Cirque

From Wikipedia, the free encyclopedia

For other uses, see Cirque (disambiguation).

Iceberg Cirque in Glacier National Park, USA

The Lower Curtis Glacier, 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.

A cirque is an amphitheatre-like valley (or valley head) of glacial origin, formed by glacial erosion at the head of the glacier. Cirques are typically partially surrounded by steep cliffs. The highest cliff is often called a headwall. They are also known as a cwm in Wales, a coomb or coombe in England, and a corrie in Scotland and Ireland.

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.

Cirques form in conditions which are favorable for glacier formation - where snow can accumulate into a thick and large mass.

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.
Should three or more cirques erode toward one another, a pyramidal peak or glacial horn would be created.