Editing Cirque (section)

==Formation==
[[File:Glacial Tarn Formation EN.svg|upright=1.2|thumb|Formation of cirque and resulting [[Tarn (lake)|tarn]]]]
[[File:Marichin_cirkus_IMG_1452.jpg|thumb|[[Maritsa]] cirque in [[Rila]] Mountain, [[Bulgaria]]]]

===Glacial-erosion===
Glacial cirques are found amongst mountain ranges throughout the world; 'classic' cirques are typically about one kilometer long and one kilometer wide. Situated high on a mountainside near the [[firn line]], they are typically partially surrounded on three sides by steep [[cliff]]s. The highest cliff is often called a [[headwall]]. The fourth side forms the ''lip'', ''threshold'' or ''sill'',<ref name="Chorley">{{cite book|last=Evans|first=I.S.|title=Introduction to fluvial processes|editor=Chorley R.J. & Carson M.A.|publisher=Routledge|year=1971|series=University paperbacks|volume=407|page=218|chapter=8.11(i) The geomorphology and Morphometry of Glacial and Nival Areas|isbn=978-0-416-68820-7 |chapter-url=https://books.google.com/books?id=X_ENAAAAQAAJ&q=cirque+geomorphology&pg=PA157|access-date=2010-01-24}}</ref> the side at which the glacier flowed away from the cirque. Many glacial cirques contain [[tarn (lake)|tarns]] dammed by either till (debris) or a bedrock threshold. When enough snow accumulates, it can flow out the opening of the bowl and form valley glaciers which may be several kilometers long.

Cirques form in conditions which are favorable; in the Northern Hemisphere the conditions include the north-east slope, where they are protected from the majority of the Sun's energy and from the prevailing winds. These areas are sheltered from heat, encouraging the accumulation of snow; if the accumulation of snow increases, the snow turns into glacial ice. The process of [[nivation]] follows, whereby a hollow in a slope may be enlarged by [[ice segregation]] weathering and glacial erosion. Ice segregation erodes the vertical rock face and causes it to disintegrate, which may result in an avalanche bringing down more snow and rock to add to the growing glacier.<ref name=Sanders2012 /> Eventually, this hollow may become large enough that glacial erosion intensifies. The enlarging of this open ended concavity creates a larger leeward deposition zone, furthering the process of glaciation. Debris (or till) in the ice also may [[Abrasion (geology)|abrade]] the bed surface; should ice move down a slope it would have a 'sandpaper effect' on the [[bedrock]] beneath, on which it scrapes.

[[File:Lowercurtis.jpg|thumb|The [[Lower Curtis Glacier]] in [[North Cascades National Park]] is a well-developed [[cirque glacier]]; if the glacier continues to retreat and melt away, a lake may form in the basin]]

Eventually, the hollow may become a large [[Bowl (vessel)|bowl]] shape in the side of the mountain, with the headwall being weathered by ice segregation, and as well as being eroded by [[Plucking (glaciation)|plucking]]. The basin will become deeper as it continues to be eroded by ice segregation and abrasion.<ref name=Sanders2012 /><ref name=Rempel2001 /> Should ice segregation, plucking and abrasion continue, the dimensions of the cirque will increase, but the proportion of the landform would remain roughly the same. A [[bergschrund]] forms when the movement of the glacier separates the moving ice from the stationary ice, forming a crevasse. The method of erosion of the headwall lying between the surface of the glacier and the cirque's floor has been attributed to freeze-thaw mechanisms. The temperature within the bergschrund changes very little, however, studies have shown that ice segregation (frost shattering) may happen with only small changes in temperature. Water that flows into the bergschrund can be cooled to freezing temperatures by the surrounding ice, allowing [[Frost weathering|freeze-thaw]] free mechanisms to occur.

[[File:Lake Seal Mt Field NP edit.jpg|thumb|left|Lake Seal, [[Mt. Field National Park]], [[Tasmania]] – a cirque formed from a glacier is visible in the walls around Lake Seal<ref>{{cite web|url=http://www.parks.tas.gov.au/file.aspx?id=6757|title=Mt Field National Park: Landforms, Flora and Fauna|publisher=Parks and Wildlife Service Tasmania|access-date=2009-05-12|archive-url=https://web.archive.org/web/20110609013854/http://www.parks.tas.gov.au/file.aspx?id=6757|archive-date=2011-06-09}}</ref>]]

If two adjacent cirques erode toward one another, an [[arête]], or steep sided ridge, forms. When three or more cirques erode toward one another, a [[pyramidal peak]] is created. In some cases, this peak will be made accessible by one or more arêtes. The [[Matterhorn]] in the European [[Alps]] is an example of such a peak.

Where cirques form one behind the other, a [[cirque stairway]] results, as at the [[Zastler Loch]] in the [[Black Forest]].

As glaciers can only originate above the snowline, studying the location of present-day cirques provides information on past glaciation patterns and on climate change.<ref name=Barr2015 />

===Fluvial-erosion===
{{Further|Steephead valley|Makhtesh}}
[[File:BoutDuMonde2.jpg|thumb|The [[Cirque du Bout du Monde (Côte d'Or)|Cirque du Bout du Monde]]]]
Although a less common usage,{{refn|This concern is not new, see Evans, I.S. & N. Cox, 1974: ''Geomorphometry and the operational definition of cirques,'' Area. Institute of British Geographers, 6: 150–53 regarding term usage.|group="nb"}} the term cirque is also used for amphitheatre-shaped, fluvial-erosion features. For example, an approximately {{convert|200|km2|mi2}} anticlinal erosion cirque is at {{coord|30|35|N|34|45|E|name=Negev anticlinal erosion cirque}} on the southern boundary of the [[Negev|Negev highlands]]. This erosional cirque or ''{{Transliteration|he|[[makhtesh]]}}'' was formed by intermittent river flow in the [[Makhtesh Ramon]] cutting through layers of limestone and chalk, resulting in cirque walls with a sheer {{convert|200|m|ft}} drop.<ref name=Ward2000 /> The [[Cirque du Bout du Monde (Côte d'Or)|Cirque du Bout du Monde]] is another such feature, created in [[karst]] terraine in the [[Burgundy (region)|Burgundy region]] of the department of {{Lang|fr|[[Côte-d'Or]]|italic=no}} in [[France]].

Yet another type of fluvial erosion-formed cirque is found on [[Réunion|Réunion island]], which includes the tallest volcanic structure in the [[Indian Ocean]]. The island consists of an active shield-volcano ({{Lang|fr|[[Piton de la Fournaise]]|italic=no}}) and an extinct, deeply eroded volcano ([[Piton des Neiges]]). Three cirques have eroded there in a sequence of agglomerated, fragmented rock and volcanic [[breccia]] associated with [[Pillow lava|pillow lavas]] overlain by more coherent, solid lavas.<ref name=Upton1969 />

A common feature for all fluvial-erosion cirques is a terrain which includes erosion resistant upper structures overlying materials which are more easily eroded.
[[File:Western Cwm and Lhotse.jpg|thumb|The [[Western Cwm]] with the [[Lhotse]] face of [[Mount Everest]] in the background]]