lode' is thought to be an earlier stratiform deposit, which runs from Perran Bay for at least 7 km to the south-east and has been worked at a number of different sites, both opencast and underground.

Evidence of the pre-Quaternary landscape processes in the area is scarce, but two outliers of Cenozoic sediments are present near St Agnes Beacon. These include sand and clay and are likely to represent the products of weathering in a warm temperate environment.

The geology of the district has played an important role in the industrial and scenic evolution of Newquay, forming both the economic minerals, which led to the large-scale mining, and also the rocky shorelines and wide sandy beaches that attract the holidaymakers.

Chapter 1 Introduction

This Sheet Explanation presents an account of the geology of the district covered by the Geological 1:50 000 Sheet 346 Newquay, published in 2012. The district is mainly underlain by Upper Palaeozoic rocks of Devonian age. These have undergone deformation in the Variscan Orogeny, a northward propagating mountain building event which commenced in the Devonian in this area, and are variably folded and faulted. Minor granite intrusions of latest Carboniferous to early Permian age occur near the coast and Cenozoic deposits are present locally around St Agnes, in the west. The regional strike in the district, and across the Cornish peninsula, is east–west, so coastal exposures afford a structural cross-section that is almost at right-angles to strike.

Several notable early workers commented on aspects of the geology in the district, among them Borlase (1758), Conybeare (1817) and Sedgwick (1820), though the earliest detailed geological notes were published in 1839 by Sir Henry De la Beche, to accompany a geological map published in the same year. Further metalliferous lodes were added subsequently to De la Beche's map by Sir W W Smyth (1858), and the work was republished in 1866. Survey at the same scale (1:10 560) by Clement Reid, J B Scrivenor and D A MacAlister was carried out around 1900–1905 and published in 1906 at a scale of 1:63 360. A reprint of the Newquay geological sheet, based predominantly on previous work, but converted to the modern 1:50 000 scale, was published in 1974.

Chapter 2 Geological description

Devonian

A summary of the geology in the Newquay district is shown in (Figure 1). Elsewhere in the region, sedimentary rocks of similar age and nature have been described as the basin infill of east–west-trending, fault-bounded basins, that were generated in response to crustal extension on the passive margin of the Tethyan Ocean in Silurian and earliest Devonian times (e.g. Leveridge et al., 2002). Sediments young from earliest Devonian to late Carboniferous across the Variscan belt, indicating progressive basin formation from south to north on a regional scale. Sequences preserved in separate basins cannot be directly correlated to one another, and along-strike variations are also difficult to trace; palaeontological dates suggest deposition was simultaneously active in multiple basins (e.g. Leveridge et al., 2002; Leveridge, 2008; Shail, 1992). Within the Newquay district, evidence of sedimentation in two Devonian basins is discernable: the Dartmouth and Meadfoot groups were deposited in the Looe Basin, in the northern part of the district, whereas the Gramscatho Group was deposited in the Gramscatho Basin, to the south (Figure 2). Present-day boundaries between sedimentary rocks derived from distinct basins are tectonic, though essentially parautochthonous in nature.

At the base of the succession in the Looe Basin, the Whitsand Bay Formation (Wtd) of the Dartmouth Group comprises thinly interbedded red, green and grey mudstone with some beds of siltstone and pale grey, fine-grained sandstone. They have a very similar character to rocks described to the east (Leveridge et al., 2002). Although dominated by mudstone, the succession also contains thin beds of siltstone, and sporadic thin sandstones with planar laminations and ripple marks that provide local way-up indicators. Palynomorphs from Watergate Bay indicate a late Lochkovian–early Pragian age (Davis, 1990). These dates are in agreement with dates indicated by other fossil material, such as the pteraspids, including Pteraspis cornubieus, acanthodians and a possible Coccosteus found by Reid and Scrivenor (1906), and Protaspis, Rhinopteraspis (Althaspis) leachi and R. dunensis recorded by White (1956).

Bedding is preserved in much of the exposure and dips dominantly southwards. The earliest regional-scale cleavage is subparallel to bedding, except in tight isoclinal fold hinges. Beds are typically right way up, and young towards the south. The base of the formation is not seen; the northern limit of its outcrop is defined by a south-dipping, northward-translating thrust, which is poorly exposed close to the entrance of Watergate Bay [SW 8410 6489]. The thrust locally places the Whitsand Bay Formation vertically above the Bovisand Formation (Meadfoot Group), though a stratigraphical contact is seen to the south (see below).

The depositional environment of the Whitsand Bay Formation in the Newquay district is difficult to interpret due to a paucity of preserved sedimentary structures. Elsewhere on the peninsula, equivalent rocks have been interpreted as the deposits of a predominantly lacustrine environment (Smith and Humphreys, 1991; Leveridge et al., 2002), though the former authors suggest that phosphatic concretions towards the top of the formation may be the result of episodic marine incursions.

A transitional contact between the Whitsand Bay Formation and the overlying Meadfoot Group extends through an interval of approximately 5 m in cliffs just north of Whipsiderry Beach [SW 8326 6342] (Plate 1). At the base of the Meadfoot Group, the Bovisand Formation (Bov) mainly comprises grey slaty mudstone and siltstone with some beds of fine- to medium-grained feldspathic sandstone and limestone. Bedding in the sandstone varies in thickness from m-scale amalgamated beds to mm-scale laminations, with the former locally containing good examples of coarsening- and thickening-up sequences, cross- and parallel-lamination, loading structures and normally graded bedding. Some sandstone beds have a carbonate cement and weather to a brown honeycomb texture, for example around the north side of Pentire ([SW 7824 6167] to [SW 7952 6163]).

These units were described as limestones by Reid and Scrivenor (1906). In Watergate Bay tuffaceous beds up to 10 mm thick are present. Unidentifiable fish fragments were also found at [SW 8416 6525].

Although no diagnostic biostratigraphical assemblages were obtained during the resurvey, the formation is of Pragian to mid Emsian age farther east (Leveridge et al., 2002). A marine fauna indicating a late to mid Pragian age, identified by Evans (1981) in the Plymouth district, was attributed to the Dartmouth Beds but redefinition of boundaries now places this in the basal part of the Bovisand Formation.

Exposure inland is restricted to small quarries and road cuttings, e.g. around the gate of Kestle Mill Farm [SW 8545 5928], though evidence from brash is plentiful. Fragments of medium-grained sandstone, variably dark grey, blue-grey and less commonly grey-green, predominate, with minor mudstone and laminated siltstone. Dark grey mudstone with thin beds of fine-grained sandstone is more predominant in the southern part of the outcrop, around Coswarth [SW 8690 5973].

Deposition in a marine shelf environment, accompanied by sporadic syn-depositional magmatism (represented by thin tuffaceous beds), has been suggested for the Bovisand Formation to the east, in the Plymouth district (Leveridge et al., 2002). In the Newquay district, background sedimentation was dominated by mud, supplemented by episodic incursions of fine- and medium-grained sand, possibly representing input during higher-energy (storm) events from a nearshore environment. There is little evidence, however, that nearshore conditions prevailed for any significant amount of time, and the succession also fines upwards (to the south), suggesting progressively deeper conditions of deposition (Hollick et al., 2006). It seems likely that the Bovisand succession in the Newquay area represents a deeper-water equivalent of that seen farther east.

The Staddon Formation (StG) underlies the high ground at Denzell Downs [SW 9001 6686] and south of St Eval [SW 8850 6840] in the northern part of the district, although it is poorly exposed in the Newquay district. Pale brown, fine- to medium-grained sandstone with a sugary texture, commonly thin to medium bedded with a 'flaggy' appearance, is seen at outcrop [SW 8645 6751]. The formation forms a well-drained soil, weathering to a rich red-brown colour, in which weathered blocky fragments of sandstone are plentiful. The Meadfoot Group at its type locality in Meadfoot Bay, near Torquay, becomes increasing sandy towards the top, though the Staddon Formation cannot be differentiated (Evans, 1981). East of Plymouth, at Staddon Heights [SX 4939 5146], an extensive sequence of purple and red sandstone is present, though to the east of Plymouth Sound, beds of Staddon Grit become less distinct (Lloyd, 1933). Dineley (1961) suggested that westward thickening of the Staddon Formation occurs across the Cornish peninsula, possibly at the expense of the lithologies attributed to the Bovisand Formation.

The Trendrean Mudstone Formation (Trd) (Hollick, 2006) incorporates the 'Perran Shales' and some of the 'Meadfoot Beds' as recorded in this district by Reid and Scrivenor (1906). Extending from west to east across the district, the outcrop reaches a maximum width of 7 km in the west, reducing to approximately 5 km in the east. The type locality is within the Newquay district, in a low road cut along an abandoned highway north of Trendrean Farm at [SW 8340 5752]. A more extensive, though heavily vegetated section is present along a disused railway cutting at Shepherd's Farm [SW 8135 5375]. On the coast, its northernmost exposures are at Pentire Point West [SW 7730 6125], extending south to near Perranporth [SW 7578 5471]. Dark grey, locally black mudstone, with sharp-based siltstone lamellae that fine upwards into mudstone and dark grey to pale grey sandstone beds, are characteristic of this formation. East–west elongate bodies, comprising dark grey siltstone in beds up to 20 mm thick, lenses/layers of dark mudstone, some fine-grained sandstone layers, and layers of carbonate with brown/beige weathering, are seen to the north-west of Entrance Wood [SW 8810 5355] and north of Arrallas [SW 8841 5443]. A mid Emsian or younger age is indicated by a palynomorph assemblage (Molyneux, 2006), suggesting that the formation may be temporally equivalent to the top part of the Bovisand Formation in the Plymouth district (Leveridge et al., 2002). Sedimentary facies indicate a combination of hemipelagic and fine-grained turbiditic deposition. In the west, the Trendrean Mudstone Formation includes grey micaceous mudstone with local occurrences of siltstone. Field brash around the north-east of Goonhavern includes grey, finely micaceous mudstone and much tectonic quartz vein material. Mine spoil at [SW 8117 5483] includes cleaved black mudstone and vein quartz with pyrite, and nearby, at [SW 8123 5485], blocks of mine waste comprise grey, siliceous silty mudstone intercalated with black mudstone. Weathered sections are exposed in the north-east-trending, disused railway cutting between Shepherd's Farm [SW 817 544] and East Downs Bridge [SW 8096 5356], and the waste dumps of Shepherd's Mine [SW 818 537] to [SW 818 543] contain much comminuted slate, mostly argillaceous and dark grey, blue-grey and black in hue, together with vein quartz and slate breccia. Close to the boundary with the tectonically overlying Grampound Formation, the slate fragments commonly contain tectonic quartz lenticles or 'quartz eyes'; the westernmost example is at [SW 827 535], with a concentration of tectonised quartz fragments present south-east of Nantillio [SW 871 537].

Some of the rocks in the Grampound Formation (Gmp) of the Gramscatho Group were incorporated in the 'Ladock Beds' or 'Grampound Grit' by Reid and Scrivenor (1906). The latter encompassed a range of rocks with a wide geographical distribution, now assigned to the Portscatho, Carne and Roseland Breccia formations (Leveridge, 2008). Reid and Scrivenor (1906) noted a decrease in mean grain size, bed thickness and overall sandstone: mudstone ratio from east to west, and although evidence for this has been recorded during the resurvey it may be an oversimplification. The definition of the Grampound Formation used here is that given by Leveridge (2008). The lower part of the formation comprises interbedded buff siltstone and grey-green fine sandstone, with fine (mm-scale) laminations and some beds of coarser feldspathic sandstone, passing upwards (to the south) into olive-grey, bedded siltstone and fine-grained sandstone with local occurrences of coarse feldspathic sandstone and fine conglomerate. Dark grey mudstone, locally metamorphosed to 'slate' is present in variable proportions.

The coastal outcrop of the Grampound Formation is bound by faulted contacts in both the north [SW 7545 5450] and the south [SW 7260 5206], where it is in contact with the Trendrean Mudstone and Porthtowan formations, respectively. Inland, the outcrop width ranges from 2500 m to 3500 m, and the outcrop is affected by numerous prominent faults that trend between north-north-west to south-south-east and north-north-east to south-south-west. A strong topographical feature marks the boundary of the Grampound Formation with the Trendrean Mudstone Formation between Perranporth and Carland Cross, being particularly pronounced to the south and east of Goonhavern, where dark grey mudstone of the Trendrean Mudstone Formation is adjacent to fragments of siltstone and fine sandstone of the Grampound Formation (Plate 2).

The lowest parts of the formation are not well exposed on the coast. Thermal metamorphism, related to emplacement of the Cligga Head Granite, affects the rocks from Hanover Cove [SW 738 531] northwards to Shag Rock [SW 747 542], altering the country rock to hard splintery siltstone hornfels and grey chloritic slate hornfels, with abundant quartz lenses close to the faulted contacts of the intrusion. Inland, the lower parts of the formation in the west of the area comprise up to 5 m of interbedded buff siltstone and grey-green fine sandstone with fine (mm-scale) laminations, and some beds of coarser feldspathic sandstone exposed at [SW 7590 5407]. Progressively higher parts of the formation are seen in exposures to the south, typically comprising olive-grey, bedded siltstone and fine-grained sandstone at [SW 7675 5351], with local occurrences of coarse feldspathic sandstone and fine conglomerate (for example, at the site of the disused New Chiverton Mine [SW 7727 5287]).

The southern part of the coastal exposure of the Grampound Formation, between the coast to the north of Trevellas Porth [SW 721 521] and Hanover Cove, is dominated by grey silty slate, some hard grey slate and local thin beds of hard, fine-grained silty sandstone. A gradational transition at the top of the Grampound Formation, passing into the Porthtowan Formation, has been recorded in limited exposures, dump material and field brash around West Chiverton and Chiverton mines [SW 7897 5073] to [SW 7956 5112]. Beds of grey micaceous and partly laminated slate, grey silty slate and fine-grained grey sandstone pass up into 'gritstone' (coarser-grained turbiditic sandstone) to the south.

Lithologies of the Grampound Formation further inland, through the central and eastern parts of the Newquay district, are essentially similar to those described above, but with finer-grained rocks becoming more prevalent to the east. A road cutting to the north of Cost-is-lost, at [SW 8025 5200], shows brown-weathered, olive-grey to buff siltstone and fine sandstone, and thin argillaceous interbeds are present further south, in a small quarry at [SW 8071 5122]. Slightly coarser sediments are present close to the boundary with the overlying Porthtowan Formation, brash from around St Allen [SW 823 506] containing roughly equal amounts of siltstone, fine sandstone and dark grey slate. Further east, at least two examples of coarsening-up to the south are identified, though ductile folding and brittle deformation disrupt the sequences.

Laminated dark grey mudstone and grey-blue silty sandstone around Boswiddle Farm and Hay Farm [SW 8697 5146] represent the lowest part of a sedimentary cycle. Green-grey mudstone and siltstone and green-blue, fine-grained sandstone are present to the north-east, near to Treveale Farm [SW 8740 5155], and coarser lithologies at the top of a coarsening-up cycle are seen to the south in a small quarry at [SW 8760 5100], comprising laminated grey siltstone, fine- to medium-grained sandstone (20 cm-thick beds) and coarse-grained sandstone (40 cm-thick beds) with some hard dark grey mudstone. Pebbly sandstone (gritstone), with pebbles up to 1 cm diameter, is present around Nansough Farm [SW 8780 5050]; blocks have been excavated from a small quarry near to the farm, and used in the walls of the farmhouse.

The age of the Grampound Formation is not known with certainty. The stratigraphical position of the formation, below the Porthtowan Formation, may indicate a Mid Devonian age.

Sandstones within the upper parts of the Grampound Formation include the Treworgans Sandstone Member, as recognised in the Falmouth and Mevagissey districts (Leveridge et al., 1990; Leveridge, 2008). In the former area this unit was originally attributed to the base of the Porthtowan Formation, but was reclassified as a component of the Grampound Formation during the resurvey of the latter. The type locality of the Treworgans Sandstone Member lies just within the south-east boundary of the Newquay sheet at [SW 8985 4945], however, the relationship of the type section to other similar sandstone units within the Grampound Formation in the Newquay district is unclear due to the polyphase deformation. Variation in the stratigraphical level of the sandstones indicates there are a number of discrete units. The unit typically comprises an association of fine- to medium-grained beige sandstone and medium-grained, green-grey feldspathic sandstone with some coarse-grained feldspathic sandstone, locally with mud clasts/rip-up clasts up to 1 cm in diameter, for example in the southern part of Ladock Woods [SW 8890 5108]. The type section of the Treworgans Sandstone Member [SW 8985 4945] includes amalgamated, thick- to very thick-bedded, medium- to coarse-grained, yellow-weathering sandstone (locally pebbly), with subordinate dark grey siltstone and mudstone.

Leveridge (2008) suggested that the sandstones may represent sediment from a more proximal source than the rest of the Grampound Formation that has been transported within submarine canyons. The decrease in sandstone units and grain size towards the west in the Newquay district would fit with sedimentation in an east– west elongated basin.

The Porthtowan Formation (Ptn), defined by Leveridge et al. (1990), incorporates rocks that were attributed to the Falmouth, Portscatho and Grampound, and Probus series in the previous survey of the Falmouth district (Hill and MacAlister, 1906), where its type locality is situated north-east of Hell's Mouth, at [SW 6035 4289]. Only the lowest part of the formation, which has a total thickness of 2.8 km (Leveridge et al., 1990), crops out in the Newquay district. Coastal exposures around Chapel Porth are dominated by hard, slaty, dark grey to grey-green mudstone, dark grey or blue-grey, fine- to medium-grained feldspathic sandstone with bed thicknesses of 1–10 cm (though thinner beds are most common), and some siltstone. Beds of sandstone typically lack lamination, though some grading within the thicker units of sandstone and some typical turbiditic features can be seen around Chapel Porth (Plate 3). No evidence for the age of the Porthtowan Formation has been obtained from the Newquay district, though a Frasnian age is indicated for the top of the formation in the Camborne–Redruth area (Goode, 1987).

The contact between the Grampound and Porthtowan formations is the right way up and gradational around West Chiverton and Chiverton mines [SW 7897 5073] to [SW 7956 5112], supporting evidence from elsewhere in the region that both formations were deposited in the Gramscatho Basin. The Carrick Thrust, which cuts through the Porthtowan Formation, locally forms the boundary with the tectonically overlying Portscatho Formation in the south-east corner of the district (Leveridge and Holder, 1987). Exposures of the Porthtowan Formation on the coast around St Agnes Head [SW 698 515] have been affected by thermal metamorphism of the St Agnes Granite. Inland, evidence is restricted to exposure in small quarries and to brash fragments, with soils that typically consist of brown clay and silty clay containing fragments of fine sandstone and vein quartz. The lower parts of the formation in the Newquay district are dominated by dark hard slate, hard grey siltstone, fine gritstone and medium to coarse greenish grey sandstone, in varying proportions, with some units of coarse grey and grey-green sandstone, for example, to the north of Trevella around [SW 8500 5119]), though these thin and die out towards the west.

Restricted to the south-east corner of the Newquay district, the base of the Portscatho Formation in the Newquay district is defined by the Carrick Thrust, which extends for approximately 20 km across the peninsula. The Carrick Nappe is the lowermost nappe of the allochthonous terrane that crops out to the south, on the Falmouth, Mevagissey, Penzance and Lizard geological sheets (BGS, 1990; BGS, 2000; BGS, 1984; BGS, 1975).

The Portscatho Formation, which includes the previously defined Portscatho Series and Falmouth Series (Hill and MacAlister, 1906) and is equivalent to the Lower Gramscatho Beds of Hendricks (1937), consists of alternating grey and greenish grey sandstone beds, grey slaty mudstone and some thin siltstone and chert laminae. The formation is essentially a turbidite sequence deposited in a deep-water fan environment, with progradation from outer to mid-fan settings evident through the formation. Micropalaeontological data from the upper part of the formation indicate a Frasnian age (Le Gall et al., 1985).

Granite

Sir Henry De la Beche, the first Director of the Geological Survey, speculated in 1839 that the granite bodies outcropping in Devon and Cornwall might be connected at depth. Gravity survey work by Bott et al. (1958) confirmed this, and more recent geophysical work has refined and improved the models (Figure 3). Radiometric dating of the Cornubian granite batholith indicates a complex history, with polyphase intrusions dating from 295 to 270 Ma, and related mineralisation continuing until 250 Ma (Darbyshire and Shepherd, 1985; Chesley et al., 1993; Chen et al., 1993).

In the Newquay district two small granite outcrops are present, forming the granite masses of St Agnes [SW 702 497] and Cligga Head [SW 738 536].

The St Agnes Granite has an ovoid outcrop, the long axis of which is oriented north-north-east to south-south-west with a length of 700 m. Field evidence and geophysical surveys, the latter presented by Tombs (1977), suggest that the St Agnes Granite is a cupola, or apical intrusion, arising from an essentially buried granite ridge that extends northwards from the Carn Marth granite pluton in the Falmouth district. Inland, the granite is poorly exposed, seen only in isolated quarry cuttings, and in brash and mine dump material. The western margin of the granite is exposed in Cameron Quarry [SW 704 507]. Much of the granite seen in the quarry faces is medium-grained and poorly megacrystic. Biotite present in the groundmass has largely been altered by greisening and silicification, though the degree of alteration is locally very variable. Tourmaline is a common accessory mineral, and disseminated copper and arsenic sulphide minerals are present in places. Mineralisation at Cameron Quarry, including the replacement of greisened feldspar crystals by assemblages that include cassiterite, is described in detail by Hosking and Camm (1985).

Rounded or elliptical 'spots', up to 6.5 mm in diameter, are seen throughout the metamorphic aureole of the granite in the Porthtowan Formation. Under the microscope, those around Chapel Porth [SW 6970 4945] are identifiable as aggregates of biotite, muscovite, iron oxides and prisms of rutile (Reid and Scrivenor, 1906). Spotted hornfels is present at the western margin of the granite, for example in Cameron Quarry [SW 704 507], where the contact between the granite and hornfels is irregular and locally a chilled margin, accompanied by layers of pegmatite with curved K feldspar crystals up to 25 mm long, is seen. To the east, towards Newdowns Head [SW 7083 5191], exposures close to the coastal path include tough grey sandstones and siltstones, commonly showing thermal spotting and locally with considerable silicification, seen as lit-par-lit replacements, irregular pods, stringers, and joint fillings. Andalusite hornfels and andalusite-bearing schist have been recovered close to the southern granite boundary at Wheal Coates [SW 6998 5003] and Chapel Porth, respectively, and spotted silty and sandy slates, with some thin developments of thin, dark grey chiastolite slate, are present in the cliffs south of White Rocks [SW 6985 5111]. South of Tubby's Head [SW 6975 5047], the cliffs contain silicified silty slates with thin developments of chiastolite slate. Tourmalinised rocks, present in the vicinity of the granite body, are evidence for the migration of early hydrothermal fluids away from the granite body.

The Cligga Head Granite is exposed in west-facing cliffs extending 600 m south from the headland at [SW 737 538] and 250 m inland. The primary granite is typically coarse-grained, sparsely megacrystic biotite granite, which shows extensive mineralisation, metasomatism and kaolinisation and has been the subject of numerous studies (e.g. Hall, 1971; Stone et al., 1988).

The eastern contact is poorly exposed at the surface, but evidence cited by Reid and Scrivenor (1906) and from abandoned mine workings indicates a faulted margin. The southern contact, clearly visible in the cliffs near Hanover Cove [SW 7360 5325], is heavily faulted, up to 100 m wide and dipping about 60° to the north, with much argillic alteration and iron staining of the granite. The western contact is beneath the sea, and the northern contact, which is visible in the undercliff at Cligga Head, is irregular, with some evidence of stoping. A significant feature of the granite, clearly visible in the west-facing cliff, is the swarm of 'sheeted' joints, which dip steeply towards the north in the northern part of the granite, then flatten to form an antiformal structure that is truncated against the southern faulted contact zone. This pattern in the joints has been cited as evidence of deformation (folding) of the granite (Moore and Jackson, 1977; Floyd et al., 1993). An alternative explanation, that the joint pattern was dictated by the early cooling of the granite and is related to the form of the roof of the cupola, might be more plausible in view of recent evidence that the Cornubian granite batholith was emplaced in an early Permian extensional setting, well after the main compressive stages of the Variscan Orogeny. Joint structures clearly exercise control over the distribution of pervasive greisening and kaolinisation, and it seems likely that they provided pathways for the hydrothermal fluids that caused widespread alteration. An Ar/Ar plateau age of 279.9 ± 0.8 Ma (Chen et al., 1993) for muscovite from the Cligga Head sheeted greisen veins is probably close to, but later than, the emplacement age of the host granite.

The Cligga Head Granite is enclosed by siliceous and spotted metasediment of the Grampound Formation. A contact metamorphic aureole is developed to the south, extending 100 m south of the granite edge and 760 m to the east. Soft, bleached, pale grey argillaceous slate, with kaolinitic alteration and metamorphic porphyroblasts up to 4 mm diameter, is seen around Hanover Cove [SW 7360 5325]. The relatively large size of the contact aureole in relation to the granite outcrop suggests that a more extensive body of granite is present at depth in the subsurface, and low gravity anomalies in the area support this hypothesis. Reid and Scrivenor (1906) reported concentric zones of (i) tourmaline-hornfels, (ii) bleaching and spotting (probably chloritoid), and (iii) spotting only, with increasing distance from the granite margin. Silicification is sporadically present close to the granite. Immediately adjacent to the granite body, the country rock is altered to hard splintery siltstone hornfels, quartzitic hornfels, spotted metapelites and grey chloritic slate hornfels.

Part of the metamorphic aureole of the Bodmin Granite is present at the eastern margin of the sheet, indicated by the occurrence of hornfels, iron-stained mudstone and silicified sandstone and mudstone fragments in the brash. Cordierite spots up to 3 mm long are seen locally, for example around [SW 908 570]. Changes in slope gradient east of St Enoder at around [SW 907 569], may represent fluctuations in lithology or the grade of contact metamorphism.

Igneous dykes

Two categories of dyke have been identified in the Newquay area, those of quartz-porphyry or quartz-feldspar-porphyry composition (locally referred to as 'elvans') and mafic dykes, classified as basaltic, microgabbroic or lamprophyric.

Elvan dykes are generally very fine grained and of essentially granitic composition. They are identified from brash fragments across most of the area, but exposures in coastal cliff and quarry sections locally reveal flow lamination, chilled margins, and flow-parallel orientation of feldspar megacrysts and tabular xenoliths. Many also show textural gradation from a nonporphyritic marginal zone to a core that is more or less rich in quartz and/ or K-feldspar phenocrysts (Goode, 1973). Some dykes show evidence of multiple intrusion, tourmalinisation and, more rarely, impregnation by cassiterite (Dines, 1956; Hosking, 1964), and many are affected by chloritic or argillic alteration. Variation in the degree of alteration provides examples of elvans that are fresh and hard, through those that have been worked for carving, to those that are entirely altered to a mass of clay and iron oxide minerals.

Elvan dykes typically trend east-north-east to west-south-west through east–west to east-south-east to west-north-west, though an extensive elvan traceable from Watergate Bay [SW 839 646] to near Benny Mill [SW 846 576] trends north–south. This was considered by Hosking (1964) to be related to a number of hypothermal lodes north of the St Austell Granite with a similar trend, which formed comparatively early. A concentration of dykes occurs around and to the north of St Newlyn East, seemingly coincident with a subsurface granite ridge (Figure 3). The strike length of individual dykes can rarely be traced for more than 2 km and individual dykes range in thickness from a few centimetres to 12 metres or more. Some elvan dykes are displaced by brittle faulting. With a few exceptions, main-stage polymetallic mineralisation postdates emplacement of these dykes (Hosking, 1964). Field relationships suggest that dyke emplacement occurred late in the scheme of igneous events in the province, a conclusion that is borne out by isotopic evidence (Darbyshire and Shepherd, 1985; Leveridge et al., 1990). Such geochemical data as is available suggest that elvan dykes are mostly derived directly from a biotite granite parent magma, though commonly with some K enrichment (Henley, 1974). Goode (1973) described elvan emplacement in terms of fluidised events linked to hydrothermal brecciation and the exsolution of high-temperature mineralising fluids. Shail and Wilkinson (1994) and Alexander and Shail (1995) discussed the tectonic environment in which elvan dykes were emplaced, and concluded that emplacement was synchronous with, or postdated, faulting in a north-north-west to south-south-east extensional regime associated with orogenic collapse.

Mafic dykes are sparse within the district. They are classified as either basaltic (fine-grained, equigranular) or microgabbroic (fine- to medium-grained with slightly porphyritic plagioclase or pyroxene) and also trend east–west to east-north-east to west-south-west. Narrow (less than 5 m) basaltic dykes are associated with elvan dykes in the area west of Cubert (around [SW 8015 5809]). Basaltic dykes are present on the eastern margin of the sheet at [SW 908 549]. Two minor intrusions of lamprophyric rocks are present in the south of the area, around Pennare [SW 8160 4940], and trend approximately north-north-east to south-south-west. Small outcrops of mica-lamprophyre (minette) dykes have been recorded in the area, most notably a discontinuous group between Holywell Bay [SW 768 599], crossing the Gannel estuary and up to Towan Head [SW 800 360]. Another dyke [SW 874 597] is recorded to the south of Colan, and was recorded to be over 20 m wide and heavily altered. Detailed descriptions of these occurrences are given in Reid and Scrivenor (1906) together with an account of their petrography. Lamprophyres are intruded into extensional fault zones on the south Cornwall coast demonstrating that extension and igneous activity were contemporaneous. Dating of these intrusions indicates emplacement during the Stephanian (Hawkes, 1981; Rundle, 1980).

Mineralisation

An extended history of mineralisation is demonstrable in the Newquay district (Figure 4) and mineral deposits were formerly worked on an extensive scale. Mineral deposits include Sn-W-Cu-Zn-As-Pb veins in the St Agnes–Perranporth mining district; Pb-Ag-Zn veins in the Perranzabuloe district; and the Great Perran Iron Lode, a series of orebodies of complex and hybrid origin, extending over more than 4 km strike length, which have been worked sporadically for iron, lead, zinc and silver.

Mineralisation in the Newquay district is dominated by east-north-east to west-south-west-trending hydrothermal veins associated with the largely buried granite ridge that extends from St Agnes Beacon through Cligga Head to Perranporth (Dines, 1956). The granites of St Agnes and Cligga include greisen developments, with associated Sn and W mineralisation. Of these, the best known are the sheeted vein swarms at Cligga Head (Plate 4) (Moore and Jackson, 1977) and the cassiterite replacements of feldspar in the granites of Wheal Coates[SW 700 501] and Cameron Quarry [SW 704 507]. Sn-bearing veins of the St Agnes orefield include representatives of early tourmaline–quartz–cassiterite assemblages, together with later, polymetallic (Sn-Cu-Zn-As-Pb) veins that include chlorite in addition to tourmaline. The polymetallic deposits show some evidence of concentric zoning, with Sn ores predominating in the zone around the granite outcrops, an intermediate zone of Cu, and a distal zone with Pb and Zn. In addition to the granite-related east-north-east to west-south-west-trending veins, a later set of north–south or north-north-west to south-south-east-trending veins are known as 'cross-courses'. These mostly yielded Pb-Zn-Ag ores.

Greisen-bordered veins and replacement bodies

At Cameron Quarry [SW 704 507] the St Agnes Granite shows the effect of greisening. In places where this replacement by white mica, quartz and topaz is well developed, the K-feldspar crystals may be replaced by a variable assemblage of cassiterite– tourmaline–quartz, giving the rock a speckled appearance (Hosking and Camm, 1985). In the better examples, the cassiterite assemblage forms well-defined epimorphs of the primary porphyritic feldspar crystals. Other economic minerals disseminated as minor species in the granite at Cameron Quarry include chalcopyrite, sphalerite, pyrite and arsenopyrite. Similar replacement of K-feldspar by cassiterite has been recorded from the nearby tin mine of Wheal Coates where fine-grained cassiterite forms perfect pseudocrystals after orthoclase feldspar (Embrey and Symes, 1987). These historically famous specimens were first discovered in 1828 and have since been the subject of much interest by mineral collectors.

Within the Cligga Head Granite, a quarry [SW 738 536] has been developed in a swarm of subparallel, greisen-bordered quartz veins, with minor cassiterite–wolframite mineralisation (Plate 4). The veins and their associated dark grey greisen edges dip towards the north or north-north-west at 60° to 70°, and are separated by narrow zones of pale grey kaolinised granite. At this locality, the economic mineralisation is not well developed, but elsewhere in the granite, for example in the cliffs to the north, and in the underground workings beneath, there are examples of quartz veins up to 300 mm wide, with patchily distributed, coarse cassiterite, wolframite and minor sulphide minerals. Notable features of these veins, ascribed to their formation by hydraulic fracturing, are the absence of brecciation and the growth of large crystals right across the width of the vein.

Tourmaline–cassiterite orebodies

The mines of the St Agnes area, between Tubby's Head [SW 697 505] and Trevellas Porth [SW 725 519], which include Blue Hills Mine, Wheal Kitty and Polberro Mine, are notable for their workings on a series of 'flat lodes', northward-dipping structures at between 20° and 40°. These are cut by a later generation of much steeper veins, trending east-north-east to west-south-west, and also by 'slides', barren fractures that dip steeply towards the south. From the evidence of material seen on the mine dumps and from historical accounts (e.g. Dines, 1956), it is clear that these flat-lying veins were the result of a complex sequence of hydrothermal events, involving initial hydraulic fracturing followed by several phases of brecciation and mineral deposition.

The sulphides recorded, mostly as minor vein constituents, include chalcopyrite, stannite, pyrite, arsenopyrite and sphalerite. The mining records stress the patchy nature of the tin mineralisation in this area, with localised rich 'bunches' of ore alternating with almost barren ground. A notable feature of the St Agnes tin lodes is the local presence of 'wood tin', fibrous cassiterite intergrown with quartz and showing light and dark banding. In contrast, elsewhere in the district, the cassiterite may be very coarsely crystalline, as at Seal Hole Mine to the west of Trevaunance Cove [SW 717 517].

'Mainstage' polymetallic veins

As in other places in Cornwall, the polymetallic vein systems record a long history of mineralisation, involving fluids of direct magmatic origin mixing with circulating meteoric waters. The fluids formed mineral deposits in extensional fracture systems during multiple episodes of fracturing, brecciation and mineralisation (Shepherd et al., 1985; Scrivener and Shepherd, 1998). In the Newquay district, veins are commonly steeply dipping structures of east–west to east-north-east–west-south-west strike, and carry assemblages of economic minerals that vary according to the proximity of the granite at outcrop or beneath the cover of sedimentary rocks (Figure 3). A particular feature of the district is the close spatial association between the quartz-porphyry dykes and the polymetallic veins.

Steeply dipping lodes in the area east of St Agnes cut the earlier, flat-lying, tin-bearing lodes, and carry assemblages that include cassiterite and sulphides such as chalcopyrite, arsenopyrite, pyrite, sphalerite and galena in a gangue of chlorite and quartz. In general terms, the tin and copper in these lodes decrease in abundance towards the south and east, whereas lead and zinc increase. In the former mining area of Perranzabuloe, and extending towards St Newlyn East, lead, silver and zinc were the main commodities worked from the steeply dipping sulphide lodes. Relatively little is known of the detailed vein paragenesis of this area, but dump material includes brecciated slate, siltstone and sandstone with quartz as the predominant gangue mineral, and sulphides including galena, sphalerite, pyrite, chalcopyrite and traces of tetrahedrite. Dines (1956) noted that in the case of the Chiverton mines, between Callestick and Zelah, located within the upper part of the Grampound Formation and the lower part of the Porthtowan Formation, the worked ore zones of the lodes were in soft grey slates, but in intervening sandstone and siltstone the lodes were barren.

Cross-course veins

Throughout the district, a set of veins trending north–south or north-north-west to south-south-east can be mapped along the coastal exposures, and are known as 'cross-courses'. Most of these veins are barren and are composed of comb-layered quartz and minor iron oxide minerals, or clay-rich material with brecciated slate wallrock (known locally as 'fluccan' lodes). Some, however, include sulphides of lead, zinc and copper, together with spar minerals and minor silver minerals, and these have been worked locally. Descriptions of former mines (Collins, 1912; Dines, 1956) confirm that the cross-courses are late structures, displacing the mainstage mineralisation.

Cross-courses are not confined to the Newquay district, and have been recorded throughout south-west England. Districts in which Pb-Ag-Zn cross-course mineralisation has been important in economic terms include the Teign Valley (Devon), the Tamar Valley and the Menheniot area of east Cornwall. Cross-course veins in the latter two districts have been shown to be of Mid Triassic age (Scrivener et al., 1994) and to result from the movement of basinal brines in extensional fractures.

Examples of lead-bearing cross-courses include the Wheal Golden and Penhale Mine system, which can be seen exposed in the cliffs [SW 758 592] near Penhale Point and at the northern end of Perran Beach [SW 762 577]. Specimens from the latter locality are of banded quartz, with aggregates of coarse galena, granular pyrite and minor brown sphalerite. A number of lead and silver-bearing cross-courses cut the body of the Great Perran Iron Lode, as described below. Prominent barren cross-courses, essentially of comb-layered quartz, can be seen at outcrop in the cliffs near Droskyn Point [SW 753 544], and also in the slate exposures to the north of Cotty's Point.

Controls on the distribution of base metal vein mineralisation

Within the district as a whole, there is some evidence that the presence of Pb-Zn-Ag mineralisation might be influenced by stratigraphy, for example in the case of mineralisation within the upper part of the Grampound Formation and the lower part of the Porthtowan Formation in the Chiverton mines (Dines, 1956). There is also some evidence that the black and dark grey slates of the uppermost parts of the Trendrean Mudstone Formation provide a favourable redox environment for the development of stratiform Pb-Zn proto-ores, which may contribute to deposition of hydrothermal sulphides from originally syngenetic or early diagenetic concentrations of those metals. Evidence for this is provided by the records of the Shepherd's Farm boreholes (BGS Reg. No. (SW85SW/4), (SW85SW/5), (SW85SW/6), (SW85SW/7), drilled in 1978, to investigate geochemical and geophysical anomalies. All four boreholes were drilled in the southern part of the outcrop of the Trendrean Mudstone, and all reached their terminal depths within that formation. The logs recorded dark grey to black slates, carbonaceous in places, with abundant quartz shear veins and lenses. Throughout the cores, disseminations of finely divided pyrite were recorded, with more coarsely crystalline pyrite and siderite present in the quartz shears. Traces of galena were noted, and re-examination of such core material as survives confirms that both galena and sphalerite are locally disseminated within the cleaved mudrocks. Such syngenetic material, and its local redistribution during the compressive stages of the Variscan Orogeny, might have influenced the distribution of vein ore deposits during post-orogenic extension and the large-scale circulation of hydrothermal fluids. Examples of mines situated in the upper part of the Trendrean Mudstone include Wheal Albert [SW 795 537], Shepherds Mine [SW 817 541] and Cargoll Mine [SW 834 542].

The Great Perran Iron Lode

The outcrop of one of Cornwall's most interesting mineral deposits, the Great Perran Iron Lode, extends from Gravel Hill Mine [SW 765 575] (Plate 5), at the northern end of Perran Bay, south-eastwards for at least 7 km. It varies in width from 1 to 30 m and is essentially conformable with the dip of the enclosing strata. The Great Perran Iron Lode typically includes brecciated slates cemented by siderite, minor quartz and masses of black sphalerite. Oxidation of the siderite to depths of 60 m or more has produced oxides and hydrated oxides of iron (hematite, goethite and limonite), which were formerly worked by opencast and underground mining at a number of sites including Gravel Hill Mine, Mount Mine, Treamble Mine, Duchy Peru Mine and Deerpark Mine (Figure 5).

The Great Perran Iron Lode is a complex structure with a long history of metallogenesis. Henley (1971) suggested that fragments of black sphalerite included in the lode may be of stratiform origin, and Scrivener and Shepherd (1998), using an analogy with the Exmoor iron lodes, considered that the siderite mineralisation may have been pre-granite in age. Calc-silicate 'skarn' minerals are associated with the eastern part of the structure (e.g. Goode and Merriman, 1977), and in places the lode is cut by late, low-temperature north–south-trending cross-course veins containing pyrite, sphalerite and argentiferous galena. The survey of the Newquay district, together with a re-appraisal of the Duchy Peru exploration boreholes of 1973–1974, has shed new light on the early history of the Great Perran Iron Lode, and has helped to explain the nature of the mineralisation seen in the shallow workings at the present day. The Duchy Peru boreholes proved some 40 m of banded, pale grey marble, enclosed by sulphide-rich, calc-silicate rocks, in the position of the down-dip projection of the Great Perran Iron Lode. Weathering of this material in the near-surface zone, with dissolution of the carbonate-rich content by the acid waters resulting from the breakdown of sulphides, would result in the brecciated (collapsed) Fe-oxide rich material seen at the present day in the shallow workings along the strike of the structure. It is possible that the banded marble, stratiform sphalerite (described by Smyth, 1887 and Henley, 1971) and siderite, which appear to represent the earliest mineralisation, are representative of 'sedex' type deposition, contemporaneous with the enclosing sedimentary rocks, of probable Mid Devonian age. The mineralisation history of the Great Perran Iron Lode is summarised in (Figure 6).

Cenozoic and Quaternary

Sedimentary evidence of pre-Quaternary landscape evolution in south-west England is sparse, although a continuum of soil accumulation and mobilisation of weathering products occurred from the emergence of the area at the end of the Cretaceous through to the Oligocene. This is indicated by the successive phases of deposits preserved in south and east Devon (e.g. Hamblin, 1974; Edwards and Freshney, 1982). Farther west, within the Newquay district, there is a substantial unconformity within the geological succession. Deposits around St Agnes Beacon provide the only sedimentary evidence for pre-Quaternary processes. Other evidence is erosional and does not provide clear constraints on the timing of events.

Landforms in the Newquay district are characterised by the occurrence of 'flat-topped' interfluves cut by typically steep-sided valleys. Waters (1971) suggested that the relatively uniform heights of interfluves represented the base of weathering, which was affected during the Quaternary by alternating 'normal' (rain and rivers) and cryo-nival(periglacial) weatheringprocesses during interglacial and glacial cycles respectively. Implicit in this theory is that no trace of pre-Quaternary landforms exist today and, furthermore, that no estimate of the height of pre-existing landforms is possible, though removal of large amounts of derived regolith material is evident from the extensive accumulations of head near to the coasts. Dating of deposits at St Agnes Beacon [SW 7085 5050] as mid Oligocene to Miocene (see below) requires that the base-level of weathering was formed at least prior to the mid Oligocene. This concurs with proposals of Green (1985) and Bowen (1994) that landform evolution essentially occurred from the Late Cretaceous to the Palaeocene, and that the area has experienced uplift since the Miocene.

Although the geological map shows a single outcrop of the St Agnes Formation, this is now thought to represent two separate outliers. The larger, St Agnes Beacon outlier occurs around the northern flanks of St Agnes Beacon [SW 7085 5050], whereas the Beacon Cottage Farm outlier, which is only a few hundred metres in diameter, occurs south-west of the Beacon. The stratigraphy of each occurence is summarised in (Figure 7). In general, the St Agnes Formation is poorly exposed at the present day, with many of the old pits filled, degraded or overgrown by vegetation. The best exposures are seen in New Downs Pit, sporadically worked for building sand, in which a section [SW 7048 5095] shows yellow sand with a lowfines content up to 3.0 m thick, but with no trace of bedding. At the base of the section are ferruginous-cemented pipe-like structures. A section in the north-east part of the complex at [SW 7062 5111] shows the following:

Palynology indicates a mid Oligocene age for the Beacon Cottage Farm Outlier (Jowsey et al., 1992), and an indigenous pollen assemblage from the St Agnes Beacon Outlier indicates a Miocene age and a subtropical Mediterranean climate (Walsh et al., 1987). Jowsey et al. (1992) noted that the contact between the outliers and the underlying metamorphosed mudstone and granite — subhorizontal close to the coast, rising to an inclination of up to 45° in the upper parts of the Beacon — was typically marked by a zone of weathered rock. These authors proposed that the contact between bedrock and the Cenozoic deposits is a homologue of the 'Reskajeage Surface', a planation surface 75–121 m above sea level, equivalents of which may be identified in southern Ireland, west Wales and northern France (Walsh et al., 1996), so dating this surface (i.e. the contact) as at least Mid Miocene. The Reskajeage Surface may represent a tropical or subtropical etch plane, from which inselbergs or remnant mountains (such as the St Agnes Beacon) rose and on which remnants of tropical/ subtropical weathering products were preserved, having been redistributed in mid Oligocene and Miocene times. Fragmentary evidence of such marine-cut features, close to the 75 m and 100 m contours, is seen across the area, being particularly well-preserved north-east of Penhale, around [SW 8615 5224]. This interpretation implies that topography which had evolved by the Late Oligocene has remained relatively unchanged to the present day; later marine transgressions must have been restricted to a lower topographical level than that at which the St Agnes Formation sediments are preserved.

Quaternary

Although no deposits younger than the St Agnes Formation have been dated in the Newquay district, the St Erth Formation, a deposit of sand and clay outcropping at an elevation of approximately 30 m OD in the Penzance district (BGS, 1984; Jenkins et al., 1986; Goode and Taylor, 1988), give some indication of early Quaternary processes in the region. Foraminifera from the deposit have been dated to between 2.1 and 1.9 Ma (Jenkins et al., 1986), identifying this as a marine deposit of Late Pliocene age. Reid (1890) suggested deposition of these sediments in water depth of approximately 90 m, concluding that their deposition was associated with a marine retreat platform at around 130 m. More recent work has deduced a water depth of only 10 m for the deposit, giving a projected sea level 45 m above that of the present (Kidson, 1977). In either case, it is clear that marine incursions since around 1.9 Ma have been restricted to levels below that of the preserved St Erth Beds.

Raised marine deposits

Variation of sea level during the Pleistocene (Funnell, 1995) created benches and alluvial terraces, recorded in West Penwith by Wilson (1975) and dated by Bowen et al. (1985) using the amino-acid technique. Wilson (1975) postulated that the St Agnes Formation was probably extensively eroded during the Pleistocene.

Discontinuous raised beach deposits are present around the coast between Pentire [SW 7897 6168] and the west side of Towan Head [SW 7990 6288]; these deposits comprise layered sand and small rounded pebbles, held together by both carbonate and quartzite cement, similar to modern-day beach deposits. The height of the raised beach deposits is approximately 5 m above the level of the present-day beach.

Head

The term 'head' was first used in geological description by De la Beche (1839); it was originally used by quarrymen to describe the overburden present above the solid bedrock. The term now more usually refers to the thick layers of periglacially weathered, and commonly soliflucted, superficial deposits found throughout the region. In the Newquay district the thickness of the head typically varies from 0.2 to 3 m. The deposits commonly reflect the lithology of the local bedrock, though they may incorporate material from other rock types in the vicinity, usually up-slope from the location of interest, or introduced by other local factors. For example, much aeolian sand is incorporated in head east of Penhale Sands, around [SW 7835 5592].

Waters (1971) identified up to three phases of successive head development in the Dartmoor region, and near to the coast on the Lizard Peninsula Roberts (1985) deduced a four-phase evolution of (i) soil formation, (ii) active solifluction, (iii) solifluction accompanied by loess accumulation and (iv) coastal erosion from the Devensian through to the Holocene.

Detailed inland geological mapping suggests that at least a two-fold distinction of head deposits may be possible in parts of the Newquay district, although not distinguished on the map: perched head, located on interfluves and locally on flat hill-tops; and valley head, often widespread on the less steep slopes within local catchment systems and reworked by fluvial processes in valley bottoms. Truncation of perched head by apparently younger valley head is seen at several locations, e.g. around Penhale at [SW 8640 5189], near to Coswarth, at [SW 8648 5985] and around Kestle Mill [SW 8509 5936].

The perched head deposits are typically located on flat or very gently sloping hilltops, bound by a distinctive break of slope marking the transition to solid rock below. On the Grampound Formation, for example around [SW 8640 5189], the head deposits comprise stiff red-brown clay, patchy beige sand, with fragments of Grampound mudstone, sandstone and siltstone. Extensive developments of head present north of Coswarth [SW 8685 5970] are characterised by lots of vein quartz fragments, with varying amounts of angular and subrounded material, suggesting a dominant residual component. In the south-east of the district, around Ladock [SW 8930 5090], there is a notable absence of perched head deposits; this may be related to the more coarse-grained nature of the bedrock in this area.

Perched head deposits are sometimes truncated by valley head. This type of head is extensively developed along north– south-trending valleys. Around Trewerry Mill [SW 8371 5805], as elsewhere, an uneven, hummocky deposit, commonly waterlogged with some patches of reeds, suggests that fluvial processes have locally reworked head deposits in the lower parts of the valley. An example is seen to the east of Mithian, where a stream bank section at [SW 7469 5057] shows 1.9 m of clay-rich head resting on brown-weathered grey slate (Porthtowan Formation). Away from active watercourses, wide areas of valley head seem prone to develop on gently sloping surfaces, for example in the 'head basin' near to Tredinnick Farm [SW 8650 5632], and in a tract more than 1 km wide trending north–south across Trefullock Moor [SW 8980 5640], which encloses a 'rise' of bedrock (Trendrean Mudstone Formation) at [SW 8992 5813] on which Ennis Barton Farm is located. Superficial deposits on Trefullock Moor were drilled at [SW 8989 5689], where 1.5 m of head deposits, comprising clay with subangular to subrounded clasts, overlie more than 2.5 m of very weathered Trendrean Mudstone Formation laminated mudstone. Valley head in the west of the area, close to the adit of Wheal Leisure [SW 7638 5345], comprises 2.5 m of brown clay, including fragments of grey slate, siltstone and vein quartz, resting on brown-weathering grey slate interbedded with discrete beds of grey siltstone and fine-grained sandstone.

Present-day distribution of head is likely to be a function of the pre-Quaternary weathering profile, in part determined by the nature of the Palaeozoic bedrock, and the evolution of slope gradients within local catchment systems throughout the Quaternary. Evidence that these systems are still active includes apparently incipient valley head development, e.g. near to Penhale, around [SW 8630 5172], and uneven ridges close to the edge of a head tract south-east of Higher Trelassick Farm, around [SW 8830 5250], indicative of stream sapping.

Intertidal deposits

Intertidal deposits are restricted to the Gannel estuary, which runs to the south of Newquay. There are two distinct types of intertidal deposits, saltmarsh and estuarine, although these are not distinguished on the map. Saltmarsh deposits are characterised by the occurrence of halophilic flora, typically marsh grass, and comprise silt and estuarine mud with layers of organic matter. These are restricted to two main sites on the Gannel estuary; the upper part of the estuary, from Tregunnel Saltings [SW 8084 6078] upstream as far as Trevemper Bridge [SW 8194 5990] and a small area of saltmarsh located in the confines of Penpol Creek [SW 7981 6080]. Both of these areas are prone to tidal influence, and are locally cut by deep channels.

Estuarine deposits, comprising a heterogenous but locally stratified deposit of mud, sand and gravel, extend downstream from close to Tregunnel Saltings [SW 8084 6078] to the mouth of the estuary at Fern Pit [SW 7895 6113], where they pass into beach sand deposits.

Beach deposits

Beach deposits, typically dominated by sand, are widespread along the coast, except where rocky headlands, typically trending east–west, extend into the sea. The easternmost extent of the beach deposits is variously defined by blown sand (commonly as dunes), alluvium, e.g. at Porth Beach [SW 8320 6290], or a cliff line, such as that extending from the northern end of Watergate Bay [SW 8423 6612] south to Whipsiderry Beach [SW 8290 6315].

Blown sand

Blown sand deposits lie inland of beaches at Mawgan Porth [SW 8490 6745], Fistral Beach [SW 7995 6200], Crantock Beach [SW 7865 6085], Holywell Beach [SW 7860 5950] and at Perran Sands [SW 7820 5660], with sand dunes at the latter locality reaching up to 92 m above OD (Cole and Tapper, 2004).

An area of blown sand on a gently sloping headland at Kelsey Head [SW 7665 6072] sug-gests that a wave-cut platform may underlie part of this deposit. The deposit extends south to Holywell, and inland across Cubert Common [SW 7800 5960], being bound to the east in part by a steep-sided valley, and to the north and south by irregular boundaries that cross-cut the contours. A narrow tract of head to the east of Holywell separates this area of sand from the extensive deposit across Penhale Sands, which has a north– south extent in excess of 3 km, terminating at Perranporth [SW 7565 5450]. The western boundary of the Penhale Sand deposit is within beach deposits, including a partly obscured cliff line at the back of the beach; to the east, the margin is constrained by alluvium as far south as Treamble Mine [SW 7842 5615], then by a more gradational boundary (gradual thinning of the sand) on to bedrock of the Trendrean Mudstone Formation.

Just south of Porth, blown sand covers a flat plateau called Barrowfields [SW 8210 6225], at approximately 40 m above present sea level. The proximity of the deposit to the coast and the restricted area that it covers may suggest constraint from a wave-cut platform or other inherited feature. Given that the Late Pliocene St Erth deposits are 30 m above sea level, the formation of a wave-cut platform 40 m above sea level must have occurred earlier, perhaps during the Miocene. Deposition of sand on this surface, and accumulation of sand deposits elsewhere in the district, may have been contemporaneous with deposition of the Lizard Loess (Roberts, 1985) at around 15 900 years B P (Wintle, 1981).

Alluvium

Water-lain deposits are commonly located along all but the most minor river drainage channels; they have a characteristic flat or slightly hummocky landform, and comprise light to mid grey clay, brown clay and silt with some thin layers of sand, and subrounded to rounded gravel, e.g. along the River Menalhyl, around [SW 8598 6666]. Identifiable lithologies within the coarser fractions include vein quartz, granite, sandstone and siltstone. A distinction can be made between 'true' alluvium and areas of head that have been remobilised by fluvial processes, the latter showing no stratified layering.

The lower parts of the River Gannel estuary contain intertidal deposits; alluvium is prevalent from upstream of Trevemper Bridge [SW 8194 5990].

River terrace deposits

A single terrace was noted on the north side of the Gannel estuary, at Tregunnel Saltings [SW 8090 6079], comprising a flat plateau area of sand, with a slight slope towards the rear, approximately 1–2 m above the level of the adjacent saltmarsh.

Structure

Overview

The Devonian rocks in the Newquay district form part of the Variscan Orogen, which extends over 1000 km through central and western Europe, and includes the Rheinisches Schiefergebirge in Germany (Franke and Engel, 1982; Holder and Leveridge, 1986). Referred to as the Rhenohercynian Zone, this area is thought to lie near the southern margin of the Eastern Avalonia plate (Franke, 2000), which separated from Gondwana in the Early Palaeozoic. Deformation of Devonian and older rocks in south-west England during the Variscan Orogeny commenced in the south of the region around the middle of the Devonian Period (385 ± 2 Ma) (Clark et al., 1998). Deformation then propagated from south-south-east to north-north-west through a nappe pile that was generated by the successive closure of basins on a passive margin (Leveridge and Hartley, 2006), affecting the northernmost (Culm) basin during Westphalian times about 310 Ma (Lloyd and Chinnery, 2002).

In the south-east corner of the Newquay district the Carrick Thrust forms the boundary between the parautochthonous terranes of the Variscan orogenic belt, to the north, and allochthonous terranes, to the south. It extends east-south-east to the north of Pentewan Beach [SX 2021 0471] and south-west towards Truro (BGS, 1990; BGS, 2000). Collapse of the Variscan edifice ensued during end-Carboniferous/early Permian times (Leveridge and Hartley, 2006; Alexander and Shail, 1995, 1996), causing deformation of pre-existing structural fabrics, reactivation of existing discrete structures, and promoting the emplacement of the Cornubian granite batholith.

Folding

Coastal exposures in the Newquay district provide an excellent structural section, almost at right angles to the orientation of regional strike. An axial planar cleavage (S1) related to close-to-isoclinal, mainly north-verging and north-facing folds is the earliest fabric identified, though Shail and Leveridge (2005) have identified some south-verging D₁ folds around Perranporth. Fold hinges trend east-north-east–west-south-west to east–west and S₁, a penetrative or spaced cleavage in mudstone or sandstone, respectively, is parallel to bedding (S₀) except in fold hinges. These relationships are clearly seen at Watergate Bay [SW 8412 6520] (Plate 6). Dominant northward vergence of D₁ folds indicates that this phase of deformation resulted from northward-directed compression, and K-Ar dating of mica from rocks just to the north of the Newquay district shows that the deformation occurred in the interval 340–320 Ma (Dodson and Rex, 1971).

Continuing (or renewed) northward compression is indicated by the local occurrence of S₂, a spaced penetrative cleavage with northward vergence that clearly cross-cuts and progressively transposes bedding and the early cleavage, S₁, over an interval of 500 m from Crantock Beach [SW 778 608] to the south side of Porth Joke [SW 769 607]. Thin (<5 mm) quartz veinlets are commonly parallel to S₂. D₂ fabrics are evident along the coast (and locally inland, for example south of Tregerles Farm [SW 8635 5532] and at Gunnamanning Farm [SW 8929 5149]; Hollick, 2006) as far south as Cligga Head, where contact metamorphic effects of granite emplacement obscure the structural fabrics. Tectonic inliers comprising bodies of the Grampound Formation within the Porthtowan Formation at [SW 8571 5076], and [SW 8765 4974] probably represent the cores of D₂ folds. Formation of S₂ fabrics is estimated to have occurred around 295–280 Ma (Warr et al., 1991; Pamplin, 1990).

From the southern part of Holywell Bay [SW 7633 5922] to the northern end of Perran Sands [SW 7530 5759], a post-D2 phase of deformation has caused folding and steepening of pre-existing (i.e. S₀, S₁ and S₂) fabrics into a monoclinal structure, previously described by Alexander (1997), Holdsworth (1989) and Steele (1994), among others. S₃ is typically a spaced crenulation that dips gently to the north and verges to the south; it locally transposes S₂, for example at the southern end of Holywell Bay. Intersection of S₃ with thin quartz veins parallel to S₂ creates the 'quartz eye' fabrics previously described inland by Reid and Scrivenor (1906). Detailed mapping as part of this resurvey traced an east– west-trending band of brash containing 'quartz eyes', up to 900 m wide, inland from Ligger Point; the band is offset by three north–south-oriented faults and emerges on the eastern edge of the district north of Nankilly [SW 9015 5160] (Figure 1).

The south-verging relationship of S₃ with S₂ indicates that S₃ was not formed by the northwards-directed compression responsible for the D₁ and D₂ structures; Alexander and Shail (1995) have suggested that D₃ was a phase of orogenic extensional collapse that occurred during latest Carboniferous–early Permian times. A pre-Variscan basement feature, perhaps a sub-basin margin within the Looe Basin, may have acted as a 'focus' for the formation of S₂ in the vicinity of the Penhale Point–Ligger Point section during D₂ north-directed thrusting. Steeply dipping S₂ planes would promote formation of extension-related fabrics during orogenic collapse (D₃), accompanied by brittle–ductile extensional fault reactivation and generation and reactivation of discrete faults. Formation of ductile–brittle to brittle late-phase extensional normal faulting across the peninsula (Shail and Wilkinson, 1994; Alexander and Shail, 1995) would have accompanied this latest phase of orogenic evolution.

Faulting

The pattern of brittle faulting established in the Newquay district is set out in (Figure 8).

Discrete thrust planes, formed in response to compression in latest Devonian times, were almost certainly reactivated later in the orogenic history. Locally, evidence of reverse movement is preserved e.g. the northern contact of the Whitsand Bay Formation of the Dartmouth Group and the Bovisand Formation of the Meadfoot Group, exposed on the coast at Watergate Bay [SW 841 649]. This structure represents a significant revision to the antiformal structure shown on the 1906 survey map (GSGB, 1906).

The non-stratigraphical nature of the contact between the Bovisand and Trendrean Mudstone formations of the Meadfoot Group is evident from detailed inland mapping: the intersection of the boundary with the topography indicates that the orientation of the boundary varies from west to east, fluctuating between a dip of approximately 50° to the north around [SW 884 589], to approximately 16° to the north/north-east around [SW 870 584]. Were the Bovisand Formation in the Newquay district to have the same, early Emsian age as that determined to the east, in the Plymouth district (Leveridge et al., 2002), this would be consistent with the Bovisand– Trendrean Mudstone Formation boundary in the Newquay district being a tectonic contact along which early thrust movement and subsequent extensional reactivation have been accommodated.

Attribution of the Trendrean Mudstone Formation and the Grampound Formation to the Looe Basin and Gramscatho Basin successions, respectively (Leveridge, 2008; Leveridge et al., 1990), implies that the contact between them was originally a pre-Devonian, basin-bounding, east– west-trending structure. This probably accommodated thrust movement during D₁.

'Quartz eye' fabrics locally present close to the boundary in the west of the area, south of Lanteague Farm [SW 804 538], and wide variations in lithologies of the Grampound Formation along the boundary with the Trendrean Mudstone Formation (indicating that the boundary cross-cuts S₀ and S₁), suggest reactivation of the boundary after D₁ deformation. The intersection of the boundary with the topography suggesting that the inclination of the boundary varies from 5° north to 23° south (Figure 1) supports this hypothesis. This modification of the boundary probably occurred during D₂ and D₃.

Relationships north of Resparveth, around [SW 9075 5080], indicate that formation of west-north-west-trending faults may have accompanied development of a post-D₁ regional fabric; a spaced to pervasive cleavage at high angles to S₀ and S₁ is very common in the brash around this area. Alexander and Shail (1996) have suggested that reactivation of pre-existing structures during D₃ extension gave rise to east-north-east-trending brittle faults, indicating a likely setting for the formation of these fabrics.

Faults trending north-west to north-north-west were generated at least locally before the end of the emplacement of elvan dykes. North-north-west-trending faults south from White Cross at [SW 8928 5855] and near Kestle Mill [SW 8460 5910] appear to be cross-cut by quartz porphyry dykes. Cross-cutting relationships indicate that north–south-through to north-east–south-west-trending faults were the most recent phase of regional deformation. A north-north-east-trending fault near Perranporth [SW 7545 5430] offsets east–west-trending elvan dykes by approximately 400 m in a dextral sense.

Chapter 3 Applied geology

Geotechnical considerations

Foundation conditions

The behaviour of geological units in the Newquay district with regard to foundations, use as fill, stability in slopes and behaviour in excavations is summarised for both bedrock and superficial deposits in (Figure 9a), (Figure 9b),(Figure 9c). The information is derived from limited ground investigations in the Newquay district, supplemented by data from adjacent map sheets. Description of strength and density are as described in BS5930 (BSI, 1999).

The folding, faulting, weathering and mineral alteration of bedrock has produced unpredictable engineering ground conditions near the surface, with soil-like, fully weathered rock, a mixture of soil and rock, and less weathered rock. In some places moderately to highly weathered rock, comprising a mixture of engineering soil and rock, may occur at depths of 10 m or more. Differentiating between head and highly weathered or residual rock may be difficult, but might be important as clay-rich head could contain shear zones whereas the weathered rock will probably not. The variation in engineering behaviour affects the design of ground investigation drilling methods, cut slopes, fill, embankment material and the drilling method used for sampling. Gravel-sized lithorelicts make some laboratory tests difficult or impossible to carry out. Fault zones will tend to be more deeply weathered and altered, resulting in weakened rock at greater depths.

Fresh mudstones range from weak to moderately weak, but are occasionally moderately strong. Siltstones range from moderately weak to moderately strong, or occasionally strong, and sandstones are generally strong. Highly weathered mudstones generally comprise stiff clay with fine to medium gravel-sized particles of very weak to weak mudstone. The proportion of gravel-size lithorelicts increases with decreasing weathering grade and increasing depth, grading into fractured rock with a little clay. The depth of weathering is highly variable; in some areas very weak, thinly laminated mudstone with very closely to closely spaced joints may occur near surface, whereas in others they are found at 20 m below ground level. Sandstone and siltstone tend not to weather so deeply. Where mudstone, siltstone and sandstone are interbedded, there can be rapid changes in strength between very weak mudstone and strong sandstone. The metasediments show similar weathering characteristics to the sedimentary rocks, but do not generally weather so deeply. Thicker quartz veins in the fine metasediments commonly have low recovery from rotary boreholes.

Fresh granite is generally strong with medium to widely spaced joints. Near surface, there is much more variation due to weathering and alteration, and joints are closely to medium spaced. Kaolinised granite is commonly firm to stiff sandy clay with gravel or larger granite particles. Greisen-lined veins in granite may have a lower frictional angle than granite joints.

Mineral lodes may contain rocks that are strong thus slowing drilling rates. Drilling flush will be lost in voids associated with mining, reducing drilling efficiency and increasing the cost and time taken to drill. Worked-out lodes may be partially backfilled with waste material sorted from the ore, and the zone of disturbed ground may increase because of the partial collapse of overlying strata into the mine. Subsidence resulting from the collapse of mine shafts and mine workings is well known in Cornwall, for instance in St Ives and Gunnislake near Tavistock. Although subsidence has not occurred recently in Newquay, the mining history of the area should be considered during the desk study stage of a construction project.

The area of blown sands in Perran Bay is currently used as a caravan and camping holiday park and golf course. There are a few permanent buildings on the landward side of this area. Seaward areas may be inundated with sand during storms. Although most of the sand is currently stable, sand inundations have been documented during the 10th century and the late 18th and early 19th century, leading to the 'old' and 'new' St Piran churches being abandoned to the sand. Draining of the local stream during mining operations may have been responsible for increasing the area of mobile sand.

Artificial ground

The characteristics of artificial ground will vary, depending on its origin and content (Figure 9a), (Figure 9b), (Figure 9c). Most of the worked and/or made ground in the area relates to either mining activity or extraction of rock from (typically) small quarries. Extensive areas of 'disturbed ground' created by mining activity are present around St Agnes, and just inland from Cligga Head. Inland, areas once intensively worked and now partially relandscaped include those around Deerpark Mine [SW 8075 5540], East Wheal Rose [SW 8380 5550] and Wheal Albert [SW 7945 5365]. The nature of the infill material for these and other areas of made and worked ground is not always apparent; where information is available, most of the fill is mine waste (a very heterogeneous term). Mine waste will be associated with known mine shafts, adits and mining areas. Granular mined material is likely to be loose and fine-grained material, soft to stiff. Some of the mine waste may have been engineered, and some areas have been landscaped. Quarries tend to be infilled with domestic refuse. Waste disposal sites are documented to the south-east of St Newlyn East, and may contain a variety of domestic and industrial waste, which can be loose, highly compressible, contain voids and may emit noxious fluids such as methane and leachate.

Engineered made ground includes embankments designed for a construction purpose, including road, railway and other embankments and some construction sites for large buildings and may contain both natural and man-made material such as brick or concrete. Urban areas contain a variety of artificial deposits varying from engineered material, the remnants of old buildings, old waste tips, and landscaped ground etc., each having a variety of engineering characteristics.

Minor bodies of made and worked ground relating to prehistoric features are present throughout the area; a detailed study of the Penhale area was carried out by Cole and Tapper (2004).

Mass movement deposits and coastal erosion

Large coastal rock falls are evident in Watergate Bay and also near to Porth Joke/ Holywell Bay, where the coastal path has been diverted. Smaller falls are common along the coast where the sea has undermined the cliffs and exploited faulting and joint lines. Headland masses typically trend east–west and the northern and southern limits of these features, typically parallel to the regional strike of the bedding and foliation, are particularly vulnerable to erosion and collapse. Inland, there is little evidence of landslide, except as soil terrace formation at sub-metre scale on steep slopes and in superficial deposits (head) in the lower parts of valley accumulations.

Natural radon emissions

Radon is a natural radioactive gas produced by the radioactive decay of radium and uranium. Radon is found in small quantities in all rocks and soils, although the amount varies from place to place. Geology is the most important factor controlling the source and distribution of radon (Appleton and Ball, 1995).

Approximately 99 per cent of the Newquay district is classified as a radon affected area, with over 90 per cent of the district being classified as having greater than 10 per cent of homes over the Action Level of 200 Bqm⁻³ (Green et al., 2002). The government recommends that houses in radon affected areas should be tested for radon. Radon protective measures may need to be installed in new dwellings, and extensions to existing ones, in most of the district (see plate 12 in BRE, 1999).

The main bedrock units in the district prone to high radon levels are the mudstones and sandstones of the Grampound, Porthtowan and Portscatho formations, the slates and sandstones of the Meadfoot Group, the siltstone of the Staddon Formation, and the slates, siltstones and sandstones of the Dartmouth Group, all of which are of Devonian age. The sandstone of the Staddon Formation may have a slightly lower radon potential.

Mineral and energy resources

Metalliferous mining

The district has a long history of metalliferous mining activity, with substantial production of tin, copper, lead, zinc, silver and iron ores. There has also been smaller production of arsenic, pyrite, tungsten and mineral pigments (ochre and umber). As noted in the discussion of mineralisation in the geological description, the mineral deposits show some evidence of geographical separation, with tin and copper ore production predominating in the St Agnes to Perranporth area, and lead, zinc and silver mostly in a zone extending from Callestick to St Newlyn East. The greatest production of iron ores was from the Great Perran Iron Lode.

Records of production are known to be incomplete, as many mines were active before their general publication in the middle of the 19th century. The statistics provided for the district by Reid and Scrivenor in the 1906 Geological Survey Memoir (at which time most of the mines were closed) are given in (Figure 10). This clearly shows the importance of Pb-Ag-Zn production in the district in the second half of the 19th century.

In the early part of the 20th century, there was a dwindling production of metals, mostly tin, from the mines of the St Agnes area, together with small amounts of copper and tungsten. There was also some production of iron ore from the Great Perran Iron Lode. Exploration of the mines around Cligga Head during the Second World War resulted in a small production of tin and tungsten concentrates; production ceased in 1945. During the same period, mining activity at the Treamble section of the Great Perran Iron Lode resulted in the production of some 20 000 tonnes of iron oxide ore.

Since the Second World War, the greisen-bordered vein swarms and associated mineralisation of the Cligga Head Granite have provided targets for exploration with a view to extracting tin and wolfram by mining large volumes of low-grade ores. The last such project was undertaken in the 1970s, and was abandoned prior to the collapse of the tin market in 1985. There have also been exploration projects to investigate the St Agnes tin veins and possible depth extensions of the Great Perran Iron Lode. There is no production of metals by mining in the district at present, apart from the extraction of tin from dump and superficial deposits for heritage tourism.

Construction materials

Building stone

Locally quarried building stones have contributed greatly to the architectural heritage of the Newquay district, particularly the various sandstones and igneous rocks. As is the case elsewhere in the peninsula, the advent of modern and inexpensive building materials such as kiln-fired bricks and concrete blocks have led to a greatly reduced demand for building stone. As a result, there is no active extraction in the district at the present day.

Sandstones and the harder siltstones of the Grampound Formation have been worked from a number of quarries across the outcrop and have found extensive use. Around Goonhavern, fine-grained buff sandstone and laminated soft-weathering siltstone are common, while farther east, the coarser and better-weathering sandstones, have been preferred. Large quarries were opened around Ladock and near the railway line at Treworgans [SW 898 494].

The Porthtowan Formation has yielded durable sandstone in places. Pale beige, fine-grained sandstone was extracted from Body's Quarry [SW 7307 5114] and was much used for house construction around St Agnes; similar, rather coarser-grained rock was worked from nearby quarries and this material was largely used for railway bridges and structures along the former GWR line from Newquay to Chacewater.

There are no good sources of roofing slate in the Newquay district, but thin-parting beds of mudstone from within the Bovisand Formation of the Dartmouth Group were extracted close to the present location of the Watergate Bay Hotel [SW 8437 6491] for building stone, and also south of Penvose [SW 8569 6469]. The thin-parting shales of the Trendrean Mudstone Formation have also been used, principally for hedgerow construction.

Quartz-porphyry dykes ('elvans') have been quarried at numerous locations for building stone. The extensive, north–south-trending elvan that crops out in Watergate Bay and can be traced for over 6 km was extracted from quarries around [SW 8425 5915] and used to construct Trerice Manor House [SW 8412 5850]. Numerous small quarries in similar rocks are present across the area. Larger-scale extraction was undertaken near to Hanover Cove, for example at [SW 7373 5303], where in contrast to similar rocks elsewhere in the district, very few joint planes are developed, making the stone favourable for facing buildings in the parish of Perranzabuloe. Altered rock has been locally extracted for use in carving and, where extensively altered to clay minerals, used for brick production (e.g. south-east of Columb Minor).

Aggregate and roadstone

No quarries are actively producing aggregate or roadstone in the district. Historical production was mostly on a small scale, from the various Devonian sandstones or igneous rocks. Relatively small-scale extraction of the Staddon Formation was undertaken in the late 1890s for use as road stone; seven quarries were identified, including locations south-east of Dayman's Farm, at [SW 8898 6703], and two quarries on Denzel Downs ([SW 9088 6724] and [SW 9079 6742]). A microgabbro dyke exposed in the banks of the Gannel estuary at [SW 7948 6122] was also quarried for roadstone, as was a felsic dyke near St Agnes Head, which has been subject to extraction at several locations, including a sizeable quarry at [SW 7136 5168]. Coarse sandstone of the Grampound Formation was extracted from quarries in and around Ladock, including the large, dormant quarry adjacent to the railway at Treworgans Farm [SW 8985 4948].

Sand

The largest resources of sand in the district are the considerable accumulations of beach and dune material, which were used extensively in the past for building and agricultural purposes; the high carbonate content of the dune sands was useful for regulating soil pH values. At a more specialised level, sand from the St Agnes Formation that is unstained by iron minerals was formerly exported within the UK for pottery manufacture. Other uses for the St Agnes sand have included gardening and mortar manufacture.

Clay

The historical requirements for clay in the district were mostly for small-scale brick manufacture, and the source was generally the superficial head deposits. Chiverton House [SW 7969 5114] was built using bricks manufactured from the local clay-rich head. More specialised clays were worked from the kaolin-rich clay components of the St Agnes Formation; these included fireclay used for the production of tobacco pipes, and 'candle clay', a stiff, plastic, blue-grey variety used to affix candles to walls and to miners' helmets, and also to plug water leaks in mine workings. The manufacture of tobacco pipes had ceased by the late 19th century. A clay-rich material was formerly worked from altered wallrocks at Treamble Mine and sold for use as a cleaning agent. Although described as 'Fullers Earth', the material was essentially impure kaolinitic clay (Sabine, 1968).

Water resources

The mean annual rainfall varies from less than 1000 mm around Newquay to over 1100 mm in the south-east part of the district (Meteorological Office, 1977). For the period 1976 to 2006, the mean annual precipitation was 1111 mm, the mean annual potential evapotranspiration 639 mm and the mean annual actual evapotranspiration 590 mm (MORECS methodology, Hough and Jones, 1997).

Bedrock

Primary permeability is low in the Devonian rocks with water occurring mainly in secondary fractures because of their age and degree of induration. Groundwater has been obtained in the past from most of the formations, and topographical maps indicate the presence of wells and springs throughout the district. Yields are generally low and as the fractures tend to close up with increasing depth, productive boreholes are generally less than 60 m deep. However, some boreholes have not struck water until depths of as much as 50 m. The water table reflects topography, with water often rising above where struck to lie within 10 m of the ground surface. Boreholes can encounter several water strikes. The infrequent reports of dry boreholes — only one is known, from Tresithick [SW 8460 4896] — is likely to reflect under-reporting.

Reported yields from wells and boreholes are often less than 0.5 l/s, and rarely exceed 1 l/s. Historically, pumping data was obtained from blow-out tests of one hour duration and hence did not represent long-term yields. Some longer duration pumping tests have been carried out more recently. Transmissivity values vary from less than 1 to over 50 m2/d but due to the absence of observation boreholes, storage coefficients cannot be calculated. Specific capacity values also vary due to significant variations in drawdown as well as yield. The Meadfoot Group has produced some of the highest yields and transmissivities appear to be lowest (up to 1 m2/d) in the Grampound Formation.

The highest yields in the district have been obtained from springs and adits with a 150 m-deep borehole with adits at 36 and 42 m into the Meadfoot Group at St Columb Minor [SW 8490 6116] pumped at 25 l/s for 12 hours/day (1080 m3/d) to supply Newquay until 1960. A second 13 m-deep well [SW 8500 6154] produced a similar volume. The 19.8 m-deep 2.4 m-diameter Treamble Well, Perranzabuloe [SW 7845 5609] into the Trendrean Mudstone Formation was pumped at 5.1 l/s for 31 days for 15.5 m of drawdown. The Mount Mine adit nearby [SW 7810 5666] produced 909 m3/d, and together they supplied Cubert and Holywell. Springs issuing from the Grampound Formation at Blowing House, Perranzabuloe [SW 7488 5144] had yields that varied seasonally from zero to 227 m³/d and Ladock Quarry [SW 8925 5105] is fed by a spring with a yield of 100 m³/d. At Perranzabuloe, three springs known as the Silver Well [SW 75 49] yielded 136 m³/d in summer and 272 m³/d in winter from the Porthtowan Formation, other springs at Gollawater [SW 760 506] yielded 45 to 90 m³/d. An adit at Gooninis, St Agnes [SW 7292 5042], in conjunction with five other wells and adits in the Gover valley in the adjacent district to the south, provided the public supply for St Agnes. They collectively produced 5.7 l/s for 18 hours/day in the summer months, at other times the yield was over 17 l/s.

The limited outcrop area of the igneous and Oligocene age rocks, and the latter's elevation, mean that they are of little hydrogeological significance.

Superficial deposits

These are relatively thin but some water may be obtainable from the extensive deposits of blown sand between Holywell and Perranporth. The Holy Well [SW 7640 6020] at the northern end of Holywell Beach produced very hard water due to percolation through the shell sand; this has deposited significant amounts of tufa. Water in the alluvium is likely to be in hydraulic continuity with the associated surface water courses. The head deposits are preferentially developed on the less sandy strata and are unlikely to contain significant volumes of groundwater. The beach and tidal flat deposits are likely to be saturated with brackish water.

Groundwater quality and protection

Groundwater from the bedrock is often acidic, with pH values locally less than 5 (e.g. in an adit in the Porthtowan Formation where the low value is likely to be caused by pyrite oxidation). Any water intended for a potable supply should be tested for metals (arsenic, copper, lead, zinc and possibly tin) in areas where mineral veins are present. Iron and manganese may also cause problems. Sodium and chloride concentrations are elevated for several kilometres inland due to the maritime influence on rainfall composition, with chloride locally around 100 mg/l.

Some alkaline waters do occur. Reid and Scrivenor (1906) stated that water from the Meadfoot Group in the immediate neighbourhood of Newquay was 'objectionably hard' with high calcium carbonate and sodium chloride concentrations; the main water supply for the town subsequently came from disused mine adits into 'baked killas' surrounding the St Austell granite in the adjacent district to the east where the groundwater was softer.

The groundwater sources in the district obtain water from shallow aquifers. Shallow groundwater is highly vulnerable to contamination from the ground surface from both diffuse (e.g. nitrate and pesticides) and point source (e.g. fuel storage tanks) pollutants, particularly in fractured aquifers. Locally the nitrate concentration is close to the EC limit for drinking water.

Conservation sites

Several areas with special protection fall within the boundary of the Newquay district.

Penhale Dunes have been designated as a SSSI since 1953 to preserve the extensive dune system and its associated flora and fauna. It has an area of 1070.4 ha. Calcareous sand forms dunes up to 93 m above OD.

The St Agnes mining district, covering an area of around 130 km2, is a subsite of the 'Cornwall and West Devon Mining Landscape' UNESCO World Heritage site, a designation that protects mining-related landscapes and remaining infrastructure. The St Agnes district was included as it is representative of coastal mining, and preserves mining heritage from prehistory to the early 20th century.

The National Trust protects several coastal areas from the edge of the Gannel south-westwards, including: The Rushy Green [SW 787 609], an area of grassland behind the dunes on Crantock Beach; The Kelseys [SW 7696 6008], an ancient area of springy turfed grassland; Cubert Common [SW 7817 5964], registered common ground and also an SSSI; and the dune environment at Holywell Bay [SW 7673 5928]. Inland, the local high point of St Agnes Beacon [SW 7092 5038] is also protected by the National Trust, as is an area to the west of the Beacon, around the remainder of Wheal Coates mine [SW 7000 5008].

Information sources

Sources of further geological information for the Newquay district are listed below. Geological advice for the area should be sought from BGS offices at Keyworth or Cardiff. Other geological information includes borehole records, fossils, rock samples, thin sections, geochemical samples, geophysical data and hydrogeological data. Indexes of some BGS data sources can be found on the website (www.bgs.ac.uk) and are available in BGS libraries. The BGS catalogue of maps, books and other products is available on the BGS website on request (addresses on back cover). The BGS hydrogeology enquiry service may be contacted at: BGS, Maclean Building, Crowmarsh Gifford, Wallingford, Oxfordshire, OX10 8BB. Tel: 01491 838800.

Maps

• Geological maps
• 1:1 000 000
• Pre-Permian geology of the United Kingdom, 1985
• 1:625 000
• Solid Geology map of the UK: South Sheet, 2007
• 1:250 000
• 50N 06W Land's End: Solid geology, 1985; Solid and Quaternary, 1987
• 1:50 000
• Sheet 351/358 Penzance, Solid and Drift, 1984
• Sheet 352 Falmouth, Solid and Drift, 1990
• Sheet 353/354 Mevagissey, Solid and Drift, 2000
• Sheet 347 Bodmin, Solid and Drift, 1982
• Sheet 335 and 336 Trevose Head and Camelford, Solid and Drift, 1994
• Sheet 346 Newquay, 2013
• 1:10 000
• The 1:10 000 scale sheets were surveyed mainly between 2003 and 2005 by C E Burt, L M Hollick and R C Scrivener, with some later work up to 2007 by L M Hollick.
• Copies of the fair-drawn maps are available for inspection at BGS offices. Uncoloured dyeline copies are available for purchase from the BGS Sales Desk at the Keyworth Office.
• Digital geological map data
• In addition to the printed publications, many BGS geological maps are available in digital form. Details are given on the BGS website. National coverage of digital geological map data (DiGMapGB) is derived from geological maps at scales of 1:625 000, 1:250 000 and 1:50 000. Selected areas also have digital geological data derived from 1:10 000 scale geological maps. Digital geological data for offshore areas is derived from 1:250 000 scale geological maps.
• Geophysical maps
• 1:1 500 000
• Colour shaded gravity anomaly map of Britain, Ireland and adjacent areas, 1997 Colour shaded magnetic anomaly map of Britain, Ireland and adjacent areas, 1998
• 1:250 000
• 50N 06W Land's End: Aeromagnetic Anomaly, 1977; Bouguer Gravity Anomaly, 1975
• Geochemical maps
• 1:625 000
• Methane, carbon dioxide and oil susceptibility, Great Britain (South Sheet), 1995
• Radon potential based on solid geology, Great Britain (South Sheet), 1995
• Distribution of areas with above national average background concentrations of potentially harmful elements (As, Cd, Cu, Pb and Zn), Great Britain (South Sheet), 1995
• Hydrogeological maps
• 1:625 000
• England and Wales, 1977
• Minerals maps
• 1:1 000 000
• Industrial minerals resources map of Britain, 1996

Books

• Durrance, E M, And Bristow, C M. The geology of Cornwall and the Isles of Scilly, 1998. Edited by selwood, e B. ISBN 0859894320
• Brenchley, P J, And Rawson, P F. The Geology of England and Wales (2nd Edition). (London: The Geological Society). ISBN 1862392005.
• Dines, H G. 1956. The metalliferous mining region of South-West England (first edition). Volumes one and two. (HMSO, London.)
• Offshore regional reports
• The Geology of the English Channel, 1992
• Memoirs
• Sheet 351/358 Penzance, 1988 Sheet 352 Falmouth, 1990
• Sheet 353/354 Mevagissey, 2007 Sheet 335 and 336 Trevose Head and Camelford, 1998
• Sheet 346 Newquay, 1906*
• *out of print, but photocopies may be purchased from the BGS library, Keyworth
• Technical reports
• Geological:
• Goode, A J J. SW 64 NE, Camborne– Redruth: WA/DM/87/10.
• Goode, A J J. SW 74 NW and NE, Chacewater. WA/DM/87/12.
• Leveridge, B E, and Holder, M T. SW 84 NW and NE, 94 NW, Truro. WA/DM/87/14.
• Goode, A J J, and Leveridge, B E. SW 86NW and NE, Trevose Head and St Breock Downs. WA/92/11.
• Hollick, L M. SW 85 SW and SE, 95 SW, Mitchell and Grampound Road. IR/06/119R.
• Holder, M T, and Leveridge, B E. A framework for the European Variscides. WA/94/24.
• Tandy, B C. Radiometric anomalies near St Columb Major, Cornwall. WF/AG/74/322.
• Strong, G E. Particle size analyses of silt fractions of Palaeogene sediments from the St Agnes area, Newquay, Cornwall. WG/PE/74/78.
• Geophysical:
• wAlker, A B. SW England seismic monitoring for the HDR Geothermal Programme in Cornwall 1989 to September 1991. WL/91/36.
• Beer, K E, Burley, A J, and Tombs, J M. The concealed granite roof in south-west Cornwall: Mineral Reconnaissance Programme Report, Institute of Geological Sciences, No. 1.
• Biostratigraphical reports Biostratigraphical reports are held as restricted reports; interested parties are recommended to contact the BGS Keyworth office for access to these reports and to the palaeontological collections.

Documentary collections

Boreholes

Borehole data for the district are catalogued in the BGS archives (National Geological Records Centre) at Keyworth on individual 1:10 000 scalesheets. Forfurtherinformation, contact: The Manager, National Geological Records Centre, BGS, Keyworth.

BGS photographs

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 at BGS libraries and on the BGS website (GeoScenic). Copies of the photographs can be purchased from BGS.

References

Alexander, A C. 1997. Late- to post-Variscan deformation in South Cornwall. Unpublished PhD thesis, University of Exeter.

Alexander, A C, and Shail, R K. 1995. Late Variscan structures on the coast between Perran-porth and St Ives, Cornwall. Proceedings of the Ussher Society, Vol. 8, 398–404.

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, 72–78.

Appleton, J D, and Ball, T K. 1995. Radon and background radioactivity from natural sources: characteristics, extent and the relevance to planning and development in Great Britain. British Geological Survey Technical Report, WP/95/2.

Borlase, W. 1758. The Natural History of Cornwall. ISBN 0025166067

Bott, M H P, Day, A A, and Masson-Smith, D. 1958. The geological interpretation of gravity and magnetic surveys in Devon and Cornwall. Philosophical Transactions of the Royal Society of London, Vol. A251, 161–191.

Bowen, D Q. 1994. Late Cenozoic Wales and south-west England. Proceedings of the Ussher Society, Vol. 8, 209–13.

Bowen, D Q, and six others. 1985. Amino acid geochronology of raised beaches in South West Britain. Quaternary Science Reviews, Vol. 4, 279–318.

Building Research Establishment. 1999. Radon: guidance on protective measures for new dwellings. Building Research Establishment Report, B R 211 (1999).

British Geological Survey. 1975. Lizard. England and Wales Sheet 359. Solid and Drift. 1:50 000. (Southampton: Ordnance Survey for British Geological Survey.)

British Geological Survey. 1984, Penzance. England and Wales Sheet 351/358. Solid and Drift. 1:50 000. (Southampton: Ordnance Survey for British Geological Survey.)

British Geological Survey. 1990. Falmouth. England and Wales Sheet 352. Solid and Drift. 1:50 000. (Southampton: Ordnance Survey for British Geological Survey.)

British Geological Survey. 2000. Mevagissey. England and Wales Sheet 353. Solid and Drift. 1:50 000. (Southampton: Ordnance Survey for British Geological Survey.)

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Chen, Y, Clark, A H, Farrar, E, Wasteneys, H A H P, Hodgson, M J, and Bromley, A V. 1993. Diachronous and independent histories of plutonism and mineralisation in the Cornubian Batholith, south-west England. Journal of the Geological Society of London, Vol. 150, 1183–1191.

Chesley, J T, and five others. 1993. Thermo-chronology of the Cornubian batholith insouth-west England: implications for pluton emplacement and protracted hydrothermal mineralisation. Geochimica et Cosmochimica Acta, Vol. 57, 1817–1835.

Clark, A H, and eight others. 1998. 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.

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Dineley, D L. 1961. The Devonian System in South Devonshire. Field Studies, Vol. 1, 121–40.

Dines, H G. 1956. The metalliferous mining region of south-west England (first edition). Vol. 1 and 2. (London: H MS O.)

Dodson, H, and Rex, D C. 1971. Potassium-Argon ages of slates and phyllites from south-west England. Quarterly Journal of the Geological Society of London, Vol. 126, 465–499.

Edwards, R A, and Freshney, E C. 1982. The Tertiary sedimentary rocks. 204–237 in The geology of Devon. Durrance, E M, and Lamming, D J C (editors). (Exeter: University of Exeter Press.)

Embrey, P G, and Symes, R F. 1987. Minerals of Cornwall and Devon. (London: British Museum (Natural History).

Evans, K M. 1981. A marine fauna from the Dartmouth Beds (Lower Devonian) of Cornwall. Geological Magazine. Vol. 118, 517–523.

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Franke, W. 2000. The mid-European segment of the Variscides: tectonostratigraphical units, terrane boundaries and plate tectonic evolution. 35–61 in Orogenic processes: quantification and Modelling in the Variscan belt. Franke, W, Haak, V, Oncken, O, and Tanner, D (editors). Geological Society of London Special Publication, No. 179. I SB N 1862390738.

Franke, W, and Engel, W. 1982. Variscan sedimentary basins on the Continent and relations with south-west England. Proceedings of the Ussher Society, Vol. 5, 259–269.

Funnell, B M. 1995. Global sea-level and the (pen-)insularity of late Cenozoic Britain. 3–13 in Island Britain: a Quaternary perspective. Preece, R C (editor). Geological Society of London Special Publication, No. 96. I SB N 1897799403.

Geological Survey of Great Britain. 1906. Newquay. England and Wales Sheet 346. Solid and Drift. 1:63 360. (London: H SM O.)

Goode, A J J. 1973. The mode of intrusions of Cornish elvans. Report of the Institute of Geological Sciences, No. 73/7.

Goode, A J J. 1987. Geological notes and details for the 1:10 000 sheets S W 64 N W, N W, S W and S E (Camborne–Redruth, Cornwall). British Geological Survey Technical Report, WA/D M/87/10.

Goode, A J J, and Merriman, R J. 1977. Notes on marble and calc-silicate rocks from Duchy Peru borehole, near Perranporth, Cornwall. Proceedings of the Ussher Society, Vol. 4, 57–60.

Goode, A J J, and Taylor, R T. 1988. Geology of the country around Penzance. Memoir of the British Geological Survey, Sheet 351/358 (England and Wales). I SB N 0118843885.

Green, B M R, Miles, J C H, Bradley, E J, and Rees, D M. 2002. Radon Atlas of England and Wales. National Radiological Protection Board Report, NRPB-W26. (Chilton, Didcot: NRPB.)

Green, C P. 1985. Pre-Quaternary weathering residues, sediments and landform development: examples from southern Britain. 58–77 in Geomorphology and soils. Richards, K S, Arnett, R R, and Ellis (editors). (London: G Allen and Unwin) I SB N 0045510938.

Hall, A. 1971. Greisenisation in the granite of Cligga head, Cornwall. Proceedings of the Geologists' Association, Vol. 82, 209–230.

Hamblin, R J O. 1974. On the correlations of the Haldon and Aller Gravels, South Devon. Proceedings of the Ussher Society, Vol. 3, 103–10.

Hawkes, J R. 1981. A tectonic 'watershed' of fundamental consequence in the post-Westphalian evolution of Cornubia. Proceedings of the Ussher Society, Vol. 5, 128–131.

Hendricks, 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.

Henley, S. 1971. Hedenbergite and sphalerite from the Perran iron lode, Cornwall. Proceedings of the Ussher Society, Vol. 2, 329–334.

Henley, S. 1974. Geochemistry and petro-genesis of elvan dykes in the Perranporth area, Cornwall. Proceedings of the Ussher Society,Vol. 3, 136–145.

Hill, J B, and Macalister, D A. 1906. The geology of Falmouth and Truro and the Mining district of Camborne and Redruth. Memoir of the Geological Survey of Great Britain, Sheet 352 (England and Wales). I SB N X78001076X.

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

Holdsworth, R E. 1989. The Start–Perranporth line: a Devonian terrane boundary in the Variscan orogen of S W England. Journal of the Geological Society of London, Vol. 146, 419–421.

Hollick, L M. 2006. Geology of the Mitchell and Grampound Road district. British Geological Survey Internal Report, I R/06/119R.

Hollick, L M, Shail, R K, and Leveridge, B E. 2006. Devonian rift-related sedimentation and Variscan tectonics — new data on the Looe and Gramscatho basins from the resurvey of the Newquay district. Geoscience in south-west England (Proceedings of the Ussher Society), Vol. 11, 191–198.

Hosking, K F G. 1964. Permo-Carboniferous and later mineralisation of Cornwall and south-west Devon. 201–245 in Present views on some aspects of the geology of Cornwall and Devon. Hosking, K F G, and Shrimpton, G J (editors). (Truro: Royal Geological Society of Cornwall). I SB N O C25163778.

Hosking, K F G, and Camm, G S. 1985. The occurrence of cassiterite and other species of economic interest in the greisenised granite porphyry of Cameron Quarry, St Agnes, Cornwall. 517–533 in High heat production (H HP) granites, hydrothermal circulation and ore genesis. (Institute of Mining and Metallurgy.)

Hough, M N, and Jones, J R A. 1997. The United Kingdom Meterological Office rainfall and evaporation calculation system: M OR EC S version 2.0, an overview. Hydrogeology and Earth Systems Science, Vol. 1, 227–239.

Jenkins, D G, Whittaker, J E, and Carlton, R. 1986. On the age and correlation of the St Erth Beds, S W England, based on planktonic foraminifera. Journal of Micropalaeontology, Vol. 5, 92–105.

Jowsey, N L, Parkin, D L, Slipper, I J, Smith, A P C, and Walsh, P T. 1992. The geology and geomorphology of the Beacon Cottage Farm Outlier, St Agnes, Cornwall. Geological Magazine, Vol. 129, 101–121.

Kidson, C. 1977. Some problems of the Quaternary of the Irish Sea. Geological Journal Special Issue, Vol. 7, 1–12.

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

Leveridge, B E. 2008. Geology of Mevagissey district. Sheet Explanation of the British Geological Survey, Sheet 353 (England and Wales.)

Leveridge, B E, and Hartley, A J. 2006. The Variscan Orogeny: the development and deformation of Devonian/Carboniferous basins in S W England and South Wales. 225–255 in The Geology of England and Wales (Second Edition). Brenchley, P J, and Rawson, P F (editors). (London: The Geological Society).I SB N 1862392005.

Leveridge, B E, and Holder, M T. 1987. Geological description for 1:10 000 sheets S W84 N W, N E, S W and S E and parts of S W 94 N W and S W. British Geological Survey Technical Report, WA/D M/87/14.

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). I SB N 0118844679.

Leveridge, B E, and five others. 2002. Geology of the Plymouth and south-east Cornwall area. Memoir of the British Geological Survey, Sheet 348 (England and Wales). I SB N 0118845608.

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Moore, J M, and Jackson, N J. 1977. Structure and mineralisation in the Cligga Granite stock. Journal of the Geological Society of London, Vol. 133, 467–480.

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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 Office at the Natural History Museum, and from BGS-approved stockists and agents.

Northern Ireland maps can be obtained from the Geological Survey of Northern Ireland.

Figures and plates

Figures

(Figure 1) Simplified map of bedrock geology in the Newquay district.

(Figure 2) Passive margin basins in the region – Looe Basin and Gramscatho Basin.

(Figure 3) Magnetic (top) anomaly and Bouguer gravity (bottom) anomaly maps for the Newquay district. The areas with larger negative Bouguer anomaly values are inferred to indicate the subsurface extent of granite intrusions.

(Figure 4) Generalised sequence of mineralisation in south-west England, with examples of exploited deposits within the Newquay district.

(Figure 5) Constituent mines of the Great Perran Iron Lode.

(Figure 6) Metallogenesis of the Great Perran Iron Lode.

(Figure 7) Summary of stratigraphy of the Beacon Cottage Farm and St Agnes Beacon outliers.

(Figure 8) Sequential development of brittle faulting in the Newquay district.

(Figure 9a) Summary of geotechnical properties of rocks and other deposits in the Newquay district — bedrock.

(Figure 9b) Summary of geotechnical properties of rocks and other deposits in the Newquay district — superficial deposits.

(Figure 9c) Summary of geotechnical properties of rocks and other deposits in the Newquay district — artificial and worked ground.

(Figure 10) Metal production in the Newquay district (after Reid and Scrivenor, 1906).

Plates

(Plate 1) Whipsiderry Beach looking north to show the transition between the Whitsand Bay Formation (Dartmouth Group) and the overlying Bovisand Formation (Meadfoot Group) (P781570).

(Plate 2) A disused quarry in Quarry Lane, Bolingey, near Perranporth [SW 7675, 5351] showing thin-bedded olive-grey siltstone and fine-grained sandstone of the Grampound Formation (P781851).

(Plate 3) Turbidite sandstone and interbedded mudstone of the Porthtowan Formation at Chapel Porth (P781566).

(Plate 4) Sheeted narrow veins with prominant greisen borders in the quarry at Cligga Head (P701714).

(Plate 5) Gravel Hill Mine at the western extremity of the Great Perran Iron Lode. The view shows old workings for iron with limonite-rich ore visible in the pillar (P701754).

(Plate 6) Watergate Bay with D1 folds and S1 cleavage showing parallel relations in limbs and oblique relationships in hinges (P781573).

(Front cover) Cover photograph Penhale Point looking north-west. The faulted and dipping bedrock sequence comprises interbedded slate and sandstones of the Meadfoot Group. (Photographer: P J Witney, P701733).

(Rear cover)

(Geological succession) Summary of the geological succession of the Newquay district.

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

Figures

(Figure 4) Generalised sequence of mineralisation in south-west England, with examples of exploited deposits within the Newquay district

(Figure 5) Constituent mines of the Great Perran Iron Lode

(Figure 6) Metallogenesis of the Great Perran Iron Lode

(Figure 7) Summary of stratigraphy of the Beacon Cottage Farm and St Agnes Beacon outliers

(Figure 8) Sequential development of brittle faulting in the Newquay district

(Figure 9a) Summary of geotechnical properties of rocks and other deposits in the Newquay district — bedrock

(Figure 9b) Summary of geotechnical properties of rocks and other deposits in the Newquay district — superficial deposits

(Figure 9c) Summary of geotechnical properties of rocks and other deposits in the Newquay district — artificial and worked ground

(Figure 10) Metal production in the Newquay district (after Reid and Scrivenor, 1906)