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Cirque

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Upper Thornton Lake Cirque in North Cascades National Park

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

Peñalara cirque, Sierra de Guadarrama, Spain

A cirque is an amphitheatre-like valley, or valley head, formed at the head of a glacier by erosion. A cirque is also known as a cwm in Wales, a coomb or coombe in England, and a corrie in Scotland and Ireland, although these terms apply to a specific feature of which several may be found in a cirque.

Cirques can be up to a square kilometre in size, situated high on a mountain side, and are typically partially surrounded on three sides by steep cliffs. The highest cliff is often called a headwall. The fourth side is the "lip" which is the side that the glacier flowed away from the cirque. Many glacial cirques contain tarns dammed by either till or a bedrock threshold.

Cirques form in conditions which are favorable; which in the northern hemisphere includes the north-east slope being in shade and away from prevailing winds. These areas are sheltered from heat, and so, they encourage the accumulation of snow. If the accumulation of snow increases, the snow transforms into glacial ice. The process of nivation follows (where a hollow in a slope may be enlarged by freeze-thaw weathering and glacial erosion). Eventually, this hollow can become big enough so that glacial erosion intensifies. Debris (or till) in the ice may also abrade (glacial abrasion) the bed surface; should ice move down a slope it would have a ‘sandpaper effect’ on the bedrock beneath on which it scrapes.

Eventually, the hollow can become a large bowl shape in the side of the mountain, with the headwall being weathered by constant freezing and thawing, and eroded by plucking. The basin will become deeper if it continues to become eroded by abrasion. Should plucking and abrasion continue, the dimensions of the cirque will increase, but the proportion of the landform would remain roughly the same.

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.