Editing Tor (rock formation)

{{Short description|Large, free-standing rock outcrop on a gentle hill summit}}
{{Use British English|date=October 2013}}
{{Use dmy dates|date=December 2024}}
[[File:Скалы Саввушки.JPG|thumb|right|A tor in [[Altai Krai]], southern [[Siberia]]]]

A '''tor''', which is also known by [[geomorphologist]]s as either a '''castle koppie''' or '''kopje''', is a large, free-standing [[rock (geology)|rock]] [[outcrop]] that rises abruptly from the surrounding smooth and gentle slopes of a rounded hill [[summit]] or [[ridge]] crest. In the [[South West England|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]].<ref name="Goudie2004a">Ehlen, J. (2004) ''Tor'' in Goudie, A., ed., pp. 1054-1056. ''Encyclopedia of Geomorphology''. Routledge. London, England.</ref>

==Etymology==
Although English topographical names often have a [[Celtic languages|Celtic]] etymology, the ''[[Oxford English Dictionary]]'' lists no [[cognate]]s to the Old English word in either the [[Breton language|Breton]] or [[Cornish language]]s (the [[Scottish Gaelic]] {{lang|gd|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 {{lang|owl|tẁrr}} or {{lang|owl|twr}},<ref name="Goudie">{{cite book |last1=Goudie |first1=Andrew |title=Encyclopedia of Geomorphology |date=2004 |publisher=Psychology Press |isbn=978-0-415-32738-1 |page=1054 |url=https://books.google.com/books?id=UHRU_6nUSR4C&q=Tor+rock+word&pg=PA1054 |access-date=21 June 2020 |language=en}}</ref> meaning a cluster or heap.<ref name="OED">{{cite web|title=tor, n.|publisher= [[OED Online]]. [[Oxford University Press]]|url=http://www.oed.com/view/Entry/203486|access-date=10 December 2013}} {{subscription required}}</ref>

==Formation==
[[Image:Kit-Mikayi.JPG|thumb|right|[[Kit-Mikayi]], a celebrated tor near [[Kisumu]], [[Kenya]]]]
Tors are landforms created by the [[erosion]] and [[weathering]] of rock; most commonly [[granite]]s, but also [[schist]]s, [[dacite]]s, [[dolerite]]s, [[ignimbrite]]s,<ref name=Aguilera>{{cite book |last1=Aguilera |first1=Emilia Y. |last2=Sato |first2=Ana María |last3=Llambías |first3=Eduardo |last4=Tickyj |first4=Hugo |date=2014|chapter=Erosion Surface and Granitic Morphology in the Sierra de Lihuel Calel, Province of La Pampa, Argentina |title=Gondwana Landscapes in southern South America |editor-last=Rabassa|editor-first=Jorge|editor-link=Jorge Rabassa|editor-last2=Ollier|editor-first2=Cliff|editor-link2=Cliff Ollier |publisher=Springer |pages=393–422 }}</ref> coarse [[sandstone]]s and others.<ref>{{Cite web|title=Tor {{!}} geology|url=https://www.britannica.com/science/tor-geology|access-date=2020-06-12|website=Encyclopædia Britannica|language=en}}</ref> Tors are mostly less than {{convert|5|m|ft|sp=us}} high. Many [[hypothesis|hypotheses]] have been proposed to explain their origin and this remains a topic of discussion among [[geologist]]s and [[geomorphologist]]s, and [[Physical geography|physical geographers]]. It is considered likely that tors were created by [[Geomorphology|geomorphic processes]] that differed widely in type and duration according to regional and local differences in climate and rock types.<ref name="Goudie2004a"/><ref name="TwidaleOthers2005a">[[Charles Rowland Twidale|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}}</ref>

For example, the [[Cornubian batholith|Dartmoor granite]] was emplaced around 280 million years ago. When the cover rocks eroded away it was exposed to chemical and physical [[weathering]] processes.<ref name=DF>{{cite web|url=http://www.nationalparks.gov.uk/dnpa_shop/images/mac1076.pdf|title=Dartmoor Factsheet: Tor Formation|year=2002|publisher=Dartmoor National Park|access-date=21 December 2011|url-status=dead|archive-url=https://web.archive.org/web/20110930170008/http://www.nationalparks.gov.uk/dnpa_shop/images/mac1076.pdf|archive-date=30 September 2011}}</ref> Where [[Joint (geology)|joint]]s 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 age]]s – [[periglaciation]].

[[File:Tor on Knocknagun, Wicklow, Ireland.jpg|thumb|Tor near the summit of [[Knocknagun]], in [[Wicklow]], Ireland]]

Where [[Joint (geology)|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 [[Stack (geology)|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 [[List of Dartmoor tors and hills|tors on Dartmoor]] showed that most are surprisingly young, less than 100,000 years of surface exposure, with none over 200,000 years old.<ref>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</ref> 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.<ref>Adrian Hall, [http://www.landforms.eu/cairngorms/Cairngorm_workshop.htm "New perspectives on a classic landscape of selective linear glacial erosion"]. ''The history of the Cairngorms: granite, landscape and processes''. [[British Geological Survey]]</ref>

== See also ==
* {{annotated link|Bornhardt}}
* {{annotated link|Etchplain}}
* {{annotated link|Inselberg}}
* {{annotated link|Nubbin (landform)}}
* {{annotated link|List of geographical tors}}

== References ==
{{Reflist}}

== Further reading ==
*{{cite book
| last=Mercer| first=Ian
| title=Dartmoor – A Statement of its Time
| publisher=Collins| location=London| year=2009
| chapter=The Physical Anatomy of Dartmoor
| pages=30–78
| isbn=978-0-00-718499-6}}

{{Commons category|Tors}}

[[Category:Rock formations]]
[[Category:Weathering landforms]]