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

2016-06-21T11:28:19Z (GMT) by Colin M. Bristow
<p>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, both for local use and for export, before concrete and brick came to dominate construction in the twentieth century. Many of the types of building stone, such as elvan and sandrock, do not occur outside Cornwall, so local stone provides much character to the local built environment. </p> <p>Granites were extensively worked in the eastern part of the Carnmenellis Granite (mainly in Mabe parish), in the St Austell Granite (Luxulyan, Carn Grey and the china stone areas) and on Bodmin Moor (De Lank, Hantergantick, Cheesewring, etc.), as well as in the Kit Hill, Tregonning and Land's End granite masses. The predominant type used was the ‘coarse grained megacrystic biotite granite – smaller megacryst variant’ of Hawkes & Dangerfield. A significant trade in granite developed in the nineteenth and early twentieth centuries, employing large numbers of skilled quarrymen. Finished granite was exported all over the world; many iconic buildings in London and other major cities use Cornish granite. A tourmalinized granite, luxullianite, was an important decorative stone, and was used for the Duke of Wellington's sarcophagus in St Paul's Cathedral. Schorl rock is often found in older buildings in the granite areas. Most pre-nineteenth century granite building did not use quarried stone but used ‘moorstone’ obtained from boulders lying on the surface of the granite uplands. Large quantities of ‘minestone’ have been used in vernacular buildings, past and present, and in some medieval churches, sourced from the waste tips of metalliferous (both alluvial and vein operations) and china clay workings. </p> <p>Allied to the granites are the fine-grained elvans of granitic composition, usually intruded in the form of dykes. Greisening often improves the durability of elvans, which have been extensively used in some of the finest stone buildings in Cornwall, such as St Austell church tower, Antony House (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 fine carving. However, not all buildings described by architectural historians as being of Pentewan Stone came from Pentewan. Another important elvan was Newham Stone, widely used in the older buildings in Truro. Tremore elvan was used, together with luxullianite, mainly as a polished decorative stone to line Porphyry Hall at Place in Fowey and in other high-status buildings. </p> <p>Basic igneous rocks include an Upper Devonian metadolerite at Cataclews Point, west of Padstow, which provided the extremely durable Cataclews Stone, used from medieval times onwards for fonts and church carvings in the area around the Camel estuary. A more unusual stone, produced by carbonatization of an ultrabasic intrusion, is Polyphant Stone, mainly used for interior use and by sculptors, composed of a mixture of talc, chlorite, and various calcium and magnesium carbonates. The Polyphant Quarry was recently reopened to supply stone for the rebuilding of Newquay parish church and to supply stone for sculpting. Allied to Polyphant Stone is Duporth Stone, obtained from the cliffs of Duporth Bay, south of St Austell, which was used in the pillars of Truro Cathedral. Basic hyaloclastite was the main stone used in the great Norman Church of St German's in SE Cornwall. The Lizard ophiolite complex provided a source of serpentine for building and for the manufacture of polished slabs; ornaments made from serpentine are still produced. </p> <p>Slaty mudstones and sandstones of Devonian and Carboniferous age have been extensively used for traditional buildings throughout Cornwall, nowadays much slaty mudstone is still used for building and for Cornish hedge building. The Upper Devonian Delabole Slate Quarry has yielded high-quality roofing slate from Tudor times onwards but there are many other large active and disused roofing slate quarries in the Tintagel area and elsewhere in Cornwall, such as the underground slate workings at Carnglaze, now a tourist attraction and concert venue. Devonian sandstones, 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 youngest building stone, seen in the Newquay and Padstow areas, is a cemented bioclastic Quaternary beach sand, laid down at a time of high sea level during an interglacial as a raised beach. It is known locally as ‘sandrock’ but is a relatively weak building stone. St Carantoc's Church at Crantock and St Piran's Church on Perran sands were largely built of it. </p>