LITMIR.BIZ

      The Coast Path passes through a notch in the crumbling granite of Cligga Head (Stage 16)

      The south-west of England is a huge area, and its geology is not easily condensed into a few words. The oldest rocks are of late Cambrian age, from 490 million years ago, found on the Lizard peninsula in South Cornwall. They derive not from the earth’s crust, but from the deep-seated mantle beneath. They’ve undergone a process of metamorphosis – altered by immense temperature and pressure – and one of the most important rock-types resulting from this is serpentine, which is locally worked into delightful ornamental forms.

      The bulk of rock types in Exmoor, Devon and Cornwall are of Devonian and Carboniferous age, stretching from 420 to 300 million years ago. Vast thicknesses of sediments were laid beneath the sea, becoming gritstone, sandstone, shale and limestone. Towards the end of this period, the rock beds were being squeezed into mountain ranges, becoming crushed and folded. A huge batholith of molten rock, deep in the Earth’s crust, pushed them upwards, solidifying to become the granite that is now exposed around Dartmoor, Bodmin Moor, St Austell, Carnmenellis and West Penwith. Coastal cliff faces often display intensely contorted beds of rock, best seen around Hartland and Crackington Haven in North Cornwall.

      The heat and pressure associated with mountain-building also squeezed mineral-rich fluids into cracks in the rock, which solidified into metal-bearing veins containing tin, lead and copper, giving rise to an important mining industry. Some shales and mudstones were changed to slates, which were quarried as roofing material. Granite has been used extensively for building, as it is generally hardwearing. In places where it rots and crumbles, it provided the basic ingredient for the china-clay industry around St Austell.

      The Permian, ranging from 300 to 250 million years ago, saw extensive desert landscapes, but this was followed by the Triassic, when the land sank back under the sea. The geology of east Devon and Dorset is remarkably different. The Triassic gave way to the Jurassic, from 200 to 145 million years ago, and the abundant marine life is preserved in fossil-rich locations along the coast. Further east, the Cretaceous, from 145 to 65 million years ago, is characterised by immense thicknesses of chalk, giving rise to sheer white cliffs. The ‘Jurassic Coast’ is a World Heritage Site on account of its geological significance.

      During the last Ice Age the south-west was free of glaciers and the landscape was bleak tundra with permafrost conditions. Britain wasn’t an island, but was joined to mainland Europe. With the melting of the northern ice sheets, sea levels rose, but the whole of Britain tilted, with Scotland rising slightly, as evidenced by numerous raised beaches. The south-west of England, by contrast, sank so that river valleys were flooded by the sea. Along some parts of the coast the land is steadily being eroded, while on other parts of the coast the same eroded material is being piled up into beaches of sand and shingle.

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