Geology of Cornwall

Geology of Cornwall

The Geology of Cornwall is dominated by its granite backbone formed during the Variscan orogeny. Around this is an extensive metamorphic aureole (known locally as killas) formed in the mainly Devonian slates that make up most of the rest of the county. There is an area of sandstone and shale of Carboniferous age in the north east, and the Lizard peninsula is formed of a rare section of uplifted oceanic crust.

Coasts

Cornwall forms the tip of the south-west peninsula of the island Great Britain, and is therefore exposed to the full force of the prevailing winds that blow in from the Atlantic Ocean. The coastline is composed mainly of resistant rocks that give rise in many places to impressive cliffs.

The north and south coasts have different characteristics. The north coast is more exposed and therefore has a wilder nature. The prosaically-named "High Cliff", between Boscastle and Tintagel, is the highest sheer-drop cliff in Cornwall at 735 ft (224 m). However, there are also many extensive stretches of fine golden sand which form the beaches that are so important to the tourist industry, such as those at St Ives, Hayle, Perranporth and Newquay. The only river estuary of any size on the north coast is that of the Camel, which provides Padstow with a safe harbour.

The south coast, dubbed the "riviera", is somewhat more sheltered and there are several broad estuaries formed by drowned valleys or rias that offer safe anchorages to seafarers, such as at Falmouth and Fowey. Beaches on the south coast usually consist of coarser sand and shingle, interspersed with rocky sections of wave-cut platform.

Interior

The interior of the county consists of a roughly east-west spine of infertile and exposed upland, with a series of granite intrusions, such as Bodmin Moor, which contains the highest land within Cornwall. From east to west, and with approximately descending altitude, these are Bodmin Moor, the area north of St Austell, the area around Camborne, and the Penwith or Land's End peninsula. These intrusions are the central part of the granite outcrops of south-west England, which include Dartmoor to the east in Devon and the Isles of Scilly to the west, the latter now being partially submerged.

The uplands are surrounded by more fertile, mainly pastoral farmland. Near the south coast, deep wooded valleys provide sheltered conditions for a flora that likes shade and a moist, mild climate. These areas lie mainly of Devonian sandstone and slate. The north east of Cornwall lies on Carboniferous rocks known as the Culm Measures. In places these have been subjected to severe folding, as can been seen on the north coast [ Enfield M A et al (1985) "Structural and sedimentary evidence for the early tectonic history of the Bude and Crackington Formations, north Cornwall and Devon", Proceedings of the Ussher Society 6(2), 165-172.] near Crackington Haven, spectacularly at the "Whaleback Pericline" on the beach just south of Bude and at several other locations.

Granite

The intrusion of the granite into the surrounding sedimentary rocks [Henley S (1976) "Rediscovery of a Granite Dyke at Perranporth, Cornwall", Transactions of the Royal Geological Society of Cornwall, XX(4), 286-299.] gave rise to extensive metamorphism and mineralisation [Durrance E M et al (1982) "Hydrothermal circulation and post-magmatic changes in granites of south-west England", Proceedings of the Ussher Society, 5(3), 304-320.] , and this led to Cornwall being one of the most important mining areas in Europe until the early 20th century. It is thought that tin ore (cassiterite) was exploited in Cornwall as early as the Bronze Age. Over the years, many other metals such as copper, lead, zinc and silver have all been mined in Cornwall. [Barton D B (1963) "A Guide to the Mines of West Cornwall", 52pp.] Alteration of the granite also gave rise to extensive deposits of china clay (kaolinite), especially in the area to the north of St Austell, and this remains one of Cornwall's most important industries.

The Lizard peninsula

The geology of the Lizard peninsula is Britain's only extensive [Chandler P et al (1984) "A gravity survey of the Polyphant Ultrabasic Complex, East Cornwall", Proceedings of the Ussher Society 6(1), 116-120.] example of an ophiolite. Much of the peninsula consists of the dark green and red rock, precambrian serpentine, which forms cliffs as at Kynance Cove, and can be carved and polished to create ornaments. This ultramafic rock forms a very infertile soil which covers the flat and marshy heaths of the Goonhilly Downs.

Mining

Historically extensive tin and copper mining has occurred in the region, as well as arsenic and china-clay extraction. As of 2007 there are no active mines remaining. However, tin deposits still exist in Cornwall, and there is talk of reopening South Crofty tin mine.

ee also

*Killas
*Geography of Cornwall
*Geology of Lizard, Cornwall
*Mining in Cornwall
*Royal Geological Society of Cornwall

Links

* [http://www.geological.org.uk/ Royal Geological Society of Cornwall]
* [http://myweb.tiscali.co.uk/geologyofcornwall/ The Geology of West Cornwall]

References

Bibliography

* Edmonds E A (1975) "South West England" British Geological Survey UK Regional Geology Guide series no. 17, 4th Edition, HMSO (ISBN 0-11-880713-7).
* Evans C D R & Hillis R R (1990) "Geology of the western English Channel and its western approaches" British Geological Survey UK Offshore Regional Report series, no. 9. HMSO (ISBN 0-11-884475-X).
* Hall A (2005) "West Cornwall" Geologists' Association Guide series no. 13, 2nd Edition.


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