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cirque
A cirque (French, from the Latin word ''circus'') is an amphitheatre-like valley formed by glacial erosion. Alternative names for this landform are corrie (from Scottish Gaelic ''coire'' meaning a pot or cauldron) and cwm (Welsh for "valley", pronounced ''coom''). A cirque may also be a similarly shaped landform arising from fluvial erosion.
The concave amphitheatre shape is open on the downhill side corresponding to the flatter area of the stage, while the cupped seating section is generally steep, cliff-like slopes down which ice and glaciated debris combine and converge from the three or more higher sides. The floor of the cirque ends up bowl-shaped as it is the complex convergence zone of combining ice flows from multiple directions and their accompanying rock burdens, hence it experiences somewhat greater erosion forces, and is most often overdeepened below the level of the cirque's low-side outlet (stage) and its down slope (backstage) valley. If the cirque is subject to seasonal melting, the floor of the cirque most often forms a tarn (small lake) behind the moraine, glacial till or bedrock lip marking the downstream limit of glacial overdeepening of the basin, which serves as a dam at the outlet.〔
Another form of cirque or makhtesh, found in karst landscapes, is formed by intermittent river flow cutting through layers of limestone and chalk leaving sheer cliffs. A common feature for all fluvial-erosion cirques is a terrain which includes erosion resistant upper structures overlying materials which are more easily eroded.
==Formation==
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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