A tor is a rock outcrop formed by weathering, usually found on or near the summit of a hill. 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.
Tors are composed usually of granite or metamorphic rocks. Tors can also be found around any previously erupted volcanoes (although Devonian and Carboniferous outcrops are also found), though occasionally of other hard rocks such as quartzite, and are the result of millions of years of weathering. In prehistoric times, when the land was covered in forest, rain water seeped into the ground and gradually weathered the bedrock through its natural cracks, or joints. Once the land became exposed, the weathering was accelerated, particularly during the Ice age when freezing water expanded in the cracks. The result can be seen today in dramatic rock formations.
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.
As the weathering of the tors continues, the rock is broken down into ever smaller sizes. Many hillsides are covered with loose rocks, known as clitter, which 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.
The most distinctive granite landform in temperature countries is the tor and in tropical regions, the inselbergs. Both suggest the removal of material by solifluction and hence lead to the opinion that tors and inselbergs are relict features.
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 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.
For a full list of Dartmoor's tors see List of Dartmoor tors and hills.