A cirque (French for "circus") is an amphitheatre-like valley, or valley head, formed at the head of a glacier by erosion. A cirque is also known as a coombe or coomb in England, a combe or comb in America, a corrie (coire) in Scotland and Ireland, and a cwm in Wales, although these terms apply to a specific feature of which several may be found in a cirque. The term "comb" is often found at the end of placenames such as Newcomb and Maycomb, where it is pronounced /kəm/.
A Cirque is a landform found in the mountains as a result of alpine glaciers. They can be up to a square kilometre in size, situated high on a mountain side near the firn line, 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. When enough snow accumulates it can flow out the opening of the bowl and form valley glaciers which can be several kilometers long.
Cirques form in conditions which are favorable; which in the northern hemisphere includes 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 freeze-thaw weathering and glacial erosion. The freeze-thaw erodes at the lower rocks and causes it to disintegrate, which can result in an avalanche bringing down more snow and rock to add to the growing glacier. 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. 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 frost shattering can happen with only small changes in temperature. Water that flows into the bergschrund can be cooled to freezing temperatures by the surrounding ice allowing freeze-thaw mechanisms to occur.
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.
As glaciers can only originate above the snowline, studying the location of present day cirques provides information on past glaciations patterns and climate change.