Rough Tor - Bodmin Moor
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Rough Tor - Bodmin Moor

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Bodmin Moor was intruded as magma into overlying sedimentary rocks some 270 million years ago. Rough Tor is owned by the National Trust and is aptly named. Tors occur in rocks other than granite and ...

Bodmin Moor was intruded as magma into overlying sedimentary rocks some 270 million years ago. Rough Tor is owned by the National Trust and is aptly named. Tors occur in rocks other than granite and are found the length and breadth of the British Isles. Their formation remains a matter of some controversy but they are undoubtedly an important facet of our landscape heritage.

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  • Tors form distinctive features in granite uplands and mountains in the British Isles. Tors are particularly common on crystalline rocks but are also found on other resistant rocks such as quartzites and some sandstones.
  • “If you climb over a tor, remember that you probably perched on remnants of rock more resistant to the potent pressures exerted by water freezing in cracks than the surrounding rocks. What then determines whether a rock will be resistant?”Rita Gardner, Landshapes, 1988.
  • “The most likely cause lies in the spacing of the joints, or fractures in the granite. Where the joints are closely spaced, the rock is more easily shattered into small fragments that can be carried away in water during periods of thaw. Where the joints are widely spaced, the rock is more resistant to this form of attack.” Rita Gardner, Landshapes, 1988.
  • The joints formed in this granite mass as the original molten material cooled, contracted and solidified millions of years ago. Further development of the fractures took place as the weight of the overlying crustal rocks was slowly removed by erosion, releasing the pressure on the granite mass beneath.
  • The origin of tors has been the subject of much controversy. There are two main theories as to their formation. Linton’s ideas cannot be applied to rocks which are not subject to extreme chemical decomposition such as quartzite and sandstone.
  • Palmer and Neilson therefore came up with an alternative “periglacial” model, where frost shattering and solifluction account for the main constituents of this rugged landscape.
  • Today some chemical weathering of the granite is still taking place. In places the surface of the granite is flaking off to expose fresh rock surfaces. Some very impressive minor weathering features have developed on the surface of the the tors. The edges of the summits for example, have a curious fluted pattern (called lapiés), with small ridges and troughs.
  • The flat tops of the tors often have rock basins developed in them. The basins are shaped like frying pans (20-60 cm in diameter) and have overflow channels running from them.
  • The rock in the bottom of these basins appears to be fresh, suggesting that this form of chemical weathering is still active today.
  • Granite consists of three main minerals: quartz, a grey translucent material, felspar with its long white crystals, and thin flakes of brown mica. Despite its toughness, the granite will weather when attacked by rain and frost and ultimately will completely disintegrate. When this happens the felspar forms a soft whitish clay while the insoluble residue of quartz crystals forms a gritty sand known as growan.
  • The summit of Rough Tor is crowned by several groups of tors. The tors rise up from clitter-covered slopes that radiate for as much as 1 km from the summit. Much of the clitter is now covered in lichens and embedded in turf and is clearly a fossil feature.
  • The clitter has been incorporated into iron age hut circles on the western slope of Rough Tor.
  • The moor is the domain of ponies and sheep now - a marginal farming area.

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