Tor (rock formation)
|This article needs additional citations for verification. (November 2006)|
A tor, which is also known by geomorphologists as either a castle koppie or kopje, is a large, free-standing residual mass (rock outcrop) that rises abruptly from the surrounding smooth and gentle slopes of a rounded hill summit or ridge crest. In the South West of England, the term is commonly also used for the hills themselves – particularly the high points of Dartmoor in Devon and Bodmin Moor in Cornwall.
The word tor (Cornish: tor, Old Welsh: twrr, Welsh: tŵr, Scottish Gaelic: tòrr), meaning hill, is notable for being one of the very few Celtic loanwords to be borrowed into vernacular English before the modern era – such borrowings are mainly words of a geographic or topographical nature. Another word is crag (from Welsh craig "rock").
Tors are most common and most well known as a landform that was created by the erosion and weathering of granitic rocks. However, tor have also developed by the erosion and weathering of schists, dacites, dolerites, and coarse sandstones among other rock types. Tors are mostly less than 5 meters (16 ft) high. A variety of hypotheses have been proposed to explain the origin of tors and their origin remains a topic of continuing discussion among geologists and geomorphologists, and physical geographers. It is considered likely that tors were created by geomorphic processes that differed widely in type and duration according to regional and local differences in climate and rock types.
For example the Dartmoor granite was emplaced around 280 million years ago, with the cover rocks stripped off soon afterwards, exposing it to chemical and physical weathering processes. Where joints are closely spaced, the large crystals in the granite readily disintegrate to form a sandy regolith known locally as growan. This is readily stripped off by solifluction or surface wash when not protected by vegetation, notably during prolonged cold phases during the Quaternary ice ages - periglaciation. Where joints happen to be unusually widely spaced, core blocks can survive and escape above the weathering surface, developing into tors. These can be monolithic, as at Haytor and Blackingstone Rock, but are more usually subdivided into stacks, often arranged in avenues. Each stack can comprise several tiers or pillows, which may become separated: rocking pillows are called logan stones. These stacks are vulnerable to frost action, often collapsing, with trails of blocks called clitter down the slopes. Weathering has also given rise to circular "rock basins" formed by the accumulation of water and the repeated freezing and thawing – a fine example is to be found at Kes Tor on Dartmoor. Dating of 28 Dartmoor tors shows that most are surprisingly young, less than 100,000 years old, with none over 200,000 years old. They probably emerged at the start of the last big ice age (Devensian). By contrast in the Scottish Cairngorms, the other classic granite tor concentration in Britain, the oldest tors dated are between 200 and 675,000 years old, with even glacially-modified ones having dates of 100-150,000 years. This may reflect a dryer, more arctic climate.
Tors in Great Britain
Dartmoor represents one of the largest areas of exposed granite in the United Kingdom, covering an area of 368 square miles (954 square kilometres). It is part of a chain of granite stretching through Cornwall, as far as the Isles of Scilly.
Some of the more durable granite survived to form the rocky crowns of Dartmoor tors. One of the best known is at Haytor, on the eastern part of the moor, whose granite is of unusually fine quality and was quarried from the hillside below the tor during the 19th and early 20th centuries. Its stone was used to construct the pillars outside the British Museum in London, and to build London Bridge. The last granite to be quarried there was used to build Exeter War Memorial in 1919.
- Alex Tor, Bodmin Moor
- Hawk's Tor, Bodmin Moor
- Helman Tor, mid–Cornwall
- Rough Tor, Bodmin Moor
- Showery Tor, Bodmin Moor
There are many tors in this area, notably in the Dark Peak where the host rock is Millstone Grit:
- Back Tor, Derwent Edge (538m)
- Carl Wark, Hathersage Moor
- Dovestone Tor, Derwent Edge (505m)
- Great Tor, Bamford
- Higger Tor (384m) and Over Owler Tor (375m)
- Howshaw Tor overlooking Sheffield
- Ladybower Tor, Upper Derwent Valley
- Low Tor, Bradfield Moors
- The Salt Cellar on Derwent Edge
- Whinstone Lee Tor
- White Tor, Derwent Moors (487m)
- Tor Raynor Broomhill, Sheffield
- Almscliffe Crag west of Harrogate
- Cow and Calf, Ilkley Moor in West Yorkshire. Made from Millstone Grit.
There are numerous tors developed in the Cairngorm granite in the Scottish Highlands:
Tors in other countries
- Externsteine in the Teutoburg Forest, North Rhine-Westphalia
- Greifensteine in the Ore Mountains of Saxony
- Großer Waldstein in the Fichtelgebirge mountains of Bavaria
- Heinrichshöhe and Hohneklippen in the Harz mountains of central Germany
- Wolfenstein in Bavaria
- Angel Rocks and Granite Tors, Chena River State Recreation Area, Alaska
- Elephant Rocks in Missouri
- Apache Leap Formation in Superior, Arizona
- Texas Canyon in Arizona
- Kit-Mikayi, Kenya
- List of peaks of the Peak District
- The book Red Shift (novel) the Mow Cop Tor at the centre of its plot.
|Wikimedia Commons has media related to Tors.|
- Ehlen, J. (2004) Tor in Goudie, A., ed., pp. 1054-1056. Encyclopedia of Geomorphology. Routledge. London, England.
- Crossing's Guide to Dartmoor, 1912 Edition, 1965 Reprint (David & Charles, Newton Abbot)
- Twidale, C.R., and J.R.V. Romani (2005) Landforms and Geology of Granite Terrains. A.A. Balkema Publishers Leiden, The Netherlands. 359 pp. ISBN 978-0415364355
- "Dartmoor Factsheet: Tor Formation". Dartmoor National Park. 2002. Retrieved 21 December 2011.
- Gunnell, Y., Jarman, D. and 8 others, 2013. The granite tors of Dartmoor, Southwest England: rapid and recent emergence revealed by Late Pleistocene cosmogenic apparent exposure ages. Quaternary Science Reviews 612, 62-76
- Adrian Hall, New perspectives on a classic landscape of selective linear glacial erosion, The history of the Cairngorms: granite, landscape and processes, British Geological Survey
- Dartmoor National Park Authority website. Retrieved 12 October 2008
- Ordnance Survey 1:25,000 scale Explorer map sheets OL1 Dark Peak area & OL24 White Peak area
- Mercer, Ian (2009). "The Physical Anatomy of Dartmoor". Dartmoor – A Statement of its Time. London: Collins. pp. 30–78. ISBN 978-0-00-718499-6.