A tor is a large, free-standing residual mass (rock outcrop) that rises abruptly from the surrounding smooth and gentle slopes of a rounded hill summit or ridge crest. In the South West of England, where the term originated, it is also a word used for the hills themselves – particularly the high points of Dartmoor in Devon and Bodmin Moor in Cornwall.[1]
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The word tor (Cornish tor, Old Welsh twrr, Modern Welsh tŵr, Scots Gaelic tòrr), meaning hill,[2] is notable for being one of the very few Celtic loanwords to be borrowed into vernacular English before the modern era – such borrowings are mainly words of a geographic or topographical nature. Similar words include crag (from the Welsh word craig, meaning "rock") and avon (from the Welsh word afon, meaning "river").
Tors are usually composed of granite or metamorphic rocks, but they can also develop in volcanic rocks and occasionally other hard rocks such as quartzite. They are the result of millions of years of weathering.
For example the Dartmoor granite was emplaced around 280 million years ago and as it cooled it contracted leaving a multitude of mainly vertical cracks; these facilitated hydrothermal circulation which both chemically altered the rock surrounding the cracks and deposited minerals in them. The rocks that overlaid the granite pluton were eroded quickly (in geological terms) and the consequent release of pressure caused the formation of sub-horizontal joints which tended to follow the shape of the land and which started the separation of the upper part of the pluton into blocks of varying sizes.[3]
Further chemical attack by circulating water and later by acidic rainwater caused further weathering along the lines of these joints, resulting in the more or less complete decomposition of the granite into its constituent crystals in regions where the joints were closely spaced. Finally during the ice age the majority of the rotted granite was stripped away and the freeze-thaw cycle levered away larger blocks, leaving them scattered around the now-exposed tors as clitter,[3], where they have provided ready building materials for thousands of years. Eventually the granite is weathered down to a level equivalent to sandy gravel, known as growan, which consists of individual crystals.
Weathering has also given rise to circular "rock basins'" formed by the accumulation of water and the repeated freezing and thawing – a fine example is to be found at Kes Tor on Dartmoor.
Dartmoor represents one of the largest areas of exposed granite in the United Kingdom, covering an area of 368 square miles (954 square kilometres).[4] It is part of a chain of granite stretching through Cornwall, as far as the Isles of Scilly.
Some of the more durable granite survived to form the rocky crowns of Dartmoor tors. One of the best known is at Haytor, on the eastern part of the moor, whose granite is of unusually fine quality and was quarried from the hillside below the tor during the 19th and early 20th centuries. Its stone was used to construct the pillars outside the British Museum in London, and to build London Bridge (now in Arizona). The last granite to be quarried there was used to build Exeter War Memorial in 1919.
Ten Tors is an annual weekend hike on Dartmoor.
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