How do cirques form at the heads of mountains?

A cirque is a steep-walled, amphitheater-like basin carved by glacier ice near the head of a valley. Snow accumulates in a sheltered hollow, compresses into ice, and slowly flows downhill, plucking and grinding away the bedrock. Over time, the glacier deepens the hollow and steepens its back wall, creating a broad basin with a lip or threshold at the outlet. After the ice retreats, cirques may contain tarns, which are small mountain lakes. Well-known cirques can be found in the Alps, the Rockies, and many other glaciated ranges.