the geology of the building and decorative stones of cornwall, uk

The geology of the building and decorative stones of Cornwall, UK

COLIN M. BRISTOW

Cornwall Geoconservation Group, Ty Gwyn, 46 Chatsworth Way, Carlyon Bay,

St Austell, Cornwall, UK (e-mail: [email protected])

Abstract: Arising mainly from its exceptionally varied suites of igneous and sedimentary rocks,Cornwall has a rich variety of building and decorative stones that were extensively exploited, bothfor local use and for export, before concrete and brick came to dominate construction in the twen-tieth century. Many of the types of building stone, such as elvan and sandrock, do not occur outsideCornwall, so local stone provides much character to the local built environment.

Granites were extensively worked in the eastern part of the Carnmenellis Granite (mainly inMabe parish), in the St Austell Granite (Luxulyan, Carn Grey and the china stone areas) and onBodmin Moor (De Lank, Hantergantick, Cheesewring, etc.), as well as in the Kit Hill, Tregonningand Land’s End granite masses. The predominant type used was the ‘coarse grained megacrysticbiotite granite – smaller megacryst variant’ of Hawkes & Dangerfield. A significant trade ingranite developed in the nineteenth and early twentieth centuries, employing large numbers ofskilled quarrymen. Finished granite was exported all over the world; many iconic buildings inLondon and other major cities use Cornish granite. A tourmalinized granite, luxullianite, was animportant decorative stone, and was used for the Duke of Wellington’s sarcophagus in St Paul’sCathedral. Schorl rock is often found in older buildings in the granite areas. Most pre-nineteenthcentury granite building did not use quarried stone but used ‘moorstone’ obtained from boulderslying on the surface of the granite uplands. Large quantities of ‘minestone’ have been used in ver-nacular buildings, past and present, and in some medieval churches, sourced from the waste tips ofmetalliferous (both alluvial and vein operations) and china clay workings.

Allied to the granites are the fine-grained elvans of granitic composition, usually intruded in theform of dykes. Greisening often improves the durability of elvans, which have been extensivelyused in some of the finest stone buildings in Cornwall, such as St Austell church tower, AntonyHouse (NT), Trelowarren, Place (Fowey) and the Georgian buildings of Lemon Street, Truro.The best-known elvan quarries were at Pentewan, which yielded a freestone capable of finecarving. However, not all buildings described by architectural historians as being of PentewanStone came from Pentewan. Another important elvan was Newham Stone, widely used in theolder buildings in Truro. Tremore elvan was used, together with luxullianite, mainly as a polisheddecorative stone to line Porphyry Hall at Place in Fowey and in other high-status buildings.

Basic igneous rocks include an Upper Devonian metadolerite at Cataclews Point, west ofPadstow, which provided the extremely durable Cataclews Stone, used from medieval timesonwards for fonts and church carvings in the area around the Camel estuary. A more unusualstone, produced by carbonatization of an ultrabasic intrusion, is Polyphant Stone, mainly usedfor interior use and by sculptors, composed of a mixture of talc, chlorite, and various calciumand magnesium carbonates. The Polyphant Quarry was recently reopened to supply stone forthe rebuilding of Newquay parish church and to supply stone for sculpting. Allied to PolyphantStone is Duporth Stone, obtained from the cliffs of Duporth Bay, south of St Austell, which wasused in the pillars of Truro Cathedral. Basic hyaloclastite was the main stone used in the greatNorman Church of St German’s in SE Cornwall. The Lizard ophiolite complex provided asource of serpentine for building and for the manufacture of polished slabs; ornaments madefrom serpentine are still produced.

Slaty mudstones and sandstones of Devonian and Carboniferous age have been extensively usedfor traditional buildings throughout Cornwall, nowadays much slaty mudstone is still used forbuilding and for Cornish hedge building. The Upper Devonian Delabole Slate Quarry hasyielded high-quality roofing slate from Tudor times onwards but there are many other largeactive and disused roofing slate quarries in the Tintagel area and elsewhere in Cornwall, such asthe underground slate workings at Carnglaze, now a tourist attraction and concert venue. Devoniansandstones, usually of turbiditic origin, are widely used for vernacular building in south Cornwall,and Upper Carboniferous turbidite sandstones are used in north Cornwall. The geologically young-est building stone, seen in the Newquay and Padstow areas, is a cemented bioclastic Quaternarybeach sand, laid down at a time of high sea level during an interglacial as a raised beach. It isknown locally as ‘sandrock’ but is a relatively weak building stone. St Carantoc’s Church at Cran-tock and St Piran’s Church on Perran sands were largely built of it.

Supplementary material: A more detailed review of the various granite and elvan quarries thathave been worked in Cornwall is available at http://www.geolsoc.org.uk/SUP18675

From: Cassar, J., Winter, M. G., Marker, B. R., Walton, N. R. G., Entwisle, D. C., Bromhead, E. N. & Smith, J. W. N.(eds) 2014. Stone in Historic Buildings: Characterization and Performance. Geological Society, London,Special Publications, 391, 93–120. First published online October 14, 2013, http://dx.doi.org/10.1144/SP391.6# The Geological Society of London 2014. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics

at University of Otago on December 21, 2014http://sp.lyellcollection.org/Downloaded from

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Cornwall’s older buildings have a special characterreflecting the unusual variety of building materialsused and the exposed situation surrounded bystormy seas, battered by salt-laden gales and experi-encing high rainfall. The purpose of this paper is todescribe the building and decorative stones thathave been extracted from Cornwall in order tohelp with building restoration and new building,thereby enabling local distinctiveness and characterto be maintained.

A wide variety of building stones have beenexploited, ranging from strong granites used tobuild lighthouses in some of the most exposed situ-ations around the British Isles, to durable stonessuch as Cataclews, Pentewan and Newham, andhigh-quality slates such as Delabole. There aremany interesting and unusual types not seen inEngland, such as the fine-grained felsitic elvans,often capable of being carved as a freestone. Someof the more unusual stones, such as luxullianite,Lizard serpentine, Tremore Porphyry and Poly-phant Stone, are so beautiful when polished thatthey should properly be regarded as decorativestones and have been used in high-status build-ings all over Britain. Good places to see a widerange of Cornish building and decorative stonesare the columns surrounding the court of theOxford University Museum of Natural Historyand King Arthur’s Hall at Tintagel, although thelatter is marred by poor labelling. The collectionsof the British Geological Survey at Keyworthand the Natural History Museum also includemany specimens of Cornish building stones, somepolished.

In the past, stone production was a significantand profitable part of the Cornish economy, employ-ing large numbers of skilled workmen and export-ing large quantities of granite (Anon 1895; Stanier1999). As late as 1947, the Cornish and DevonGranite Masters Association Handbook (Anon1947) listed 40 producers of granite. Some of themost iconic structures in London such as TowerBridge and other bridges over the Thames, aswell as many high-status official and commercialbuildings, used Cornish granite. Much granite usedin pre-nineteenth century building was not quar-ried but was obtained as ‘moorstone’ from looseboulders lying on the surface. Also, much buildingstone resulted from mining activity as ‘minestone’,both from tin streaming and as waste rock frommining. The information contained in this paperhas been obtained from fieldwork and document-ary research over the last 30 years. An importantelement, in keeping with the theme of this book, isto relate the geological characteristics of the build-ing stones, including the alteration processes theyhave undergone, to the way in which they can beused and conserved in buildings.

Previous work

Early writers, such as Carew (1602) and Borlase(1758), make reference to the principal buildingstones of the County, and De la Beche (1839) pro-vided a useful review of the building stone industryin the 1830s. However, apart from short referencesby Worth (1875) and the author (Bristow 2001,2004), and reference to Cornish building stones inmore general publications (Howe 1910), there hasbeen no systematic attempt to describe the buildingstones of Cornwall. Stanier’s (1999) book, SouthWest Granite, is an excellent review of the granitequarrying industry but does not cover the manyother non-granite sources of stone in Cornwall andcontains only limited geological information. Amajor source of information are the Memoirs pub-lished by the Geological Survey of Great Britainin the early decades of the twentieth century;many of the quarries they describe are now eitherfilled in or inaccessible.

A series of detailed reports were published by theCornwall Archaeology Unit of Cornwall Council(now the Historic Environment Service) on theindustrial archaeology of quarrying locations inCornwall, such as the granite quarries on Kit Hill(Herring & Thomas 1990), the granite quarries ofthe Luxulyan Valley (Smith 1988) and the slatequarries in the cliffs near Tintagel (Sharpe 1990),although these do not cover the geology. TheMineral Planning Department of Cornwall Councilpublished The Cornish Building Stone and SlateGuide (Anon 2007), which lists most of the cur-rently active producers of building stone in thecounty, and the County Mineral Planners are cur-rently preparing a Technical Paper on the Identifica-tion of Heritage Quarries (Anon 2013). Buildingsare covered systematically in Pevsner’s classicbook (Pevsner 1951), with some limited informa-tion on the building stones used, and the Cheshers’book, The Cornishman’s House (Chesher & Chesher1968), is informative about vernacular buildingstyles. Two books on the churches of Cornwall(Cox 1912; Miles Brown 1973) contain usefulinformation but the descriptions of the stones usedare not always reliable. There are numerous refer-ences to building stone in ‘Listing Particulars’,Church guides, trade publications, etc., are a valu-able source of information about specific buildingsand quarries.

History

Neolithic settlements on Carn Brea and at HelmanTor used the local granite for making the walls forhuts, and some of the earliest standing stones andstone circles may have been erected in the Neolithic

C. M. BRISTOW94



some 5000 or more years ago (Fig. 1). Most of thestones used in Neolithic and Bronze Age ritualstructures were sourced locally as moorstone,although in some cases, such as the Penrice Long-stone (National Grid Reference SX 030/521)and the Tristram Stone (moved several times, butnow standing on the outskirts of Fowey with aDark Age inscription on it at SX 112/522), can bematched with a granite many miles away atCarn Grey.

Bronze Age hut circles, as seen in many parts ofBodmin and Penwith moors, continued the traditionof using granite for building. Later, in the Iron Age,villages such as Chysauster, in Penwith, also usedlocal granite, gathered close to the building site asmoorstone; this type of traditional building contin-ued through Romano-British times. Very littlebuilding in stone took place in the Dark Ages,with the notable exception of Tintagel, where localslate and ‘Tintagel greenstone’ was used for thefifth–sixth century settlement. Also, in the FirstMillennium, many wayside crosses were erected,some of the early ones appear to be memorials,often with a crude Latin inscription, others may besome sort of boundary or way marker often givensanctity by the carving of a cross.

By medieval times, notably after the Conquest,in the great era of Church building in the thir-teenth–sixteenth centuries, the range of stones inuse had considerably expanded. All of the granitewould still have been obtained in the form of moor-stone but quarries for some specialist stones werebeginning, such as Delabole slate, PentewanStone, Cataclews Stone and Polyphant Stone. Priorto the eighteenth century, there were few roads inCornwall and hauling stone for more than a fewmiles by oxcart or mule was prohibitively expens-ive, so the only practical way of moving stone any

distance was by sea, using Cornwall’s long coastlineand numerous inlets.

By the eighteenth century, demand for stone hadconsiderably increased, mainly due to increasingpopulation and demand for mine buildings, suchas engine houses. Minestone had become importantfor vernacular building to house the miners. Stimu-lated by the demand for harbours, lighthouses andbridges, granite quarrying took off in the early nine-teenth century and a substantial export trade devel-oped supplying stone to the rapidly growing urbancentres of Britain and overseas. This became a sub-stantial industry (Stanier 1999), ranking alongsidethe metalliferous and china clay industries interms of the numbers of skilled workers employed.In the twentieth century, the dimension stone quar-rying industry considerably contracted. Present-dayquarrying of igneous rocks is mainly to supplyaggregate for concrete and road construction, withthe supply of building stone a secondary activity(Bristow 1993). However, there are many quarriesworking Devonian slaty mudstones that supplypresent-day building activity, and quarrying is stilla significant source of employment in Cornwall.

Utilization of building stones in Cornwall

The ways in which stone can be utilized in buildingsand engineering structures are usefully reviewed byBodman (1979) and Smith (1999, pp. 327–372).Figure 2 shows the main building stone sources inCornwall.

A typical older building in Cornwall (Fig. 3) willuse elvan or granite for the window tracery andquoins, rubble ‘killas’ for the walls, and grey slatefor the roof (Chesher & Chesher 1968). ‘Killas’ isa widely used local term that covers a wide rangeof slaty rocks, most of which should properly becalled ‘slaty mudstones’. Slate is one of the mostversatile and practical building materials in Corn-wall, used not only for roofs but also for walls andfloors. It can be extremely resistant to weathering,as is evidenced by the fifth–sixth century buildingson Tintagel Island.

There were many quarries producing dressedgranite in the nineteenth and early twentieth centu-ries but only a few remain in Cornwall in the twenty-first century. Many dimension stone granite quarrieshave been converted into aggregate quarries, wherethe granite is extracted by large-scale blasting andthen crushed to produce aggregate for use in con-crete or road building. One of the main aims ofthis kind of blasting is to produce good fragmenta-tion; it also tends to produce almost invisible micro-cracks extending some distance behind the face.Any dimension stone produced from immediatelybehind the faces of a former aggregate quarry is,

Fig. 1. Trethevy Quoit, on the SE flank of BodminMoor, Cornwall, a megalithic portal grave formed ofmassive moorstone granite slabs, dating from the lateNeolithic.

BUILDING STONES OF CORNWALL 95



therefore, liable to fracture in an unpredictable way,even though it may appear whole. This is an impor-tant consideration because granite quarries that havebeen changed from dimension stone production toaggregate production cannot readily be returned todimension stone production without the removalof a great deal of blast-affected stone. However,granite from an aggregate quarry can be used forrubble masonry where mass rather than strength isthe primary consideration, so aggregate quarrieswill often produce small quantities of stone forrubble walling.

Unlike areas further north, Cornwall was not gla-ciated during the Quaternary, which means thatdeep weathering profiles, some of which may dateback to the Mesozoic (see the ‘Geology history ofCornwall’ section later), have not been whollyremoved by erosion. Together with argillic altera-tion associated with the kaolinization of the gran-ites, this means that alteration of the rocks is anever-present and unpredictable factor. Also, if thefaces in a quarry, irrespective of the kind of stoneworked, have not been worked for a long time,they may also have been slightly weathered sincebeing exposed to the elements, which will result

in, for example, changes in strength and colour.This may mean that affected stone needs to beremoved before stone production can restart. Mostquarries show an improvement in the freshnessand quality of the stone in depth.

Cornish hedges have been a characteristic fea-ture of the Cornish landscape, and there are manythousands of kilometres of them in Cornwall.They are not true hedges made from shrubby veg-etation but are, instead, composed of earth androck (Fig. 4). Many, notably in the granite areassuch as Penwith, were constructed with stonecleared from adjoining fields in prehistoric times(Balchin 1954, pp. 52–53). Cornish hedges differfrom drystone walls in having a substantial coreand capping of earth or subsoil, which means thatthey rapidly become covered with vegetation,giving the appearance of a true hedge. The widthof a typical Cornish hedge at the base is roughlyequivalent to the height (1.5–2.0 m). It is composedof large stones at the base (‘grounders’), tapering tosmaller stones at the top; the sides are concave togive strength. Any kind of stone can be used, andthe design and pattern of masonry will reflectthe kind of stone used and the hedgers style. It is

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Key

Quarry types

GraniteThe Lizard complex

GraniteElvanGreenstoneSlateSandstone and sandrockMiscellaneous

GRANITES1 Sheffield (H)2 Castallack (H)3 Lamorna (H)4 Castle-an-Dinas (H)5 Tregonning (H)6 Burthy7 Melangoose8 St Stephen’s Stone, including

Cathedral quarry (H)9 Carn Grey (H)10 Luxulyan, including Tregarden,

Carbean and Colcerrow (H)11 Boslymon12 De Lank/Hantergantick13 Tor Down14 Carbilly Tor15 Cheesewring, including

Gold Diggings (H)16 Bearah17 Kilmar Tor18 Kit Hill (H)19 Hingston Down (H)ELVANS20 Rosevear21 Nancevallen22 Newham (H)23 Brannel24 Trelowth25 Pentewan/Polrudden Cove (H)26 Penrice/Castle Gotha (H)27 Tremore (H)28 Bodwannick29 St Columb Minor30 Cubert31 Budnick32 Deer Park33 St Kew (H)34 Escall's Green35 St Erth36 Treveddoe (H)37 Kingsand rhyolite flow, including

Watergate quarry (H)BASIC IGNEOUS38 Tintagel greenstone39 Cataclews (H)40 Duporth41 Tregongeeves42 Helston greenstone

43 St German's hyaloclastite,including Sawdey's Rock.

44 Polyphant45 Greystone (H)SLATES46 Callywith47 Westwood48 Lantoom49 Carnglaze slate caverns (H)50 Kestle (H)51 Bangor52 Trecarneffynes/Merrifield53 Delabole54 Trevillet55 Boscastle (H)56 Prince of Wales57 California/Welltown/Grower/

Trevalga58 Tintagel cliffs59 Poltesco60 Trenarren61 St Blazey Gate red slate62 East Polmear63 Tredinnick (north)SANDSTONES etc.64 Trewarthenick65 Tredinnick (south) (H)66 Treworgans67 Cansford (H)68 Pigsdon (H)69 Herdbury (H)70 Fistral sandrock71 Camel estuary sandrock

MISCELLANEOUS, LIMESTONES etc.72 Glebe calc-flintas73 Rosevannion calc-flintas74 Trenault limestone75 St Mewan Beacon76 Pilsamoor chert/slateLIZARD COMPLEX77 Gwendreath, Kennack gneiss (H)78 Serpentine workings including

Trevassack (H)79 Poltesco80 Dean/Porthoustock (H)

(H) indicates the site isdesignated a HeritageQuarry by Cornwall Council

0 10 20km

NCarnmenellis

BodminMoor

The Lizard Complex

Kit Hill

St Austell

La

nd’s End

Fig. 2. A map of the main building stone sources in Cornwall. Heritage quarries contain stone of limited distributionthat has made an important contribution to the built heritage within (or in some cases beyond) the County.

C. M. BRISTOW96



the policy of Cornwall Council to flank roadimprovements with Cornish hedges, often con-structed with Devonian slaty mudstone or granite.

The Guild of Cornish Hedgers publishes a Codeof Good Practice for Cornish hedging at www.cornishhedgers.com.

Geological history of Cornwall

The general geology of Cornwall has been coveredin two recent books (Selwood et al. 1998a;Bristow 2004). The latter is an introduction toCornwall’s geology and scenery suitable for thegeneral reader, whilst the former is a more detailedaccount with each chapter written by a specialist inthe field concerned. A valuable review of theigneous rocks of SW England has been publishedas part of the Geological Conservation Review(Floyd et al. 1993). The following is a brief sum-mary of the geology.

Cornwall lies in the northern part of the Variscanfold belt, and is mainly composed of folded andfaulted Devonian and Carboniferous sediments(Fig. 5) that were intruded by Late Carboniferous–Early Permian granites. There was also extensivebasic volcanicity in the Devonian and Early Carbon-iferous, in the form of lavas, ashes and beds ofagglomerate, as well as dolerite intrusions. All ofthese basic igneous rocks are known colloquiallyas ‘greenstones’, some are ultrabasic. Some green-stones have been converted by later circulatingfluids to talc-carbonate rock or serpentine. Work-ing from south to north the following geologicalunits can be recognized.

The Lizard ophiolite complex, obducted in theDevonian, forms the southernmost unit of Cornishgeology. It is mainly composed of basic and ultra-basic igneous intrusions, often serpentinized, andalso includes some Late Cambrian–Early Ordo-vician material. There is some high-temperaturemetamorphism.

The Gramscatho Basin lies in SW Cornwall, andcontains deep-water turbiditic muddy and sandysediments of Middle and Late Devonian age.There is much basic volcanicity in west Cornwall.Submarine slump breccias form an olistostrome onthe south side of the Gramscatho Basin and separatethe basin from the Lizard Complex; they containlarge blocks up to 1 km long of quartzite and lime-stone. The north side of the basin is formed by amajor structural feature known as the Start–Perran-porth Line (SPL), probably a dextral fault, whichextends from Perranporth–Holywell Bay on thenorth coast to Polrudden Cove, near Pentewan, onthe south coast.

North of the SPL, the early part of the Devonian(‘the Dartmouth Group’) is characterized by amixture of mud and sand deposition, with some vol-canicity, particularly in SE Cornwall (see fig. 25,pp. 42–43, in Bristow 2004). It consists of

Fig. 3. A typical traditional Cornish building – CoinageHall, Truro. The walls are of a silty slaty mudstone,probably from Poltesco Quarry beside the Truro river.The quoins, window tracery and plinth are of granite,probably obtained as moorstone from the CarnmenellisGranite area.

Fig. 4. Cornish hedges, Burlawn, Carclaze. The nearesthedge was built with slaty mudstone, with a core of earthand an earth capping, about 3 months before thisphotograph was taken. The hedge behind was built to thesame design about 20 years ago and is now covered withbramble-dominated vegetation.



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Fig. 5. A map of granite types (from Dangerfield & Hawkes 1981).

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alternating layers of purple and green sediments,and was probably laid down in shallow brackishbasins on a coastal plain. The rest of the LowerDevonian (‘the Meadfoot Group’) consists of marineslaty mudstones with much volcanicity, culminat-ing in the Staddon Grit Formation. The Middleand Upper Devonian are characterized by muddeposition, with occasional limestones and somebasic volcanics.

The early part of the succeeding CarboniferousPeriod (Dinantian) saw the earliest phase of the Var-iscan Orogeny affecting the south of Cornwall,whilst to the north of the SPL we find deep-watersediments, mainly of a muddy nature, with someradiolarian cherts and occasional limestones. Exten-sive basic volcanicity occurs in the Early Carbon-iferous. A belt of country extending from Tintagelin the west through Launceston to Tavistock is char-acterized by a stack of tectonic slices, often con-taining different successions in each slice, whichseems to represent a series of sheets of sedimentthat were shed from the advancing Variscan maindeformation zone to the south and, possibly, basininversion to the north.

The Rusey Fault defines the southern margin ofthe Culm Basin in north Cornwall, which containsUpper Carboniferous sandy and muddy sediments,mostly turbiditic, that represent the final infillingand closure of the basin. The final phase of the Var-iscan Orogeny then follows. The stresses createdduring the orogeny converted the original mudsinto slaty mudstones, occasionally, as in the Tinta-gel area, the regional metamorphism reached green-schist level and produced phyllites.

After the Variscan Orogeny, granites wereintruded between 300 and 270 Ma ago (Late Car-boniferous–Early Permian), forming a series oflarge granite intrusions. There were many differenttypes of granite, a classification by Dangerfield &Hawkes (1981) and Hawkes et al. (1987) is probablythe most helpful from the point of view of buildingstones (Fig. 6). Most of the granites are biotite gran-ites, and all of the granite masses have been quar-ried for building stone at some time. However, it isnotable that most of the successful dimension stonequarries were in Dangerfield & Hawkes’ (1981)‘coarse grained megacrystic biotite-granite – smal-ler megacryst variant’. More unusual types of gran-ite containing lithium (Li) micas are found in thewestern part of the St Austell Granite and at Tregon-ning Hill. These are associated with china clay depo-sits but, where these granites are not kaolinized, theycan yield a useful pale-coloured building stone.

A final pulse of igneous activity occurred as thegranites were cooling. Deep cracks opened up in thenewly solidified granite, and magma from the still-liquid reservoir at depth was injected up thesecracks to form ‘dykes’ of elvan. These dykes were

relatively narrow, so they cooled rapidly, onlyallowing small crystals to form. The resulting fine-grained felsitic elvan is often an ideal buildingmaterial, provided that it has not been softened bylater alteration, and it has been widely exploited asa source of building stone.

Heat from the cooling granites and from radio-active minerals contained within the granite (Dur-rance et al. 1982) caused metals to be mobilizedby hydrothermal fluids from the granite and thesurrounding rocks, to form veins containing oresof tin, copper and other metals. Mining generatedlarge quantities of waste vein material, graniteand killas, which could be exploited as a source ofbuilding stone known as ‘minestone’. The hydro-thermal fluids also extensively altered the granites,so that later lower temperature fluids could pene-trate the granite and kaolinize the feldspars.

Following the emplacement of the granites in thePermian, Cornwall lay in the centre of a large con-tinental mass nearly on the Equator with a hot aridclimate. For much of the rest of the Mesozoic, theCornubian Massif appears to have been an island,although there are no rocks of this age preservedonshore today in Cornwall. Deep chemical weather-ing profiles were developed.

During the Tertiary, the present shape of Corn-wall’s scenery began to be formed, partly as a resultof further deep weathering of the rocks under a sub-tropical climate, and partly as a result of fault move-ments. The flat-topped Cornish killas plateau seenthroughout the County is a product of long periodsof planation in the Mesozoic and Tertiary.

Cornwall was not glaciated during the Quatern-ary but lowered sea levels in the cold ‘glacial’periods caused valleys to be cut into the killasplateau by the torrential mud-laden floods in thespring thaws, and, near the coast, these valleyswere cut well below present sea level. At times the

Fig. 6. The medieval font in St Petroc’s parish Church,Padstow, carved from a Upper Devonian dolerite knownas Cataclews Stone, The sculptor’s name is unknown,so architectural historians refer to him as the ‘Master ofSt Endellion’.




climate was very cold and dry, so that dust pickedup from the dry floor of the Irish and Celtic seaswas blown over Cornwall and deposited as a layerof loess. Much of the best material used for mak-ing cob walls has a loessic derivation. During thecolder phases of the Quaternary, the loess, soil andweathered material froze to form a layer of perma-frost that became filled with ice to such an extentthat it moved like a muddy glacier downslope, caus-ing all of the material to become a jumbled mass ofsoft material with large stones in it, which is called‘head’. Particularly in the granite areas, these largestones became the ‘moorstones’ that were used asbuilding stone by medieval masons.

As the climate ameliorated after each coldperiod, sea level rose and the overdeepened valleyswere filled with sediments. The first sediment to bedeposited was usually a poorly sorted clayey gravel,which, in mineralized areas, contained significantquantities of cassiterite, exploited by tin streamersfrom the Bronze Age onwards. These gravels con-tained many large non-tin-bearing stones, whichwere cast aside by the tin streamers and often laterused for building.

At times during the interglacials, sea level washigher than present, which resulted in the formationof raised beaches, some of which are largely com-posed of bioclastic sands. In places, calcium carbon-ate was dissolved away near the surface by naturallyacid rain and then redeposited lower in the sandaccumulation, resulting in the sand becomingcemented to form ‘sandrock’, which was exploitedon a small scale for building.

As the coastal cliffs were eroded to their presentform comparatively recently, fresh unweatheredstone is usually available on the shoreline justabove sea level. This explains why the early slatequarries in south Cornwall (Polmear, Trenarrenand Poltesco) and greenstone quarries (e.g. Cata-clews) were all situated close to tidewater whereno overburden of weathered stone needed removal,and transport by water was possible.

Igneous rocks

Devonian–Lower Carboniferous basic and

ultrabasic rocks

Greenstone is a convenient colloquial term for avariety of basic igneous rocks, although quarryworkers use the term ‘blue elvan’ for any darkgreenish-blue basic igneous rock, particularly inwest Cornwall. All pre-Variscan basic igneousrocks in Cornwall have been subjected to low-graderegional metamorphism, which has resulted in thedevelopment of hydrous phyllosilicates such asserpentine, chlorite and illite, together with minerals

such as prehnite, epidote and pumpellyite (Robin-son 1998, pp. 114–119). Spilitization is also wide-spread, and olivines are normally altered. Despitethese mineralogical changes, most basic rocksstand up to weathering well. Metabasites affectedby contact metamorphism around the granites canbe extremely tough and durable. A separate phaseof hydrothermal alteration, carbonatization, hasbeen responsible for forming some interesting talc-containing decorative stones but these are soft andunsuitable for exterior use.

Basic intrusions. These have been extensively quar-ried for building stone and aggregate. One of thebest known is Cataclews Stone, which was workedfrom medieval times in a quarry at CataclewsPoint (SW 873/761). This dark green rock intrudedinto Upper Devonian slates was described as ahydrous alkali dolerite by Selwood et al. (1998b,p. 33). The grain size is variable; the medievalmasons seem to have preferred the finer-grainedvariety. It has been used in a number of churches,such as St Merryn, St Endellion and St Petroc,Padstow. Many of the medieval carvings in thisarea (Fig. 6) appear to be by the same hand, nameunknown, so architectural historians refer to himas ‘The Master of St Endellion’. Where four-teenth–fifteenth century carvings in this stone havebeen exposed to the weather in external work, thisstone seems to have survived well.

A series of dolerite intrusions in the PentewanValley were quarried (e.g. Tregongeeves, SX 000/515) mainly for aggregate but some buildings suchas the Masonic Hall and the former Public Roomsin Truro Road, St Austell have used this material.A slightly weathered dark green basic rock form-ing one of the arches in the Chancel of HolyTrinity St Austell is also probably from one ofthese dolerites. Dark green igneous rocks tend togive a building a rather gloomy appearance, whichmay partly explain why lighter-coloured granite isgenerally preferred. This stone also has the repu-tation of being difficult to work, although the localsculptor Karl Williams has managed to producework using slightly weathered quartz dolerite fromnear Black Head.

Flett & Hill (1912, p. 251) reported that a ‘felds-pathic greenstone of doleritic texture’ has beenextensively used in Helston for building, andexamples of this stone can still be seen in thetown. It looks like an altered basic rock with abun-dant phenocrysts of feldspar. Although a rathersoft stone, it was said to be easy to work. The largequarries from which this stone was obtained werereported to be on the SE side of the town, andhave now been entirely built over.

There are many other large quarries in basicintrusions, such as Penlee Quarry near Newlyn and

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Stepper Point Quarry at the mouth of the Camelestuary, plus many smaller quarries, but these haveoverwhelmingly produced aggregate and onlyminor quantities of stone for rough walling.

Carbonatized basic and ultrabasic intrusions. Acarbonatized ultrabasic intrusion has been workedin two quarries at Polyphant, near Launceston(SX 256/826 and 260/826), since Norman times.Originally an ultrabasic rock the ferromagnesianminerals were carbonatized by hydrous carbondioxide containing fluids to a mixture of talc, chlor-ite and dolomite, but the alteration is very localized(Power & Scott 1995). The resulting rock is quitesoft but is a superb medium for carving and can bepolished to a handsome dark green shiny surface.Many churches in Cornwall have interior featuresmade of Polyphant Stone. The Boer War Memorial,part of which is shown in Figure 7, adjacent tothe West Door of Truro Cathedral, is a fine exam-ple (Cartwright 1997). Polyphant Stone has beenextensively exported out of Cornwall, particularlyfor use in churches; for example, the tomb ofArchbishop Temple in Canterbury Cathedral and asculpture in Christchurch Cathedral, New Zealand.Recently, the quarry at Polyphant was reopened

and sufficient stone for restoration work at New-quay Parish Church and for sculpting obtained,demonstrating that small-scale reopening of quar-ries to supply stone for restoration work andspecial projects is a realistic proposition.

Polyphant Stone does not weather well in exte-rior use because it is too soft, as can be seen at Laun-ceston Castle and Priory. Care must also be takento exclude any stone that has been slightly weath-ered, even imperceptibly. Dewey (in Reid et al.1911, p. 128) reported that spheroidally weatheredmasses of Polyphant Stone were used to make afont at Launceston Roman Catholic Church; recentreports say that this font is showing signs of dis-integration. St Paul’s Church in Truro appears tohave been built with a form of Polyphant Stonethat has not weathered well, although it is not clearwhether this stone was a less altered stone fromthe main Polyphant Stone Quarry or from one ofthe two dolerite quarries at Polyphant, neither ofwhich shows evidence of significant carbonati-zation. A small area of carbonatized dolerite wasalso seen in the western part of the active GreystoneQuarry (Power & Scott 1995) near Launceston (SX360/804) but has now been quarried away.

Another carbonatized basic igneous rock thathas been used is Duporth Stone (Power & Scott1995). The stone occurs in the cliffs of DuporthBay, near St Austell (SX 036/513), and forms a sill-like feature in the Lower Devonian Meadfoot Groupslates. It appears to have originally been intrudedas a dolerite, and has subsequently been carbona-tized to a mixture of talc, chlorite and various car-bonates, as at Polyphant. Duporth Stone is palegreyish-green and often has a coarse speckled tex-ture. It has been extensively used in the pillars ofTruro Cathedral (Worth 1888; see also WaringtonSmith 1886, pp. 25–26), and has also been used inthe Rood wall at St Paul’s, Charlestown.

Volcaniclastic rocks. Lower Carboniferous basicvolcaniclastic rocks of the Tintagel Volcanic For-mation (Selwood et al. 1998b) were some of the ear-liest building stones used in Cornwall, and can beseen in the Tintagel and Launceston areas in medie-val structures such as Tintagel castle where it isknown as Tintagel Greenstone. Whether theserocks originated as tuffs, agglomerates or someform of submarine pyroclastic flow such as a hyalo-clastite is not clear because they occur in the Tinta-gel high-strain zone, and are metamorphosed tolower-greenschist facies, with a strong structuralfabric. They weather well and were presumablyobtained from coastal exposures, such as TrebarwithStrand, where there is some evidence of exploitation(SX 049/865). Just over the Devon border, in theTavistock area, the similar Lower Carboniferous

Fig. 7. The Cornish shield, forming part of the Boer WarMemorial in Truro Cathedral, carved from PolyphantStone.




Hurdwick Stone was extensively used from medie-val times onwards.

The great Norman Church at St German’s –Cornwall’s first cathedral – is largely built of abasaltic hyaloclastite, presumably obtained from anearby quarry in the St German’s tuff member ofthe Middle Devonian Saltash Formation (Leveridgeet al. 2002, p. 48 & 87). The texture of this interest-ing rock is well seen in the Lych Gate and theNorman pillars inside the church. Hyaloclastitewas also quarried at Sawdey’s Rock (SX 364/609)but whether it was used as a building stone isunknown.

The Lizard Complex. The Lizard Complex yields awide range of rock types, many of which havebeen used for building purposes. The gabbros anddolerite dykes of the Crousa Downs Unit havebeen extensively quarried for aggregate productionin Dean and Porthoustock quarries, and exporteddirectly via quays adjacent to the quarries, buthave not seen much use in building, apart for roughwalls and farm buildings, probably because themuch easier to work serpentine is available nearby.

Serpentine is the main building stone in theLizard and has been exploited in many old quarriesscattered over a wide area, each yielding a stonewith particular characteristics of, for example,colour and texture. It has also been used as a decora-tive stone for turning to make ornaments or forpolished slabs. Serpentine originated from perido-tite formed at depth (Bristow 2004, pp. 70–80),and the original peridotite was a mixture of oliv-ine, orthopyroxene and spinel. As it was broughtcloser to the surface and into contact with hydro-thermal fluids, various mineral changes occurred,culminating in the serpentine seen at the surfacetoday. Serpentine group minerals are hydrated mag-nesium silicates. It is possible that this hydrationprocess is still happening slowly.

There are three principal varieties of serpentine(see the map opposite p. 66 in Flett & Hill 1912).Most of the serpentine used for turning is enstatite-or bastite-serpentine, and was originally a lherzolitecomposed of a fine-grained matrix of olivine withlarge crystals of bastite (enstatite pyroxene) scat-tered through it. This has been variably alteredto a dark green or reddish serpentinous mass butmost specimens contain some fresh olivine. Thetremolite-serpentine has been quarried mainly foraggregate use. A third variety of serpentine isdunite-serpentine, originally largely composed ofolivine, which occurs along the northern margin ofthe Lizard Complex. Some small quarries appearto have exploited this stone for building.

Extraction of blocks of serpentine for buildingdates back to medieval times, as the Churches atGrade, Landewednack and Ruan Minor testify.

Many of the large blocks of serpentine used inthese ‘Listed Churches’ may have come from moor-stone-type blocks lying on the surface of CrousaDowns. The alternation between pale-colouredgranite from Carnmenellis and the dark-colouredserpentine makes these buildings visually attractive.However, it is widely believed in the district thatwalls built of serpentine are rarely waterproof(Flett & Hill 1912, p. 251).

Ornament manufacture in workshops in LizardTown has been going on since the early nineteenthcentury and continues up to the present. Sagar-Fenton (2005) provided, in his book on serpentine,a useful description of the history and currentmethods of working; he quotes the following listof quarries that have provided serpentine suitablefor turning:

† Signal Staff Quarry, Cadgwith, famous for its redstriped stone and Flagstaff Point, nearby;

† Long Alley Quarry, opened in 1854;† Kellawyn and Poltesco Quarries, close to the

Carleon Cove work;† Holestrow Quarry, near Kynance Cove;† Balk Quarry, north of Church Cove;† Gwendreath Quarry, north of Kennack Sands;† Ruan Major, near the ruined church;† Treal Quarry, by Ruan Minor.

Of these, Balk Quarry (SW 713/130), which nowbelongs to the National Trust, is the oldest andprobably most significant. Flett & Hill (1912,pp. 254–257) provided a helpful account of the ser-pentine industry in the first decade of the twentiethcentury, with many of the small quarries identified,together with their characteristics when polished.At Poltesco (SW 722/158), a serpentine factoryexisted in the late nineteenth century that was capa-ble of producing, for example, polished serpentineslabs, panels, columns and mantelpieces; there wasalso a serpentine works in Penzance that exportedproducts all over the country. Shop fronts and theBank of England used the facing slabs but, after awhile, they acquired a reputation for cracking thatcaused a decline in the industry, so that few, ifany, of these polished slabs survive in externalsituations.

Late Carboniferous–Early Permian granites

Granite is the most important building stone inCornwall, both in terms of its use in Cornish build-ings and its use in buildings outside Cornwall. It isa very strong stone, especially when really fresh andunweathered but is not an easy stone to carve intointricate shapes. Because of its strength, graniteis the stone of choice for buildings in the mostexposed situations, such as lighthouses where inter-locking blocks of granite are used, so that the loss ofstrength due to mortared joints is reduced. Granite

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is capable of being used to form ashlar masonry, butis not a true freestone, although it is frequently usedin older buildings for quoins, window tracery andplinths. Granite can also be polished to yield slabsfor facing buildings and kitchen work surfaces.

The granite batholith. Geophysical studies suggestthe granite masses merge at a depth of 10–15 kmto form a continuous large batholith. However,radiometric dating shows that the granites wereintruded as a series of separate intrusive eventsover a 30 Ma period from 300 to 270 Ma, straddlingthe Carboniferous–Permian boundary (Bristow2004). Even single granite plutons, such as StAustell, were intruded over a 15 Ma period from285 to 270 Ma. Consequently, there are many dif-ferent types of granite of varying suitability forbuilding purposes. The map in figure 5 from Dan-gerfield & Hawkes (1981) is one of the best guidesto the different types, although this does not takeinto account the major variations in chemistry.

Composition of granites. The commonest type ofgranite is a coarsely crystalline biotite granite, com-posed of quartz, feldspar and biotite mica withvarious accessory minerals.

There are usually two types of feldspar: apotassium-rich feldspar, normally orthoclase(KAlSi3O8); and a sodium-rich feldspar which isusually albite or oligoclase (NaAlSi3O8). The ortho-clase is often microperthitic, with alternatinglamellae of soda- and potash-feldspar. Usually, theorthoclase is predominant and forms the continuousphase with needles of soda-feldspar set in it; thereverse is a microantiperthite (Bristow & Exley1994).

The commonest type of mica is a dark-colourediron-rich biotite mica (K(Mg,Fe)3(AlSi3O10)(OH)2)often accompanied by the colourless mica musco-vite (KAl2(AlSi3O10)(OH)2). Some of the later gran-ites, notably in the western part of the St Austellmass, contain lithium-rich micas such as zinnwal-dite (KLiFe3+Al(Al,Si3)O10(F,OH)2), which is usu-ally pale brown. A greenish mica, often referredto by the field term ‘gilbertite’, occurs in greisenedareas and is similar in composition to muscovitebut contains more hydroxyls in the lattice. Fine-grained mica, usually with a similar compositionto muscovite, is usually referred to as illite or‘clay mica’. Many of the micas are intermediatebetween the principal types mentioned above.Deferruginization of biotite is common in alteredor weathered granites, the iron released can thenstain the granite, yellow, brown, pink or red. Othercommon accessory minerals that occur in thegranite include the black boron-containing min-eral tourmaline, the phosphate-containing mineralapatite and the fluorine-containing mineral topaz.

Where killas country rock has been absorbed intothe granite, andalusite and cordierite (usuallypartly or wholly altered to pinite) may be found.

Many granites contain significant amounts ofU and Th in minerals such as uraninite and mona-zite, with levels in the St Austell Granite of 13.5–24.2 ppm U and 14–21 ppm Th (Allman-Ward1985). The release of radiogenic heat has stronglyinfluenced the cooling history and hydrothermalactivity (Sams & Thomas-Betts 1988). Also, radi-ation levels from some granites are significant andarchitects need to be aware of this factor, especiallyin buildings where radiation associated with radonis known to be a problem.

Granites can be coarse grained or fine grained.A fine-grained granite can be termed a microgra-nite. Sometimes the long axes of the minerals inthe granite show a tendency to be oriented in onedirection, so the granite appears to have a kind of‘grain’; this is called ‘foliation’, an example of thisis the granite currently being worked in De LankQuarry. This probably indicates that the granite wasunder stress as it crystallized. Sometimes the gran-ite contains feldspar crystals that are larger thanthe groundmass feldspars, normally K-feldspar;these are known as phenocrysts or megacrysts, andthe granite containing these extra large crystals isknown as a porphyritic or megacrystic granite. Insome cases, these large crystals will have formedin the magma before it flowed into its final placeor sometimes they will have grown in the graniteat the same time, or shortly after, it crystallized.When the long axes of these phenocrysts are orien-tated parallel to one another, it can suggest theywere being carried along in the granite magma asit flowed into place. Phenocrysts of feldspar oftenshow twinning and growth rings of dark inclusions,formed as the crystal grew. Polished granite surfaceswill often show this latter feature beautifully.

A great variety of classification systems havebeen proposed for the granite types of the Cornubianbatholith, some based on texture and some basedon chemistry. As mentioned earlier, one of the sim-plest is that proposed by Dangerfield & Hawkes(1981) (Fig. 5). It is noteworthy that most of thesuccessful granite quarries lie in one type ofgranite – the ‘coarse grained megacrystic biotite-granite – smaller megacryst variant’. In the tradeliterature, this is usually referred to as ‘silver-greygranite’ (Anon 1947, p. 55). However, in areassuch as the western part of the St Austell Granite,total confusion reigns with different researchersgiving different names to the same type of granite,and the same name to demonstrably differentkinds of granite. Contacts between different granitesare seen in nearly every china clay pit, so it is clearthat the western part of the St Austell Granite wasformed by repeated episodes of granite intrusion.




Fluids evolving from the still-crystallizinggranite often form veins of very coarsely crystallinematerial, known as pegmatite. Crystals in pegma-tites can be very large, up to 0.5 m in some cases.Alternatively, these late fluids can form materialthat is finer grained than the granite, this is thenknown as aplite. Many dimension stone quarries inCornwall contained veins of aplite. Aplites andelvans can superficially appear similar but theelvans (see later) are a later separate kind of mag-matic intrusion. When there are large feldspar crys-tals set in a fine-grained granitic matrix, someresearchers will call it a ‘porphyry’ but this termshould be used with care as some of the elvanshave also been described as porphyries.

The metamorphic aureole. The heat of the graniteintrusion caused thermal metamorphism of thekillas for a distance of 1 km or more around thegranite to form what is known as ‘the metamorphicaureole’, containing high-temperature minerals,such as cordierite (frequently altered to a mixtureof white mica and chlorite called pinite) and anda-lusite. The result of this thermal metamorphismwas to convert the killas into hornfels, which hassometimes been used as a building stone, notablyin the Penwith area. Boron-containing fluids ema-nating from the granite sometimes cause the rocksof the metamorphic aureole to become impregnatedwith the boron-containing mineral tourmaline – theso-called ‘tourmaline schist’ of the older writers.Rocks containing a mixture of silica and calciumcarbonate, such as calcareous sandstones or chertylimestones, reacted strongly to thermal metamorph-ism, sometimes at a distance of many kilometresfrom the nearest granite, to produce a rock com-posed of calcium silicate minerals, known to theolder writers as calc-flintas, which has only rarelybeen used as a building stone. Occasionally, espe-cially in the Goss Moor area, kaolinization willaffect the rocks of the metamorphic aureole(Bristow & Scott 1998).

Granite emplacement and xenoliths. As the hotgranite flowed into place, thin veins of moltengranite were injected along lines of weakness,such as joints and faults, into the killas. If theseveins surrounded a block of killas, so that itbecame detached, it could form a xenolith. Thegranite at Lamorna showed some fine examples ofxenoliths, although the quarrymen thought thesewere disadvantageous to the appearance of thegranite. Sometimes an area in the granite willcontain an abnormally high content of dark mineralssuch as biotite, perhaps with some metamorphicminerals such as cordierite; this will indicate anarea where a xenolith has been entirely melted andabsorbed into the molten granite.

Vein formation, mineralization and alteration. Gra-nites are subject to many forms of alteration. Theearliest forms of alteration took place at or soonafter crystallization. Pressurized fluids (Allman-Ward 1985; Bromley 1989; Bristow 2006a) hydrau-lically fractured the granite to form parallel cracksperpendicular to the direction of least stress. Hotsilica-laden hydrous fluids separating out from thecrystallizing granite circulated along these parallelcracks to form sheeted vein systems. If the hotfluid was rich in silica (SiO2), it altered the granitealongside the crack by converting the feldspar toa mixture of quartz and mica – a process knowas greisening – and if there was much boron andiron, then tourmaline would have been produced –a process known as tourmalinization. Vein for-mation will often cause the granite in the vicinityto be unsound for building, so granite quarrymentended to avoid these areas. However, many of theelvans that were affected by greisening showimproved resistance to weathering (see later).

In a few cases boron-rich material could pro-duce a special type of granite, such as luxullianite,composed of black tourmaline and pink feldspar(Wells 1946; Hatch et al. 1949, p. 469), which washighly valued by the Victorians, including QueenVictoria herself, as an ornamental stone (Fig. 8).Schorl rock is composed of tourmaline and quartzand is a stable material, although difficult to work;much schorl rock and greisened granite occurs inthe large boulders left over as waste from chinaclay working (‘stent’), and in the larger stones castaside from tin streaming. They are therefore fre-quent components of minestone, especially in theSt Austell china clay area. Roche Rock is a largetor-like feature entirely composed of schorl rock(Fig. 9) and St Michael’s Chapel, on top of therock, is entirely built of schorl rock.

However, the most important process to affectthe granites was kaolinization. This is the processthat formed china clay by converting the feldsparin the granite to kaolinite, the main mineral form-ing commercial china clay. The processes that ledto the formation of the world-class deposits ofchina clay in Cornwall and Devon are described inBristow & Exley (1994) and Bristow (2006a). Kao-linization destroys the value of granite as a build-ing stone. However, very slight kaolinization canmake the granite easier to work; St Stephen’sStone would be an example of this, although thismay be at the expense of long-term resistance toweathering. Sometimes an apparently sound gran-ite will crumble and disintegrate after only a fewseasons of weathering, this is due to earlier geologi-cal processes having made the granite permeableto water, so that frosts can break it up. Hard graniteboulders, removed as waste from china clay pits,often show this tendency and this is why apparently

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sound granite from china clay pits is rarely suitablefor building.

Weathering of the granite. Deep chemical weather-ing in the Mesozoic and Early Tertiary has alreadybeen mentioned but a less intense form of weather-ing has continued up to the present day. Most of thegranite near the surface has been affected by this

kind of weathering, which causes the feldspars tobecome cloudy due to incipient kaolinization and,more noticeably, the biotite begins to release itsiron (deferruginization), which disperses throughthe granite, so the originally silver-grey granitebecomes buff or some other rusty shade due to stain-ing by iron oxide. Several writers report that thisslightly weathered buff granite is preferred for

Fig. 8. Luxullianite from Tregarden Quarry, Luxulyan. The specimen is about 10 cm across, and consists of pinkorthoclase and black tourmaline, with some quartz (grey).

Fig. 9. Roche Rock in mid-Cornwall is composed of schorl rock (quartz + tourmaline). The fifteenth centurySt Michael’s Chapel on top of this tor-like feature is also entirely built of schorl rock.




building, notably for rubble walling, partly for aes-thetic reasons and partly because it is slightlyeasier to work. Most of the moorstone obtained asloose surface blocks on the granite moors will bein this slightly weathered category and, hence,pale buff in colour. Any fresh silver-grey graniteintroduced during restoration of old buildings com-posed of moorstone can therefore look incongruous,in spite of both types being petrologically the same.The granite paving from Bodmin Moor quarriesrecently installed in St Austell town centre showsthe difference between the silver-grey and the buffgranite clearly. The shallower faces in quarrieswill also show this kind of slightly weatheredstone. Most dimension stone quarries that exportedstone for major buildings and structures, wherestrength was important, are deep in order to accessthe freshest silver-grey granite at depth. The tipsof large boulders alongside many old dimensionstone quarries are mostly composed of slightlyweathered granite that was rejected by thequarrymen.

Joints. Joints run through the granite, breaking it upinto approximately rhombohedral blocks. They arean important factor affecting quarrying for buildingstone. There are three main sets of joints in most ofthe granite plutons:

† ‘master’ joints in a NNW–SSE or north–southdirection, approximately parallel to the cross-course mineralization and faulting;

† WSW–ENE or west–east joints, approximatelyparallel to the axis of the batholith and the main-stage mineral veins and elvans;

† subhorizontal joints, increasing in frequencytowards the surface.

Jointing in other directions is sporadically devel-oped and sometimes a conjugate pair of joints mayreplace the main joints listed above, particularlythose in the WSW–ENE direction. The NNW–SSE joints are usually close to vertical; the WSW–ENE joints are steeply dipping but may show morevariation in dip. Thin coatings of minerals such asquartz, tourmaline and fluorite on many joint sur-faces indicate that many of the joints were formedwhile hot fluids containing silicon, boron and fluor-ine were still in circulation. However, it is impor-tant not to confuse true joints with the earlierformed greisen-bordered sheeted vein systems,which developed under quite different condi-tions. Sometimes slickensides or displacement offeatures within the granite will indicate that therehas been a small amount of horizontal fault displace-ment, usually dextral, along the NNW–SSE joints.By an increase in the movement, the joint canbecome a fault. Dextral NNW–SSE strike-slipfaults are an important feature of Cornish geology.

The more significant faults are usually associatedwith much shattering of the rock adjacent to thefault, and the fault itself may be occupied by amixture of ground-up rock and clay, known to theold miners as ‘fluccan’.

Sometimes the subhorizontal joints are parallelto the long axes of the crystals within the granite,which is taken to indicate the direction of flow atthe time of emplacement. Sheffield Quarry nearPenzance (SW 454/268) shows this kind of fea-ture (Reid & Flett 1907, p. 41). The spacing betweenthe ‘flooring’ joints often appears to become closertowards the surface, suggesting that the release ofvertical load as the overlying granite was removedby erosion has induced their formation. This ideais supported by the fact that these kind of jointsare often orientated parallel to the land surface andappear to follow its undulations.

The largest single block of granite ever brokenout from its bed was in Polkanuggo Quarry, nearLongdowns, and was estimated to weigh over2700 tonnes. However, although most blocks weremuch smaller, they nevertheless needed to be cutup before they could be moved from their bed andout of the quarry. In breaking up these large blocksof granite, the quarrymen would use various termsto describe the direction of break. Thus, theNNW–SSE direction is known to quarry workersas the ‘cleaving way’, the WSW–ENE direction isknown as the ‘tough way’ and the horizontal direc-tion was referred to as the ‘bedding’ or ‘flooring’.Much additional information about the graniteindustry and the operation of quarries will befound in Stanier (1999).

Granite occurrences. Granite quarrying in theLand’s End Granite was mainly in the quarriesabove Lamorna Cove, which worked a coarse-grained porphyritic granite with randomly orien-tated large orthoclase phenocrysts, which made thegranite unusually tough with no pronounced direc-tion of splitting. The granite was shipped from asmall pier in the cove; uses include the Wolf RockLighthouse and the Embankment, London, as wellas in the Royal Geological Society of Cornwallbuilding in Penzance, which forms part of StJohn’s Hall. Granite was also worked in the Shef-field and Castallack quarries. The Castle-an-DinasQuarry, in the centre of the granite pluton, exploitsa finer-grained granite, worked nowadays mainlyfor aggregate, although small amounts of buffslightly weathered granite for rubble walling arealso produced.

The small Tregonning–Godolphin granite masscontains a variety of granite types, including somethat are Li-rich. It is variably kaolinized but hasyielded a granite building stone similar to the StStephens Stone of the St Austell pluton. Worth

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(1875) noted that many of the older churches in thePenwith area used Tregonning Granite.

The most important granite-producing area inCornwall was the SE part of the CarnmenellisGranite, notably in the parishes of Mabe and Con-stantine (Fig. 10). The granite includes coarse- andmedium-grained varieties, usually with abundantmegacrysts; occasionally it shows foliation. Theouter part of the granite mass yielded a finer, morebiotite-rich kind of granite, whereas the innergranite was more porphyritic, coarser grained andpaler in colour (Hill & MacAlister 1906, pp. 52–64; Leveridge et al. 1990). The northern part ofthe granite, where there is significant metalliferousmineralization, produced relatively little dimensionstone. Proximity to loading wharves in the Falestuary led to a substantial dimension stone industryin the nineteenth century (Stanier 1999), and manyimportant buildings in London and all over theworld have used granite from Carnmenellis (seeStanier 1999). There are still several active quarriesthat now only produce crushed aggregate for generalconstructional work and rubblestone. CarnsewQuarry, near Mabe, provided the stone for theexterior of Truro Cathedral. Paler granites from

the SE part of the granite have been used as a kindof china stone.

In the St Austell Granite, coarse-grained bio-tite granite with large megacrysts of orthoclasewas formerly extensively quarried in the easternpart of the granite, around Luxulyan, in the Tregar-den, Carbean and Colcerrow quarries, and manyfamous buildings and engineering structures suchas the old London Bridge (now in Arizona) andPlymouth Breakwater were constructed from it inthe 1830s and 1840s. This is an unusually coarse-grained granite, with a distinctive ‘black and white’appearance. The Treffry Viaduct in the LuxulyanValley is said to have been entirely constructedfrom moorstone, before the main quarries at Car-bean and Colcerrow were opened up. The exteriorof Porphyry Hall and Tower at Place, Fowey arealso built of Luxulyan Granite.

A striking variant of this granite is ‘luxullianite’,which is made up of black tourmaline, pink ortho-clase feldspar and quartz (Wells 1946; Hatch et al.1949, p. 210). Recently obtained samples of luxul-lianite from Tregarden aggregate quarry alsocontain masses of pyrite. Luxullianite can be cutand polished, and many of the slabs used to line

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KeyGraniteMargin of inner graniteElvan dykeGranite quarryElvan quarry

GRANITE QUARRIES1 Carn Brea (same location as the

elvan quarry)2 Carn Marth (N)3 Carn Marth (S)4 Trannack5 Polkanuggo6 Lower Trolvis7 Higher Trolvis8 Rosemanowas9 Chywoon (H)10 Carnsew (H)11 Trenoweth12 Kessel Downs (H)13 Lower Spargo14 Higher Spargo15 Trevone16 Pelastine17 Goodagrane18 Maen19 Bosahan20 CarwynnenELVAN QUARRIES21 Brea22 Carn Brea23 Carn Brea (E)24 Carthew25 Polmarth26 Treservern Croft27 Praze28 Pencoose29 Bolenowe Crofts30 Gregwartha

(H) indicates the site is designateda Heritage Quarry byCornwall Council

N

Fig. 10. Carnmenellis Granite – the more significant granite and elvan quarries.




the interior walls of Porphyry Hall, Place, Fowey areof polished luxullianite. The slabs were cut andpolished at ‘The Porphyry Works’, at Fowey Con-sols Mine, where water power to drive the equip-ment was available. The Duke of Wellington’ssarcophagus in the crypt of St Paul’s Cathedral hasbeen carved from a single block of luxullianite,obtained from Trevanney Farm, and cut andpolished at the Porphyry works.

Further west in the St Austell Granite, there are aseries of granite quarries around the tor at CarnGrey, which yielded a granite intermediate in char-acter between the biotite granites of the eastern partof the St Austell pluton and the more exotic granitesof the western part of the pluton (Floyd et al. 1993).Carn Grey is the main building stone used in theolder buildings of St Austell such as the listedMarket House, built in 1844.

The Li-mica granites of the Nanpean and Hens-barrow areas, in the western part of the St AustellGranite, have been extensively exploited as chinastones for ceramic use, and can also be useful build-ing stones. Architectural historians refer to this typeof granite as ‘St Stephen’s Stone’. It is the palest-coloured granite in Cornwall, with an exceptionallylow content of dark minerals; some variants arealmost white. It is non-porphyritic, comparativelyfine grained and slightly softer than most other gran-ites due to slight kaolinization, which makes iteasier to work. St Stephen’s, Probus and St ColumbMajor church towers are made of this granite, andthe interior of Truro Cathedral is made lighter bythe use of this type of granite obtained from Cathe-dral Quarry, Nanpean. Some of the older houses inLemon Street also use this type of granite. St Paul’sChurch at Charlestown used a similar Li-mica

granite from a quarry between Hensbarrow Bea-con and Stenalees, which has the distinguishingfeature of containing veins and vughs of turquoise.

Granite is still worked in the Bodmin MoorGranite at the De Lank Quarry near St Breward onthe west side of the moor. This is a hard non-porphyritic medium-grained biotite granite, oftenwith a slight foliation, which was used in manyfamous lighthouses, such as Eddystone, BishopRock and Beachy Head, and in bridges such asTower and Blackfriars in London. It is the mainsource of dimension stone from Cornwall at pres-ent; recent contracts supplied granite for the Prin-cess Diana Memorial in Hyde Park and paving forthe courtyard at Burlington House, Piccadilly (Rob-inson 2000). On the SE side of Bodmin Moor, alarge quarry below the Cheesewring, was workedfor high-quality granite that was taken for ship-ment at Looe via the Caradon railway. CheesewringGranite and De Lank Granite were used in TowerBridge. There are also many other smaller quarriesin the Bodmin Moor Granite.

There were a series of important granite quarriesin the small Kit Hill granite mass that yielded asilver-grey granite similar to the granites fromBodmin Moor and Carnmenellis. Many importantstructures all over the world used this granite (seethe appendices in Stanier 1999 and the supplemen-tary publication).

Early Permian felsitic elvans and extrusives

Elvans are some of the most interesting and diversebuilding stones in Cornwall. A typical elvan is apale-coloured, fine-grained minor intrusion, usuallyin the form of a dyke (Figs 11 & 13), with a

Fig. 11. The pale-coloured elvan in the cliff at Polrudden Cove. This is the famous ‘Pentewan Stone’ quarried frommedieval times onwards. The elvan is 2–5 m thick.

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composition similar to a biotite granite. Elvans arevariously referred to in the geological literature asporphyries, felsites, rhyolites or microgranites.The term ‘elvan’ is widely used in the quarryingindustry by architectural historians and in geolo-gical literature, but care must be taken becausemany quarrymen use the term ‘blue elvan’, whichusually refers to a basic igneous rock of somekind. As the granite masses are of different ages,so the elvans associated with each are also of differ-ent ages. Elvans have been used in a great varietyof buildings, ranging from stately homes andchurches to humble vernacular buildings. Some arecapable of being carved as freestones (Fig. 12) andsome have been polished to form spectacular slabs.

In a study of the durability of various structuresalong the south coast of Devon and Cornwall, thePentewan elvan came out as one of the most stablebuilding stones (Mottershead 2000). Unlike gran-ites, elvans have not been exported from Cornwallto any great extent and, apart from a few occur-rences in SW Devon, have not been exploitedoutside Cornwall as building stones. In spite oftheir extensive use in older buildings, includingmany that are listed, there is no active elvan quarryin Cornwall at present, which means that salvaged

stone from derelict buildings is the only stone avail-able for restoration work. Elvans are variable inappearance, and in order to match existing stone ina building it is desirable to know which elvan hasbeen used and even from what part of the intrusionthe stone has been obtained (Fig. 13).

In the descriptions of elvans in the older BritishGeological Survey Memoirs there is less infor-mation on the end uses for elvans compared to thegranites, and it is clear that the ability to raiselarge intact blocks of stone was considered impor-tant in the building stone context. Some slightlyaltered/weathered elvans seem to have been pre-ferred to granites for nineteenth century road mak-ing before roads were sealed with tarmacadam.Elvans with closely spaced joints were preferredfor aggregate production and for road making.

Genesis. Elvans appear to have been intrudedseveral million years after the pluton with whichthey are associated. The chemical composition ofthe elvans indicates that they were derived from abiotite granite magma at depth. Computer model-ling indicates that the cooling of the granite wouldhave been prolonged by radiogenic heat releasedfrom U and Th in the granite, so a mass of subsolidusgranite could have existed at depth many millionsof years after the upper part of the granite hadcooled and crystallized (Sams & Thomas-Betts1988). Steeply dipping fractures formed in theextensional regime associated with the mainstagemineralization, allowing the magma to rush uptowards the surface to form dykes. Some may bestraightforward magmatic differentiates, othersmay be derived from a fluidized system (Hawkeset al. 1975). Sperulitic devitrification textures inthe margins of some elvans indicate rapid initialchilling to form glass at the margins in some cases.

Form. There is a generalized spatial relationshipbetween the areas where elvans occur and the mainareas of metalliferous mineralization. Some elvanscut across the veins of the mainstage mineralization,whereas others are cut by the mineralization. As thegranite masses are of different ages, so the elvansassociated with each are also of different ages.

Elvans run for a considerable distance beyondthe granites and their metamorphic aureoles. Anelvan at the south end of Watergate Bay is some10 km from the nearest granite, and an elvan25 km long reaches the Camel estuary north ofRock, at which point it is some 15 km from thenearest part of the Bodmin Moor Granite. In themineralized areas, the elvans generally run parallelto the lodes. However, in the area south and westof the St Austell Granite, the SPL seems to haveexerted a strong influence on the orientation of theelvans, as a near-continuous elvan extends from

Fig. 12. A bust of Asklepios, the Greek God of medicine,recently carved from Pentewan Stone by Karl Williams.The dark flecks in the stone are small inclusions ofslaty mudstone wall rock. This bust is now in thereception area of the Peninsula Medical School, Truro.




the northern end of Perran Sands to near Mitchell,and then, after a short break, continues from nearSt Stephen to the area near Sticker and Polgooth,where it splits into two before reaching the southcoast. Also, a north–south elvan extends southfrom Watergate Bay and is one of the few examplesof an elvan following this trend, possibly along theline of a cross-course fault. As might be expected,elvans swell and pinch in a manner difficult topredict (Fig. 13), so any proposed extension to aquarry needs the presence of the elvan to be verifiedby drilling before development starts. Just as withthe granites, the quality of most elvans generallytends to improve with depth. In a few cases,elvans may have been worked underground for

building stone, either on their own or as a by-product of metalliferous mining.

Most elvans (Fig. 13) show a clear zonationacross their width. At the margin, flow banding isusually seen that can make the stone fissile anduseless as a building stone. This marginal chilledzone may have originally been a glass, now devitri-fied. Some elvans are so massive that the centre ismore coarsely crystalline and is best described as amicrogranite (e.g. the Wheal Budnick elvan nearPerranporth). The distinction between a true elvanand an apophysis of a granite pluton may be diffi-cult to draw, as was seen in the excavations forthe Highgate interchange on the A30 (Bristow &Scott 1998).

Fig. 13. Diagram to illustrate the main factors that will affect the working of an elvan.

C. M. BRISTOW110



Petrology. Most elvans are porphyritic with cor-roded phenocrysts of quartz, feldspar (usuallyK-feldspar) and mica (usually biotite). The ground-mass is very fine grained (in places, cryptocrystal-line) and, when fresh, consists of quartz, feldsparand mica. Tourmaline is frequently present, some-times as individual acicular crystals but often asspherulitic growths. Good petrographical descrip-tions of the mineralogy may be found in the recentlypublished British Geological Survey Memoirs(Penzance: Goode & Taylor 1988; Trevose Headand Camelford: Selwood et al. 1998b; Falmouth:Leveridge et al. 1990; Mevagissey: Leveridge2008). Elvans are often affected by kaolinization,greisening and/or tourmalinization. Kaolinizationof an elvan can be recognized by the loss on igni-tion (LOI) in a whole-rock chemical analysis.An unkaolinized elvan will have a LOI of around1% or less, a fully kaolinized elvan will have aLOI of around 5%; anything above 2% should beregarded with suspicion as elvans affected by sig-nificant kaolinization are generally unsuitable asbuilding stone. Some (e.g. Trelowth) contain kaoli-nized feldspar phenocrysts that weather out toform pseudomorphic hollows in the exposed faceof the stone. Greisening converts the feldspar inthe elvan to a mixture of mica and quartz, whichcan result in a hard rock composed of little elsebut fine quartz and mica. In a study of the durabilityof building stone in old buildings along thesouth coast of Devon and Cornwall, Mottershead(2000) concluded that there was a positive corre-lation between durability and quartz + mica con-tent. This could explain why greisened elvans areso stable under long-term weathering conditions, aconclusion also hinted at by Flett (in Ussher et al.1909, pp. 78–79). Sometimes the feldspar pheno-crysts will be kaolinized or greisened preferentiallyto the groundmass. Some elvans (e.g. Tremore:Fig. 14) contain abundant spherulitic growths oftourmaline up to 5 mm across, or patches rich intourmaline and quartz (e.g. Escall’s Green). Otherelvans contain xenoliths of country rock or frag-ments of granite (e.g. one of the branches of the Pen-tewan elvan at Polrudden Cove). Iron staining iscommon, giving shades of red, pink, brown andyellow, the iron arising from mineralization ordeferrugunization of biotite (e.g. Tremore). Thiscan make matching stone difficult.

Elvan occurrences. The most famous elvan that isused as a building stone is Pentewan Stone, whichforms a dyke of feebly porphyritic elvan, exposedin the cliffs about a quarter of a mile NE of Pente-wan, at Polrudden Cove (Fig. 11). This stone wasobtained in medieval times from quarries in thecliff and at the top of the cliff (SX 026/475). Theelvan exposed in the cliffs at Polrudden can be

traced inland for about half a mile to a large over-grown quarry in the valley behind Pentewanvillage (SX 022/478), which is probably wheremost of the extraction of stone in the eighteenthand nineteenth centuries took place.

St Austell church tower, with its famous fifteenthcentury carvings of the Holy Trinity (Rowse 1960),and the exterior of Place, Fowey are composedof this stone. Pentewan Church is built of the localPentewan Stone obtained from less than 0.5 kmaway. Pentewan Stone is a true freestone (Fig. 12)and is usually a pale straw or yellow colour, oftenwith tinges of brown and pink. It stands up to weath-ering surprisingly well because it is not slowlydissolved by rainwater, nor does it react with atmos-pheric pollutants, as limestone would. Prolongedexposure to the weather, over many centuries ascan be seen at the base of St Austell church tower,can lead to the surface layer of the stone developinga honeycomb texture due to the less resistant stonebeing washed away by the rain. There are also manyother minor occurrences of elvan SW of the StAustell Granite. None of the many occurrences ofelvan within 2 km of the granite margin have beenworked for building stone, presumably because ofpervasive kaolinization emanating from the granite.Often an elvan will appear to be hard and intactbut, after several years of weathering, will disinte-grate; for example, the elvan boulder in the BoulderPark at Wheal Martyn Museum (Bristow 2006b).

Many buildings described by architectural his-torians as being built of ‘Pentewan Stone’ are builtof similar stone from locations such as Polgooth,Sticker or Penrice. An example of this is StLevan’s Church at Porthpean, near St Austell; theelvan here superficially looks similar to PentewanStone but closer inspection shows large feldsparphenocrysts, and it was eventually traced to asmall quarry nearby in Penrice Woods (SX 023/505). Not far away is the Georgian Penrice House,which is also built of a Pentewan-type stone, thistime a very-fine-grained whitish variant. Investi-gation revealed that it is likely that this stone camefrom a pair of infilled quarries less than half amile away at Castle Gotha (SX 027/498). When itwas proposed that a conservatory be added toPenrice House, the Planning Authority requiredthat it be constructed from Pentewan Stone. So aPentewan-type stone recovered from an old barnwas used but it does not match the original andprobably did not come from the quarries at Polrud-den Cove or Pentewan. Planners and bodies suchas English Heritage need to understand the geo-logy of the stones concerned before they insiston conditions about the kind of stone to be used.

Tremore is a particularly handsome elvan occur-ring north of the St Austell Granite. It has promi-nent phenocrysts of white orthoclase feldspar and




quartz set in a red, pink or grey fine-grained matrixwith spherulitic growths of black tourmaline (Bar-row & Flett in Ussher et al. 1909, pp. 74–78). Itshould be regarded as both a decorative stone andas a building stone, and was extensively quarriedat Tremore (SX 010/648), near Withiel (De laBeche 1839, p. 501). Joseph Treffry used polishedslabs of the blood-red Tremore elvan to line theinterior of Porphyry Hall (Fig. 14). Queen Victoriaand Prince Albert so admired the stone on a visitto Cornwall in 1846 that polished slabs of Tremoreelvan were used in Osborne House, Isle of Wight.Polished Tremore elvan can be seen as tiles in the

floor of the Baptistry in St Austell Parish Church,as star-like features in the floor of King Arthur’sHall at Tintagel and as a large polished slabforming a table at the rear of Lanlivery Church.An example of Tremore used as a building stone isthe front of West Hill Baptist Church, St Austell.

Another important source of building stone wasthe Newham elvan, which was worked in a quarryabout 1 km south of Truro (SW 830/437). This issimilar to Pentewan but is usually paler and oftenfoliated, with quartz veins in it, so that on casualexamination it can superficially look like a siltstonefrom the Portscatho Formation. The dominant stone

Fig. 14. A polished Tremore elvan column in Porphyry Hall, Place, Fowey. White orthoclase phenocrysts are set in ared iron-stained fine matrix with black spherulitic growths of tourmaline.

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used in the Georgian buildings of Lemon Street isNewham elvan, much praised by Pevsner (1951,p. 212): ‘Lemon Street is one of the most completelyGeorgian streets preserved anvwhere, all two-storeyed, stone fronted houses of uniform charac-ter’. However, it does not appear to carve so well asPentewan-type stone. The early sixteenth centurySt Mary’s Church, now much restored and incorpor-ated into the Cathedral, is a wonderful hotch-potch of Newham Stone, Pentewan-type Stone andBath Stone. Nineteenth-century accounts (Thomas1889) record that an additional elvan-type stoneused in St Mary’s is called ‘Wild Duck’ but thesource of that stone is unknown.

Elvans have been exploited for building stonebetween Truro and the eastern margin of the Carn-menellis Granite at Nancevallen, Saveock, Pen-coose, Creegbrawse, Kerley and Enys, and therewere some significant elvan quarries in the cen-tral area of the Carnmenellis Granite at locationssuch as Polmarth, Tresevern Croft, Praze, Carvath,Carthew and Bolenowe. All of these quarries musthave produced large quantities of stone for buildingin the urban centres of Truro, Falmouth, Redruthand Camborne in Victorian and Edwardian times.Most have now been filled with landfill; furtherdetails will be found in Hill & MacAlister (1906).

An example of the local use of elvan is seen atSt Bartholemew’s Church and in the village of War-leggan, where a porphyritic elvan is extensivelyused, probably obtained in the form of ‘moorstone’,although a quarry was opened later nearby atTreveddoe.

Early Permian extrusive rock has been used forbuilding in the vicinity of Kingsand. Leveridgeet al. (2002) reported this rock to be a rhyoliticlava but the coarse columnar jointing and the solesuggests it may really be a welded tuff. The stoneis fine grained and easily worked; it is probablynot as resistant to weathering as the better elvansand granites, although its chemical compostionshows similarities to the elvans of the BodminMoor Granite. It has been stained a reddish-browndue to the desert weathering conditions in EarlyPermian times. The rock outcrops on the foreshorewere a source of this stone; there is also WatergateQuarry above the village. For more details see theSupplementary publication.

Veinstones arising from post-Variscan

mineralization

The older buildings in the mineralized areas oftenuse various kinds of veinstone, much of whicharose from metalliferous mining. Early medievalbuildings often use stones that look as if they havearisen from tin stream works. Even today,

minestone is still used, a wall built in 2011 madeof veinstone from Charlestown United tin mine isshown in Figure 15 and much vernacular buildingused this type of stone.

Porphyry Hall in Place House, Fowey has skirt-ing and floor panels made of schorl rock – a vein-stone composed mainly of tourmaline with somequartz. Possibly, Joseph Treffry would haveobtained this from one of the tin mines in the StAustell Granite, such as Rocks or Carclaze. Thecentral floor rose also contains panels of topaz-fels from St Mewan Beacon, which is a high-temperature hydrothermal veinstone from thecontact between the granite and killas just south ofBlackpool china clay pit (Manning & Exley 1984;Floyd et al. 1993, p. 191). Typically containing25% topaz, this rock has been used elsewhere inPlace for flooring, presumably because the topazand quartz make an extremely hardwearing combi-nation. Collins & Coon (1914) reported that theSt Mewan topazfels was used for the wearingfloors of mills used to grind china stone because ofits hardness, and a small quarry (SW 985/535) atSt Mewan Beacon was worked for this purpose.

China clay working yields a waste material com-posed of the harder rocks that cannot be brokendown by monitors to yield china clay, known asstent (Bristow 2006a, b), which is composed oflarge fragments up to 2 m in diameter of weaklykaolinized granite, greisened and tourmalinizedgranite, schorl rock and veinstone. The granite israrely suitable for building as it tends to disintegratewhen subjected to weathering but the harder quartz-tourmaline vein material, schorl rock and somegreisen are often used in older industrial buildingssuch as clay dries and in vernacular building.

White quartz stone is often used in building inCornwall north of the Camel estuary and SW of

Fig. 15. Cassiterite Close, near St Austell. The‘minestone’ used to build the wall in the forground wasobtained from a waste tip belonging to CharlestownUnited tin mine. The wall was built in 2011.




Truro in the Kea area. Some of this may be true vein-stone but some could be either silica segregationsgenerated during regional metamorphism or, poss-ibly in some cases, a form of silcrete developedduring deep chemical weathering in early Tertiarytimes.

Sedimentary rocks

Slates (slaty mudstones)

Slate is extensively used in Cornwall for buildingand roofing, and is the most natural material to usein most of the non-granite areas. Churches andgrander houses may have a facing of elvan orgranite ashlar but with rubble masonry forming thecore of the wall. Strictly speaking, modern geologi-cal terminology uses the term ‘slaty mudstones’for what have traditionally been called ‘slates’.Relatively silt-free mudrocks with a strong regularcleavage are best for roofing slates. However, thebest ‘slate’ for building walls seems to have beenderived from a silty or sandy mudrock, such as mayhave been formed by a turbidity current. Recrys-tallization of the clay minerals under stress duringthe Variscan Orogeny consolidated the originalmud into a hard rock and produced the cleavage.Redistribution of silica from the original detritalgrains to form a cement to the rock is an importantfactor in forming a tough rock that will make agood building stone. There is some evidence thatmild thermal metamorphism associated with thegranites also helps to make the rock tougher andbetter for building. Unfortunately, most of Corn-wall’s mudrocks have suffered from more thanone episode of deformation, resulting in severalsets of cross-cutting cleavages that makes the slateuseless for roofing. In places, the deformation hasbeen sufficiently intense to convert the slate into aphyllite, as in the Tintagel high-strain zone and atDodman Point in south Cornwall.

Many Cornish slate formations (such as theLower Devonian Meadfoot Group slate in the Tren-arren Slate Quarry) are pyritic, which can present aproblem as, on exposure to the weather, the pyrite(FeS2) oxidizes, which can result in acid attack ofthe mortars. This is analagous to the better-knownproblems caused by ‘mundic blocks’, which weremade with pyrite-bearing mine waste (Bristow1993). In south Cornwall, it is noteworthy that thelarge early slate quarries (Poltesco, Trenarren andPolmear) are all close to sea level. This undoubtedlywas partly for ease of transport by water but, also,on the higher ground inland there would have beena greater thickness of weathered slate to remove inorder to reach unweathered slate.

Some of the more significant sources of slateare described below; there are many other small

slate quarries that space does not permit to bementioned.

Lower Devonian. There are fewer Lower Devonianslate quarries than Middle and Upper Devonian,partly because multiple deformations in the south-ern part of Cornwall have led to multiple superim-posed cleavages and also because really thicksequences of mudrocks are not found. In addition,Meadfoot Group slates tend to have a high pyritecontent.

The earliest Devonian sediments are the purpleand green slates of the Dartmouth Group, whichwere exploited at East Polmear in two quarries(SX 089/534 and 089/535). Menabilly House, theformer home of the Rashleighs and Daphne duMaurier, is built of purple and green slate that mayhave come from these quarries. St Nunn’s Churchat Pelynt also has much purple and green slatymudstone in the tower. Trenarren Slate Quarry(SX 039/489) is practically unknown, even tolocal residents, but is referred to as the ‘the greatquarry of Trenyaren’ in the Parliamentary Surveyof the Manor of Tewington of 1650. It worked well-cleaved dark blue pyritic slate belonging to theHallane Formation(?) of the Meadfoot Group,the slaty cleavage is at a considerable angle to thebedding; it was probably worked for roofing. Onerather unusual feature of a number of buildings inthe Holmbush–St Blazey Gate area is that they arebuilt with a red slate. St Mary’s Church at StBlazey is also built of this unusual stone. The red-dening is due to the local slates belonging to theMeadfoot Group being impregnated by iron oxide,possibly due to iron in solution moving out fromthe St Austell Granite along an adjacent strike-slipfault known as the Great Cross-course. PoltescoQuarry (SW 831/445) is situated on the east sideof the Truro river, just below the city. The quarryexploits Portscatho Formation silty mudstones,which were used extensively in pre-twentiethcentury buildings in Truro.

Middle Devonian. Slates that are called in the build-ing trade ‘St Issey Stone’ are quarried for generalbuilding and walling use near the Royal CornwallShowground, Wadebridge in Tredinnick Quarry(SX 935/688) and Cannalidgey Quarry (SX 938/700). Neither the Memoir (Reid et al. 1910) northe 1895 List of Quarries (Anon 1895) lists eitherof these two quarries, so one must assume thatthey are of no great age. When freshly broken, theslaty mudstone is greyish-green but most of thestone that is recovered from these quarries is weath-ered to a rusty brown colour. Much stone from Tre-dinnick has been used for Cornish hedges builtalongside new road schemes, for example. There

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are a large number of other small quarries in theSt Issey area.

Kestle Quarry, 3 km SE of Sladesbridge, is alarge disused quarry in the Trevose Slate Forma-tion, which has been intruded by two doleriteswith associated metamorphic adinoles. CallywithQuarry (SX 080/683) is a large active quarry inthe Bedruthan Formation, which is 2 km NE ofBodmin, just to the west of the A30. The latestMemoir (Selwood et al. 1998, p. 87) attributes theslates being worked in this quarry to the BedruthanFormation; however, the accompanying geologi-cal map indicates that the slates belong to theTrevose Slate Formation. The slate is a fineuniform greyish-green. There is dark brown ironstaining on joints but only rarely on the cleavages.Cut faces are a greyish-green, which weathers toan attractive paler khaki colour. Westwood andLantoom quarries are active quarries in the MiddleDevonian Saltash Formation. Westwood Quarry(SX 185/645) is 3 km west of Dobwalls, immedi-ately south of the main railway line in the Fowey(Glynn) Valley, and Lantoom Quarry (SX 225/649) is 1 km east of Dobwalls, immediately southof the A38. Both extract dark grey slaty mudstonefrom the Saltash Formation for decorative walling,landscaping and paving. Carnglaze Slate Caverns,near St Neot, were a series of underground work-ings, primarily for roofing slate, dating back to atleast the early nineteenth century (SX 188/668).Underground operations ceased in about 1903.The slates belong to the area where the TrevoseSlate Formation is changing its name to becomethe Saltash Formation, and were said to be blue incolour. The slate was used over a wide area fromPlymouth to Penzance. Although outside the meta-morphic aureole of the Bodmin Moor Granite, as

marked on Sheet 336, nevertheless these slateswere, to an extent, probably affected by the gra-nite, which may explain their special quality. Nowa-days, the slate caverns are a tourist attraction and avenue for concerts.

Upper Devonian. Delabole Slate Quarry is situatedimmediately SE of the village of Delabole (SX075/840) and was mentioned by Carew (1602),who writes of a considerable export trade toBritish and Continental ports. Borlase (1758,pp. 93–94) speaks of an excavation 300 yards longby 100 yards wide and says: ‘that for its lightnessand enduring of weather, it is generally preferredto any slate in Great Britain’. A high proportion ofthe older roofs in Cornwall use Delabole Slate.Present activity in the 150 m-deep quarry is concen-trated in the northern part, which yields a durablegrey slate, verging on a phyllite, which is suitablefor roofing, building, ornamental and monumentaluses (Fig. 16). The quarry works the DelaboleMember of the Tredorn Slate Formation of Famen-nian age. In present-day terminology, the slate isseen to belong to the epizone, based on illite cry-stallinity (Robinson 1998, pp. 115–117). Distortedspecimens of Cyrtospirifer verneuilli are sometimesfound (‘the Delabole butterfly’) and are much prizedby collectors.

Trevillet Quarry is 2.5 km east of Tintagel(SX 082/882) and dates back to at least the nine-teenth century. In contemporary terminology, theTrevillet Quarry would be regarded as belongingto the Upper Devonian (Famennian) Tredorn SlateFormation. It produces greyish-green slates withmuch brown oxide staining on cleavage planes andjoints. The Prince of Wales Quarry, 3 km SE ofTintagel (SX 072/860), exploits a similar slate.

Fig. 16. Delabole Slate Quarry. High-quality Upper Devonian slate, verging on a phyllite (see the text).




The Tintagel cliffs (SX 050/870–SX 053/887)have a series of quarries along the cliffs betweenTrebarwith Strand and Tintagel where good-qualityroofing slate was extracted, possibly dating backto the seventeenth century (Sharpe 1990). Work-ing from south to the north the quarries were:West, Lanterdan, Dria, Gull Point, Lambshouse,Long Grass and Gillow. The last quarry to work(Long Grass) closed in 1937. The slates were gener-ally greyish-green well-cleaved slates belongingto the Upper Devonian Tredorn Slate Formation,with occasional thin beds of limestone and silt-stone. Photographs show that it must have been ahazardous place to work. There are a number ofother active quarries in the Tredorn Slate Formationin this area, including Trecarne Quarry (SX 069/847, two sites), which is 1 mile WNW of DelaboleQuarry, and currently produces building and hedg-ing stone; Tynes Quarry (SX 045/819), 3.5 kmSW of Delabole Quarry, also currently producesbuilding stone (non-roofing slate); Trebarwith RoadRustic Quarry (SX 069/850) and Merrifield Quarryare also in the same area.

A group of quarries exploited greenish-greyslates belonging to the Upper Devonian TredornSlate Formation along the coast west of Boscastle:

California (SX 090/908), Welltown (SX 088/908),Grower (SX 085/907) and Boscastle (SX 082/905).The slate was quarried from the unweathered rockat the base of the cliffs and hauled up the cliff bya cable arrangement to the cliff top.

CarboniferousBoscastle black slate quarries (above village)

and sandstones. Most of the buildings in the Bos-castle area use black slates and sandstones belong-ing to the Boscastle Formation, which is mainlyof Lower Carboniferous age but may range downinto the Upper Devonian and up into the Namur-ian. The black coloration is due to the fact that theslates and sandstones contain pyrite and carbon.Black cherty rocks are also present in the area.As roofing slates, these beds are not of any specialquality; nevertheless, much of the distinctive char-acter of the built environment of Boscastle derivesfrom the use of these black sandstones and slates,often accentuated by white vein quartz cappingthe walls.

Bangor Slate Quarry, near Launceston. Therehas been significant quarrying of slate in the Laun-ceston area, notably at Bangor Slate Quarry (SX319/833), 1 mile SE of the town. This quarry isnow filled in and used as a recycling centre byCornwall Council. The quarry is in CrackingtonFormation (Namurian–Westphalian) siltstones andslates, although Reid et al. (1911, pp. 24–25 & 129)pointed out that the cleavage is weak, and comment:‘A good deal of roofing slate was at one time madehere; but although of good colour and fairly sound itdoes not stand the weather well and cannot competewith the Delabole slate; the material is better suitedfor flagstones’.

Sandstones

Sandstone has been worked on a small scale froma few localities in the Portscatho Formation(Middle–Upper Devonian) in mid-Cornwall, suchas Tredinnick Quarry just north of Grampound(SW 931/492) and Treworgans Quarry (SW 899/495). The Tredinnick Sandstone Quarry (do notconfuse with the other Tredinnick Slate Quarrynear the Royal Cornwall Showground, referredto earlier) is blue when first broken, weatheringbrown. Mottershead (2000) studied the durabilityof various building stones used in south Cornwallin coastal locations. Somewhat surprisingly, theMiddle–Upper Devonian Portscatho Formationsandstones, as used in sixteenth century St Mawescastle, turned out to be one of the most durable,syntectonic recrystallization of all finer-grainedmaterial in these turbidite sandstones (Leveri-dge et al. 1990) has created a very tough rock

Fig. 17. Pigsdon sandstone quarry in the UpperCarboniferous Bude Formation. Although mainly anaggregate quarry, small quantities of building stone arealso supplied.

C. M. BRISTOW116



that is resistant to the weathering conditions inthis exposed situation. Trewarthenick House alsouses a fine-grained turbiditic Portscatho sandstone,obtained from a nearby quarry in the Fal Valley(SW 908/439). Interestingly, the sandstone was cutby hand to form brick-shaped stones, which werethen painted red, in order to avoid the eighteenthcentury brick tax. There are many other small quar-ries in the Portscatho turbiditic sandstones and slatymudstones that have been used to supply buildingstone in the Roseland area.

Ordovician quartzites from the Roseland BrecciaFormation (e.g. small quarries at Carne, SW 913/380) have been used locally and have been con-sidered as sources of high-purity silica. The Stad-don Grits in SE Cornwall have also been quarriedin a small way; the main use has been as a localbuilding and walling stone.

Quarries in north Cornwall (e.g. Pigsdon, SS278/092 (Fig. 17); and Cansford Quarry, SS 168/931) work massive sandstones in the WestphalianBude Formation. They provide a useful source ofaggregate, together with small quantities of buildingstone.

Limestones, cherts, etc.

Limestone is almost absent from Cornwall and, con-sequently, little locally derived limestone has beenused for building. A recently published book onlime kilns and lime burners in Cornwall (Isham2000) provides details of the few limestone occur-rences in Cornwall.

In the past, much limestone from the Plymoutharea was brought into the coastal areas of Cornwallfor lime-burning (Isham 2000) and, occasionally,for building; for example, the station building inFowey, Methleigh House in St Austell (now demol-ished) and St Mary’s Church in East Looe). Presum-ably, these represent a back-load of limestone forships taking granite up to Plymouth for the dockyardand breakwater.

The main structural stone used in Truro Cathe-dral (built 1890–1900) is Carnsew Granite fromMabe Parish, which shows little deterioration from100 years of weathering. However, the carved stoneextensively used in the facade is a Jurassic limestonefrom Bath, which has weathered badly, partly due tolocal environmental factors such as salt-laden gales.At the time of construction, many prominent localgeologists strongly advocated using local stone(Barham 1976, p. 14), and 72 varieties of localstone were sampled and considered, including themany elvans available only a few miles away.However, the architect insisted on using BathStone which has required extensive replacement; adecision that has cost the Diocese many millionsof pounds.

A peculiar variant of calcareous rock is calc-flinta, which is found in a series of east–west bandsin and around the Goss Moor depression (see theearlier ‘The metamorphic aureole’ section). Thestone is usually closely jointed and relatively brit-tle, and cannot be obtained in good-sized blocksand so is not widely used for building. However, ithas been widely exploited as a source of aggregate,as at Glebe Quarry near Roche (SW 988/593).Other small quarries are found on the north sideof the Belowda–Castle an Dinas ridge near StColumb Major at Rosevannion Quarry (SX 929/649) and at Tremore (SX 010/648) – alongsidethe more famous Tremore elvan.

Radiolarian cherts occur in the Lower Carbon-iferous of North Cornwall, and are seen in PilsamoorQuarry (SX 276/857). The chert splits into con-veniently sized blocks for building and has beenmarketed for this application. Cherts were also for-merly exploited in the now abandoned BarracadoesQuarry, near Launceston (SX 322/862).

Quaternary sandrocks

Present-day soils in Cornwall are generally acid,which explains the magnificent displays of

Fig. 18. A thirteenth-century arch in St Carantocus’sChurch at Crantock, built using Quaternary sandrock.




acid-loving plants such as camellias, rhododen-drons and magnolias seen in the County. The def-iciency in lime-bearing rocks was overcome,notably near the north coast, by using beach sandsthat have a high bioclastic carbonate content. Some-times these Quaternary sands, particularly wherethey form a raised beach, are cemented by calciumcarbonate, as at Godrevy Point, near Hayle, in thePadstow estuary and on the north side of FistralBay. This ‘sandrock’, as it is known, is just aboutrobust enough to be used as a building stone.

Sandrock has been used in some of the olderchurches, as at Padstow and for the font at StEnodoc (John Betjamin’s Church), the historic StPiran’s Church in Perran Towans and in St Caran-toc’s Church at Crantock (Fig. 18). Borlase (1758)recounts that the Perranporth sandrock is poorlycemented; the poor state of the masonry in StPiran’s Church bears this out (Bristow 2007). It ispossible that the source for the sandrock used inCrantock Church may lie behind Crantock beach,having been covered with blown sand since it wasworked in medieval times. St Carantoc’s once hada tower but it collapsed shortly after it was builtin the fourteenth century, suggesting that thereare probably structural limitations with sandrock.However, it must be one of the geologically young-est building stones to be used anywhere in Britain,probably having been laid down as a raised beachduring the last interglacial.

The author acknowledges with gratitude the helpand encouragement provided by A. Lea, D. Pascoe andA. Pattison of the Minerals Planning Department ofCornwall Council, and also the help with drafting thediagrams provided by J. Brinkhoff, also of CornwallCouncil. My thanks also go to the many quarry and prop-erty owners who allowed access to their premises to lookand photograph the stones involved. I am particularlygrateful to the late D. Treffry for allowing access to Por-phyry Hall in Fowey to study and photograph the magnifi-cent polished examples of granite and allied stones in thattour de force of the granite mason’s art. The two refereesare also thanked for many useful comments.

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