A cirque is an amphitheatre-like valley of glacial origin, formed by glacial erosion at the head of the glacier. Also known as a cwm in Wales, a coomb in England and a corrie in Scotland and Ireland. Cirques form in conditions which are favorable, which in the northern hemisphere includes the north-east slope being in shade and away from prevailing winds. These conditions are sheltered from heat, and so encourage accumulation. Once accumulation has begun to occur, and snow is forming, a process of nivation occurs. This is where a hollow in a slope is enlarged by freeze-thaw weathering, and meltwater (from melted snow) transporting the broken debris away. Eventually this hollow will become big enough to let glacier ice begin to form (where glacier ice is compacted snow). Debris in the ice begins to abrade (glacial abrasion) on the bed surface which further enlarges the hollow into a small basin. Eventually the hollow is a large bowl shape in the side of the mountain, with the headwall being weathered by freeze-thaw and eroded by plucking, and the basin becoming deeper as it is eroded by abrasion.