Buckfast. Report from the Geology Section

Buckfastleigh is to the south of the granite of Dartmoor but the granite has heavily influenced the underlying geology. This underlying geology includes the slates and thin limestones of the Upper Devonian and some Middle Devonian limestones. There are also rocks of the Buckfastleigh Volcanic Series in the area, some of which have been important economically.

The intrusion of the granite and related mineralisation affected the existing country rock; copper was worked at the Brookwood and adjacent Wheal Emma Mines by the South Devon United Copper Mine until the 1880s and prospecting continued into the twentieth century. In 1918 the Kings Wood mine, about 2 km southeast of Buckfastleigh, was also explored for copper. The mineral veins there are in Devonian slates and probably result from late hydrothermal emissions from the granite. Uranium (in the form of pitchblende) is also present resulting in high levels of radioactivity, though it does not appear to have been of any commercial value. Umber was worked nearby at the grandly named ‘Devon and Cornwall Umber Works’ at Ashburton and the Bully Cleaves Quarry just behind Buckfastleigh Hill was quarried for building and roadstone and supported a thriving graphite industry.

The dolerite sill of Whitecleaves Quarry, near Buckfastleigh, is intruded into the grey mudstone sequence of the Devonian Saltash Formation and was worked for aggregate for road building. The quarry has recently been the subject of some concerns locally with plans to use it as a disposal site for ash from a Plymouth incinerator.

The Devonian limestones, formed in the Middle Devonian in warm shallow seas around 390 million years ago, have been exploited both for liming and for decorative building stone, in particular the local “Ashburton Marble” from the Linhay Quarry which is a very attractive black limestone that takes a good polish. It was much used for fire surrounds and flooring and can still be seen at County Hall, Exeter, and was previously on the ground floor of the old Debenhams building in Exeter. It was used commonly in London – in the foyer of the Post Office Tower and in the bathrooms of the London Hilton! It is the only ‘marble’ recently produced in Britain and it was an important export, to South Africa, Hong Kong and particularly to the United States where it is seen in public buildings including the President Roosevelt Memorial.

Of great importance geologically in Buckfast are the Joint Mitnor Caves, looked after by the William Pengelly Cave Studies Trust. These limestone caves are hugely important in the Pleistocene history of the south west as the site is one of a handful of places where mammal bones have been found. The limestone caves were filled with detritus from above giving a layered history of the period. Hippopotamus and elephant bones are present from the lpswichian stage (approx 125,000 years ago) when conditions were warmer than they are today.

Jenny Bennett and David Allen
(and thanks for comments from Sheila Phillips,
William Pengelly Cave Studies Trust).

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