CIRQUE, a French word used to denote a semi-circular amphitheatre, with precipitous walls, at the head of a valley in a glaciated mountain region (Lat., circus, ring), generally due to the basal sapping and erosion beneath the bergschrund of a glacier. The bergschrund is a large crevasse, in the form of a great sym metrical arc, parallel to the head of the neve (see GLACIER) ; it lies at a short distance from the exposed rock surface and sep arates the stationary from the moving ice, and in early summer, when the glacier commences to move, it opens and exposes the rock at its base to diurnal changes of temperature. Frost action then causes rapid disintegration downwards at its base and back wards upon such part of the rock surface as is exposed in the bergschrund beneath the stationary ice, thus producing the characteristic form of the cirque.
The formation of cirques has played an important part in the development of the scenery of glaciated mountain tracts. Aretes (sharp ridges) are formed by the intersection of two cirques, and pyramid-like peaks such as the Matterhorn and Snowdon are remnants left by the recession of three or more cirques. Cirques frequently contain lakes, for, owing to the action of the bergschrund, the floor slopes toward the mountain mass. W. D. Johnston (Journ. of Geol. vol. xii. p. 569) first recognized the processes giving rise to cirque formation by actually descending a bergschrund on the Mount Lyell glacier. Hollows of similar shape to cirques occur in limestone regions which may not have been glaciated. These are formed by aqueous solution and are not true cirques.