A tor, which is also known by geomorphologists as either a castle koppie or kopje, is a large, free-standing 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, the term is commonly also used for the hills themselves - particularly the high points of Dartmoor in Devon and Bodmin Moor in Cornwall.
Tors are landforms created by the erosion and weathering of rock; most commonly granites, but also schists, dacites, dolerites, ignimbrites, coarse sandstones and others. Tors are mostly less than 5 meters (16 ft) high. Many hypotheses have been proposed to explain their origin and this remains a topic of discussion among geologists and geomorphologists, and physical geographers. It is considered likely that tors were created by geomorphic processes that differed widely in type and duration according to regional and local differences in climate and rock types.
For example, the Dartmoor granite was emplaced around 280 million years ago, with the cover rocks eroded away soon afterwards, exposing it to chemical and physical weathering processes. Where joints are closely spaced, the large crystals in the granite readily disintegrate to form a sandy regolith known locally as growan. This is readily stripped off by solifluction or surface wash when not protected by vegetation, notably during prolonged cold phases during the Quaternary ice ages - periglaciation.
Where joints happen to be unusually widely spaced, core blocks can survive and escape above the weathering surface, developing into tors. These can be monolithic, as at Haytor and Blackingstone Rock, but are more usually subdivided into stacks, often arranged in avenues. Each stack can comprise several tiers or pillows, which may become separated: rocking pillows are called logan stones. These stacks are vulnerable to frost action and often collapse leaving trails of blocks down the slopes called clitter or clatter. Weathering has also given rise to circular "rock basins" formed by the accumulation of water and repeated freezing and thawing. An example is found at Kes Tor on Dartmoor.
Dating of 28 tors on Dartmoor showed that most are surprisingly young, less than 100,000 years old, with none over 200,000 years old. They probably emerged at the start of the last major ice age (Devensian). By contrast, in the Scottish Cairngorms which is the other classic granite tor concentration in Britain, the oldest tors dated are between 200 and 675,000 years old, with even glacially-modified ones having dates of 100-150,000 years. This may reflect a dryer, more arctic climate.