Cirques (alternatively known as Corries in Scotland and Cwm's in Wales) are masses of ice that carve out an arm-chair shaped hollow with a steep back wall and a steep headwall. Snow fall collects in the already evident (if not small) hollow on the mountain side. They are prevalent on north facing slopes as they recieve less sunlight so accumulation exceeds ablation. A series of processes collectively known as Nivation (processes that occur under and near a snow patch) including freeze-thaw, solifluction, erosion, weathering etc. These processes cause the underlying rocks to disintegrate, and the hollow deepens. As the snow patch grows, its layers become increasingly compressed to form firn and eventually ice. Plucking steepens and scours the sidewalls and headwall, as the material is removed from the valley sides. A rotational movement of ice flow enables abrasion to deepen the hollow further. Cirques often form valley glaciers as they fill up the hollow and spill over the rock lip.