Cirque

For other uses, see Cirque (disambiguation).

For the erosion cirques found in the Israeli and Egyptian deserts, see Makhtesh.

Upper Thornton Lake Cirque in North Cascades National Park

A cirque (French for "circus") is an amphitheatre-like valley, or valley head, formed at the head of a glacier by erosion. A cirque also is 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" often is found at the end of placenames such as Newcomb and Maycomb, where it is pronounced, /kəm/.

Formation

Formation of cirque and resulting tarn

A cirque is a landform found among mountains as a result of alpine glaciers. They may be up to a square kilometre in size, situated high on a mountainside near the firn line, and typically are partially surrounded on three sides by steep cliffs. The highest cliff often is called a headwall. The fourth side is the "lip," the side at which the glacier flowed away from the cirque. Many glacial cirques contain 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 freeze-thaw weathering and glacial erosion. The freeze-thaw cycle erodes at the lower rocks and causes it to disintegrate, which may result in an avalanche bringing down more snow and rock to add to the growing glacier. Eventually, this hollow may become large enough that glacial erosion intensifies. Debris (or till) in the ice also may 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.

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 shape in the side of the mountain, with the headwall being weathered by constant freezing and thawing, and as well as being 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 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 freeze-thaw mechanisms to occur.

Lake Seal, Mt. Field National Park - a cirque formed from a glacier is visible in the walls around lake seal

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.

Notable cirques

• Blue Lake Cirque, New South Wales, Australia
• Mount Ashland, Oregon, United States
• Chandra Taal, Himachal Pradesh, India
• Circo de Gredos, Sierra de Gredos, Spain
• Cirque de Gavarnie, Pyrenees, France
• Cirque d'Estaubé, Pyrenees, France
• Cirque de Navacelles, Massif Central, France
• Cirque of the Towers, Wyoming, United States
• Cirque Valley, Hindu Kush, Pakistan
• Great Basin, Maine, United States
• Iceberg Cirque, Montana, United States
• Ramon Crater, Israel
• Śnieżne Kotły, Karkonosze, Poland
• Tuckerman Ravine, New Hampshire, United States
• Western Cwm, Khumbu Himal, Nepal

See also

• Glacial landforms

External links

• Photographs and case study of corrie glaciers