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A corrie is an armchair shaped hollow, high on a mountain with steep back and side walls. After glaciation, the hollow may be filled by a small lake or tarn.

The diagram below shows an example of a tarn.

Snow gathers in mountain hollows, especially north facing hollows, where there is more shade. This snow builds up and compacts to ice (neve).

The action of gravity means the ice moves down the hill. As it goes, it sticks to back walls and plucks rock from the surface. Rocks on the backwalls are loosened by freeze-thaw action. A gap between the wall and the ice develops, called a bergschrund.

Ice moving with loose rock acts like sandpaper and deepens the hollow by abrasion. Most erosion is where the weight of the ice is the heaviest. Stones frozen in the base of the ice grind or abrade the corrie base, deepening it.

Ice in a corrie has a rotational movement which means that the front of the corrie is less eroded, and a lip forms. The glacier retreats and melts, often leaving a tarn/glacial lake in the base of the corrie.

An arête is a narrow knife-edged ridge where two corries have eroded back to back. The back walls of a corrie have been eroded back so far that only a narrow ridge separates them.

Pyramidal peaks or horns have a sharp summit and steep slopes on at least three sides. A pyramidal peak may form where three or more corries erode back so far that they produce arêtes with a pyramidal peak in between.