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Tor (rock formation)

January 01, 1970

A tor, which is also known by geomorphologists as either a castle koppie or kopje, is a large, free-standing 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.[1]

A tor in Altai Krai, southern Siberia

Etymology edit

Although English topographical names often have a Celtic etymology, the Oxford English Dictionary lists no cognates to the Old English word in either the Breton or Cornish languages (the Scottish Gaelic tòrr is thought to derive from the Old English word). It is therefore accepted that the English word Tor derives from the Old Welsh word tẁrr or twr,[2] meaning a cluster or heap.[3]

Formation edit

 
Kit-Mikayi, a celebrated tor near Kisumu, Kenya

Tors are landforms created by the erosion and weathering of rock; most commonly granites, but also schists, dacites, dolerites, ignimbrites,[4] coarse sandstones and others.[5] Tors are mostly less than 5 meters (16 ft) high. Many hypotheses have been proposed to explain their origin and this remains a topic of 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.[1][6]

For example, the Dartmoor granite was emplaced around 280 million years ago. When the cover rocks eroded away it was exposed to chemical and physical weathering processes.[7] 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.

 
Tor near the summit of Knocknagun, in Wicklow, Ireland

Where joints happen to be unusually widely spaced, core blocks can survive and remain 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 may include several tiers or pillows, which may become separated: rocking pillows are called logan stones. These stacks are vulnerable to frost action and often collapse leaving trails of blocks down the slopes called clitter or clatter. Weathering has also given rise to circular "rock basins" formed by the accumulation of water and repeated freezing and thawing. An example is found at Kes Tor on Dartmoor.

Dating of 28 tors on Dartmoor showed that most are surprisingly young, less than 100,000 years of surface exposure, with none over 200,000 years old.[8] They probably emerged at the start of the last major ice age (Devensian). By contrast the Scottish Cairngorms, which is the other classic granite tor concentration in Britain, the oldest tors dated are between 200 and 675 thousand years of exposure, with even glacially-modified ones having dates of 100–150,000 years.[9]

See also edit

References edit

  1. ^ a b Ehlen, J. (2004) Tor in Goudie, A., ed., pp. 1054-1056. Encyclopedia of Geomorphology. Routledge. London, England.
  2. ^ Goudie, Andrew (2004). Encyclopedia of Geomorphology. Psychology Press. p. 1054. ISBN 978-0-415-32738-1. Retrieved 21 June 2020.
  3. ^ "tor, n." OED Online. Oxford University Press. Retrieved 10 December 2013. (subscription required)
  4. ^ Aguilera, Emilia Y.; Sato, Ana María; Llambías, Eduardo; Tickyj, Hugo (2014). "Erosion Surface and Granitic Morphology in the Sierra de Lihuel Calel, Province of La Pampa, Argentina". In Rabassa, Jorge; Ollier, Cliff (eds.). Gondwana Landscapes in southern South America. Springer. pp. 393–422.
  5. ^ "Tor | geology". Encyclopedia Britannica. Retrieved 12 June 2020.
  6. ^ Twidale, C. R., and Romani, J. R. V. (2005) Landforms and Geology of Granite Terrains. A. A. Balkema Publishers Leiden, The Netherlands. 359 pp. ISBN 978-0415364355
  7. ^ (PDF). Dartmoor National Park. 2002. Archived from the original (PDF) on 30 September 2011. Retrieved 21 December 2011.
  8. ^ 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
  9. ^ 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

Further reading edit

  • 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.
 
Wikimedia Commons has media related to Tors.

January 01, 1970
rock, formation, which, also, known, geomorphologists, either, castle, koppie, kopje, large, free, standing, rock, outcrop, that, rises, abruptly, from, surrounding, smooth, gentle, slopes, rounded, hill, summit, ridge, crest, south, west, england, term, commo. A tor which is also known by geomorphologists as either a castle koppie or kopje is a large free standing 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 1 A tor in Altai Krai southern Siberia Contents 1 Etymology 2 Formation 3 See also 4 References 5 Further readingEtymology editAlthough English topographical names often have a Celtic etymology the Oxford English Dictionary lists no cognates to the Old English word in either the Breton or Cornish languages the Scottish Gaelic torr is thought to derive from the Old English word It is therefore accepted that the English word Tor derives from the Old Welsh word tẁrr or twr 2 meaning a cluster or heap 3 Formation edit nbsp Kit Mikayi a celebrated tor near Kisumu Kenya Tors are landforms created by the erosion and weathering of rock most commonly granites but also schists dacites dolerites ignimbrites 4 coarse sandstones and others 5 Tors are mostly less than 5 meters 16 ft high Many hypotheses have been proposed to explain their origin and this remains a topic of 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 1 6 For example the Dartmoor granite was emplaced around 280 million years ago When the cover rocks eroded away it was exposed to chemical and physical weathering processes 7 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 nbsp Tor near the summit of Knocknagun in Wicklow Ireland Where joints happen to be unusually widely spaced core blocks can survive and remain 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 may include several tiers or pillows which may become separated rocking pillows are called logan stones These stacks are vulnerable to frost action and often collapse leaving trails of blocks down the slopes called clitter or clatter Weathering has also given rise to circular rock basins formed by the accumulation of water and repeated freezing and thawing An example is found at Kes Tor on Dartmoor Dating of 28 tors on Dartmoor showed that most are surprisingly young less than 100 000 years of surface exposure with none over 200 000 years old 8 They probably emerged at the start of the last major ice age Devensian By contrast the Scottish Cairngorms which is the other classic granite tor concentration in Britain the oldest tors dated are between 200 and 675 thousand years of exposure with even glacially modified ones having dates of 100 150 000 years 9 See also editBornhardt A large dome shaped steep sided bald rock Etchplain Plain where the bedrock has been subject to considerable subsurface weathering Inselberg Isolated steep rock hill on relatively flat terrain Nubbin landform Small hill of bedrock with rounded residual blocks List of geographical torsReferences edit a b Ehlen J 2004 Tor in Goudie A ed pp 1054 1056 Encyclopedia of Geomorphology Routledge London England Goudie Andrew 2004 Encyclopedia of Geomorphology Psychology Press p 1054 ISBN 978 0 415 32738 1 Retrieved 21 June 2020 tor n OED Online Oxford University Press Retrieved 10 December 2013 subscription required Aguilera Emilia Y Sato Ana Maria Llambias Eduardo Tickyj Hugo 2014 Erosion Surface and Granitic Morphology in the Sierra de Lihuel Calel Province of La Pampa Argentina In Rabassa Jorge Ollier Cliff eds Gondwana Landscapes in southern South America Springer pp 393 422 Tor geology Encyclopedia Britannica Retrieved 12 June 2020 Twidale C R and Romani J R V 2005 Landforms and Geology of Granite Terrains A A Balkema Publishers Leiden The Netherlands 359 pp ISBN 978 0415364355 Dartmoor Factsheet Tor Formation PDF Dartmoor National Park 2002 Archived from the original PDF on 30 September 2011 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 SurveyFurther reading editMercer Ian 2009 The Physical Anatomy of Dartmoor Dartmoor A Statement of its Time London Collins pp 30 78 ISBN 978 0 00 718499 6 nbsp Wikimedia Commons has media related to Tors Retrieved from https en wikipedia org w index php title Tor rock formation amp oldid 1177764402, wikipedia, wiki, book, books, library,

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