Also known as a corrie or cwm, this is a circular, armchair-shaped hollow cut into bedrock during glaciation. The side and back walls are steep, but the front opens out downslope. Cirques may be up to 2 km across. The formation of cirques remains unclear. It is suggested that frost-shattered material falls into the bergschrund and contributes to the erosion of the cirque. During glaciation, ice is thickest in the centre of the cirque and is thought to undergo rotational slipping, thus overdeepening the cirque floor but merely riding over the low bar at the mouth. Fluvial erosion and large-scale slope failures can be important for the creation of hollows, which are then deepened during glaciation.
The sides and back are subject to intense physical weathering and it is suggested that freeze-thaw occurs at the base of the bergschrund, but such a crevasse could not be deep because ice becomes plastic when it is more than 40-60 m thick (according to the elevation). Since cirques can have back walls up to 1000 m high, freeze-thaw would not seem to be the major formative process. Cirques seem to grow by headward extension, biting back into the mountain mass until only arêtes or pyramidal peaks remain.
In the temperate zones of the Northern Hemisphere, cirques face outward in directions between north-east and south-east. This is due in part to decreased insolation on north-east-facing slopes and in part to the build-up of drifted snow in the lee of westerly winds. Cirques are widely distributed in glacially eroded uplands, examples including Cwm Idwal, Snowdonia, and the cirque containing the Schwarzensee in the Zillertal Alps. There are no cirques, however, in those parts of glaciated mountains that were completely submerged by ice.
