Widecombe History Group Talk on Dartmoor Granite

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History Group August 2002

Peter Hirst gave an insight into the Geology of Dartmoor.

As this was a non-speaker evening Peter Hirst our Vice Chairman gave the meeting a short address to set the scene for the guided walk he was to give the Group, scheduled for Saturday 10th August at 12.00 noon beginning at Peek Hill.

He mentioned first the D.N.P. Exhibition currently being staged at The Postbridge Information Centre entitled ‘Dartmoor Rock’. The Exhibition is well worth a visit and there is also an audio visual presentation to witness.

We were taken on a ‘journey’ of some 500 million years!

All the rock mass that we see today has been recycled several times during the existence of this planet which is estimated to have been some 4600 million years. The rocks are weathered by wind, rain, water and other agents, form sand dunes, sediments, mud, clay etc, get compressed and layered and eventually due to the heat generated many miles below the surface, significant movement takes place, volcanic activity adds to this and the crust of the earth shifts and the pressure causes mountain ridges to develop. As the rock gets mixed with the molten masses it in turn melts and distortion takes place. As the upper cold skin begins to cool, the granite is formed by crystals of Felspar, Quartz and Mica in various degrees. The mud turns into slate and shale, the layers of sedimentary deposits give slate and limestone their particular qualities. The slate that was against the hot granite was baked and produce the type of stone found at Leigh Tor. Granite rocks are susceptible to weathering by all types of force and gets more rounded as time goes by. The pressure makes the rocks twist and this results in vertical cracks in the rock and when the pressure eases as the weathering takes place, horizontal cracks appear. These cracks all aid the weathering activity and so the process continues. Undergound the cracks fill with water and gasses and this carries with it many chemicals and as the water evaporates it leaves a layer on the sides of these fissures, of quartz and the other minerals that have over the recent centuries been the source of mining activities. Tin and Copper being the two most sought after in this area but the same applies to Iron, Arsenic, Lead and so on.

It is really the top couple of feet that interests us mainly, particularly the farming fraternity. As the rocks break up small stones fill the gaps between the larger rocks, these break down forming sand, grit and gravel. This in turn encourages vegetation, the early forms like lichen, the action of the plants help continue the process by extracting the minerals and the decomposed vegetation helps to produce a form of humus and so the whole process continues. Gradually these cracks hold extra water, the water freezes, the rocks then slide on the frozen ground beneath, and from many tors a ‘river of rocks’ can be found running down the sides of the tors. The effects and results of this will be seen on the walk scheduled for next Saturday. The book "Dartmoor Stones" by Stephen Woods is well worth reading to see the uses that Granite has been put to!

Richard Wells brought four of his watercolour painting depicting Tors for the meeting to see.

There is quite a lot of iron in the ground and the acidic rain absorbs some of this. Below water table level no air can get to the subsoil, above this, a layer of iron oxide forms and this curtails plant life as the minerals can not rise to the plant life ultimately for the benefit of livestock. The orange stain of iron hydroxide and can be seen by most rivers. The speed that granite cools, results in the size of the crystals, slow cooling means larger crystals while a more rapid cooling results in smaller crystals. Haytor granite is reputedly some of the best samples of granite to be obtained and was quarried and used for many important buildings. There is a story of the Island of Hern having a granite quarry that went into liquidation.

One of the main creditors of the company, The British Commercial Assurance Company, obtained the remainder of the lease. Some time after money was raised and the quarry reopened. Stone was exported from Hern under the name Hayter Granite Co, note the different spelling E instead of O, This inference had to stop after about 12 years of trading. For thousands of years granite has been used for buildings, menhirs, standing stones etc and then, when man was able to use iron tools, the shaping of stones began leading to troughs, rollers, saddle-stones, mullions for windows and so on. The mention of ‘logan stones’ created by weathering, led to the sad stories of how some have been deliberately tipped off their rocking base, pointless destruction. Many small quarries were opened and used for specific purposes like rail or road ballast, nearby buildings (the Princetown Prison Quarry for example). Granite is porous in spite of its hardness, and when building with granite, cement is not ideal as it cracks with time and permits water to seep in, lime mortar is better as it swells when damp and tends to create a better seal, when that dries out any water can seep out again. It was noted that granite headstones do not hold their inscriptions as well as slate. Weathering is the cause of that too. The radio active qualities of granite was mentioned and Hermon French was again mentioned. Once he had a specimen of stone that was so radio active that it was able to take a photo of itself onto a photographic plate. Hermon’s ability as a geologist was also mentioned when he was once given a stone found in a consignment of fertilizer from Czechoslovakia and he decided correctly that it was not known in Britain. Peter finished by stating that the colouring agent in the green glass produced at Meldon years ago was Uranium. Now there’s a thought!

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