Tor (rock formation)

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

The word tor is an English word referring to "a bare rock mass surmounted and surrounded by blocks and boulders", deriving from the Old English torr.[2][note 1][3]

Formation

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, with the cover rocks eroded away soon afterwards, exposing it 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 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 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 old, with none over 200,000 years old.[8] They probably emerged at the start of the last major ice age (Devensian). By contrast, in the Scottish Cairngorms which is 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.[9] This may reflect a dryer, more arctic climate.

See also

• Bornhardt – A large dome-shaped, steep-sided, bald rock
• Etchplain – A plain where the bedrock has been subject to considerable subsurface weathering
• Inselberg – Isolated rock hill or small mountain that rises abruptly from a relatively flat surrounding plain
• Nubbin (landform) – A small and gentle hill consisting of a bedrock core dotted with rounded residual blocks.
• List of geographical tors – Wikipedia list article

Notes

1. Tor is often considered to have a Celtic etymology, but the Oxford English Dictionary lists no match in Cornish or Breton; the nearest Celtic word is the Welsh tẁr, from the Old Welsh tẁrr. The Old English torr is likely cognate with the Scottish Gaelic tòrr.[3]

References

1. 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 J.R.V. Romani (2005) Landforms and Geology of Granite Terrains. A.A. Balkema Publishers Leiden, The Netherlands. 359 pp. ISBN 978-0415364355
7. "Dartmoor Factsheet: Tor Formation" (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

• 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.