The Tors of Dartmoor

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. 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 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. I have also heard the question asked, "how many tors are there"? again that is a difficult one to answer. 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.

The O.E.D. defines the word as meaning,

"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'.

However, on Dartmoor things are a bit different, some say the word derived from the Celtic word twr which meant a  'tower'. The definition of a tor is "a prominent outcrop of rock that can occur at any point between valley floor and hilltop". The majority of tors consist of granite however there are some on the moor edges that are of greenstone, elvan, and quartz-schorl. The natural weathering of the tors produces secondary features such as rock-basins, logan stones, clitters, and tolmen stones.

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. This was when the molten granite which was at a temperature of between 900 and 1000ºC cooled and began to solidify. As this process took place the minerals which make up granite crystallised as tightly knit grains which gave the rock its natural hardness. 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.

When the still hot granite was still hot but solid the contraction caused vertical open fractures to appear and eventually form the distinct 'joints'. At the same time hot water flowed through these joints and deposited a lining of quartz and tourmaline crystals in them. The orientation of the joint pattern was controlled by two things, the pressure in the earth's crust and the actual movements along faults.

The granite was covered by a layer of mostly slate and sandstone which was between 2 - 3 km thick. 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. This movement started the formation of the horizontal joints, a good example being that of Chat tor in the photograph above. 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.

The granite was then over a lengthy period of timed weakened by the process of kaolinisation. 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. As these are one of the major components of granite (30-40%) this process resulted the formation of a white clay called kaolin. You only have to drive across Lee Moor or walk up to Redlake to see the extent of this process. This began the lengthy task of shaping the tor formations.

Around about 60 - 30 million years ago Dartmoor was experiencing a sub-tropical climate and was covered with trees and vegetation. As the vegetation began to rot the rainwater percolated acidic water down through the granite. This resulted in further weakening of the rock, especially along the joints through which the water ran and once agin the erosional forces began to further shape the granite.

Between 2 million to 10,000 years ago Dartmoor was experiencing an Ice Age and this too began a process of erosion. Probably the most damaging of these forces was the 'freeze thaw' effect. This is where water lying in the joints is frozen and the expanding ice forces the granite apart and causes blocks of rock to fall away. This loose material then rolled off downhill, aided by the permanently frozen ground underneath. In warmer spells the frozen surface layers began to thaw and the resulting 'mudflow' would then carry granite debris of all shapes and sizes downhill. In some areas of Dartmoor huge blocks and boulders were carried up to 1 km during this process which eventually formed the clitters we see today. Eventually all of the loose material from around the cores of granite were eroded leaving us with the tors of today.

Ever since man has lived on Dartmoor he has utilised the granite in every conceivable way. Initially it made his home, cooked his food, enclosed his fields, and built his ritual monuments. This trend never ceased and in later years the granite made his roads, his buildings, and in some cases his fortune as the Quarry owners knew all too well. Today the tors bring in the visitors who along with many of the locals always have an irresistible urge to climb on top of a granite outcrop and stand and stare as if it was they had conquered their personal 'Everest'. They provide sheltered picnic spots, climbing challenges, hiding places for letterboxes, viewpoints, navigational points, and much much more.

So to return to the original question as to how many tors there are. William Crossing suggests 147 which when looking at the OS map is believable. Over the years I have been building a database of Dartmoor place-names and below are 433 names which have been used to name the tors of Dartmoor. That is not to say that there are 433 tors as many of them have been known as various names down through the ages and this list contains all the 'aliases' as well. Many of them no longer appear on the modern maps but they have all been used at some time or other. The sources I have used to compile this list includes maps, books, documents, and gazetteers of all ages. I believe this to be the most comprehensive list yet to be published and it is still growing. Only today I was reading a feature in the latest issue of the 'Dartmoor News' where it was saying that under the new CROW act there is now access to 'Pin Tor', a tor I had never come across

The Gazetteer of Dartmoor Tors

© Tim Sandles 2006/7