A corrie is an armchair shaped hollow high on a mountain with steep back and side walls. Snow gathers in mountain hollows, especially north facing hollows, where there is more shade. This snow builds up and compacts to ice. The action of gravity means the ice moves downhill.
As it travels, ice sticks to the back walls and plucks rocks from the surface. Rocks on the back walls are also 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 occurs where the weight of the ice is 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/corrie loch in the base of the corrie.