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From Dartmoor to the Scillies, one slab of granite

20 February 2012

In the 'Sunday Times Book of the Countryside' (1983) there is a very nice diagram of the SW peninsular (Cornwall and Devon). It seems that one giant block of granite rises from a single core, or batholith – which reaches down to an unknown depth below the surface. The upper parts of the granite slab protude from the surface on Dartmoor, Bodmin Moor, the hills around St Austell, Lands End … and the Isles of Scilly. These locations are where the granite is exposed, being points along what is a series of bumps and troughs, which include the trough between Lands End and the Scillies. In fact, the Isles of Scilly are the surviving remnant of another exposure – most of it being under water. The diagram illustrates that although the Scillies and Lands End are part of one giant granite mas there is actually a deep valley, or fold that separates them. However, Bodmin Moor is not as high as Dartmoor and Lands End is considerably less so … and the Scillies are virtually submerged. The batholith, in effect, is dipping down towards the west and in doing so has become overrun by the ocean. Who knows how far west the batholith originally extended – but what caused it to lean over? That question might also be framed in another way – is it the batholith that has tilted or is it the sea that has readjusted its surface in relation to the SW peninsular?

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