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A cirque is a deep, steepsided recess, roughly semi-circular in plan, cut into slope by erosion beneath a bank of firn (snow more than one-year old) or a glacier usually at the head of a valley. Although a cirque usually forms in half-bowl or amphitheater shape, the actual shape can vary depending on the preglacial conditions of the valley, duration of glaciation, and the rock type.
A cirque has three distinct components: headwall, basin, and threshold. The headwall is the steep-sided wall which can be 2000 to 3000 feet high. Lack of talus at the foot of the headwall suggests very little weathering action. In the center of the cirque, a depression or basin extends from the headwall to the threshold (the bedrock riser at the open end of the cirque). The basin often contains a small lake called a tarn or cirque lake.
Cirques develop more extensively when the pre-glacial valleys are widely spaced, snowfall is heavy enough to form snow fields and glaciers but not ice caps, and if the rock is fairly homogenous which causes erosion to be equal in any direction.
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