This began the lengthy task of shaping the tor formations.Īround about 60 – 30 million years ago Dartmoor was experiencing a sub-tropical climate and was covered with trees and vegetation. You only have to drive across Lee Moor or walk up to Redlake to see the extent of this process. As these are one of the major components of granite (30-40%) this process resulted the formation of a white clay called kaolin. This occurred when hot water which had been heated from within the granite by natural radiation, began to circulate and break down the feldspar minerals. The granite was then over a lengthy period of timed weakened by the process of kaolinisation. The joints tend to follow the topography of the surrounding land insomuch as on the hilltop tors the joints run horizontally and in the valley tors they may be inclined. This movement started the formation of the horizontal joints, a good example being that of Chat tor in the photograph above. In geological time scales this layer was eroded away fairly quickly and once the pressure of this overlying rock was removed the granite began to expand upwards. The granite was covered by a layer of mostly slate and sandstone which was between 2 – 3 km thick. The orientation of the joint pattern was controlled by two things, the pressure in the earth’s crust and the actual movements along faults. At the same time hot water flowed through these joints and deposited a lining of quartz and tourmaline crystals in them. When the still hot granite was still hot but solid the contraction caused vertical open fractures to appear and eventually form the distinct ‘joints’. There are three main types of mineral to be found in granite, Feldspar, Biotite, and Quartz, this combination gives the Dartmoor Granite its distinctive hues of colour. As this process took place the minerals which make up granite crystallised as tightly knit grains which gave the rock its natural hardness. This was when the molten granite which was at a temperature of between 9✬ cooled and began to solidify. So how did the tors come to be? For the beginnings we shall have to delve deep into the annals of time to around about 280 million years ago. The natural weathering of the tors produces secondary features such as rock-basins, logan stones, clitters, and tolmen stones. The majority of tors consist of granite however there are some on the moor edges that are of greenstone, elvan, and quartz-schorl. The definition of a tor is “a prominent outcrop of rock that can occur at any point between valley floor and hilltop”. However, on Dartmoor things are a bit different, some say the word derived from the Celtic word twr which meant a ‘tower’. “ a hill or rocky peak, whose origin is the Old English word torr, perhaps of a Celtic origin and related to Welsh tor ‘belly’ and Scottish Gaelic tórr ‘bulging hill‘. Mainly because what can be classified as an actual tor? There are the obvious massive granite outcrops which clearly can be described as a tor but then there are the tiny eroded remains that today exist as nothing more than a few boulders. I have also heard the question asked, “how many tors are there”? again that is a difficult one to answer. I can remember once stood in a Dartmoor Park Visitors Centre and hearing an overseas visitor ask, “who built the tors?”, well I think if we knew the answer to that the world would be a much better place than it is today. This is hardly surprising as it is the literal bedrock of the moor and what we see today are mere vestiges of what once existed. I don’t think I would be far wrong in suggesting that there are very few places on Dartmoor from which a tor of some kind is not visible.
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