Ator, which is also known bygeomorphologists as either acastle koppie orkopje, is a large, free-standingrockoutcrop that rises abruptly from the surrounding smooth and gentle slopes of a rounded hillsummit orridge crest. In theSouth West of England, the term is commonly also used for the hills themselves – particularly the high points ofDartmoor inDevon andBodmin Moor inCornwall.[1]
Although English topographical names often have aCeltic etymology, theOxford English Dictionary lists nocognates to the Old English word in either theBreton orCornish languages (theScottish Gaelictòrr is thought to derive from the Old English word). It is therefore accepted that the English wordTor derives from theOld Welsh wordtẁrr ortwr,[2] meaning a cluster or heap.[3]
Tors are landforms created by theerosion andweathering of rock; most commonlygranites, but alsoschists,dacites,dolerites,ignimbrites,[4] coarsesandstones and others.[5] Tors are mostly less than 5 meters (16 ft) high. Manyhypotheses have been proposed to explain their origin and this remains a topic of discussion amonggeologists andgeomorphologists, andphysical geographers. It is considered likely that tors were created bygeomorphic processes that differed widely in type and duration according to regional and local differences in climate and rock types.[1][6]
For example, theDartmoor granite was emplaced around 280 million years ago. When the cover rocks eroded away it was exposed to chemical and physicalweathering processes.[7] Wherejoints are closely spaced, the large crystals in the granite readily disintegrate to form a sandyregolith known locally asgrowan. This is readily stripped off bysolifluction or surface wash when not protected by vegetation, notably during prolonged cold phases during theQuaternary ice ages –periglaciation.
Wherejoints happen to be unusually widely spaced, core blocks can survive and remain above the weathering surface, developing into tors. These can be monolithic, as atHaytor and Blackingstone Rock, but are more usually subdivided intostacks, often arranged in avenues. Each stack may include several tiers orpillows, 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 calledclitter orclatter. 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 28tors on Dartmoor showed that most are surprisingly young, less than 100,000 years of surface exposure, with none over 200,000 years old.[8] They probably emerged at the start of the last major ice age (Devensian). By contrast the Scottish Cairngorms, which is the other classic granite tor concentration in Britain, the oldest tors dated are between 200 and 675 thousand years of exposure, with even glacially-modified ones having dates of 100–150,000 years.[9]
