Describe and explain the formation of a Corrie (9 marks)

A corrie is a bowl-shaped hollow on a mountain side formed during a period of glacial erosion, an example being Cwm Idwal in Snowdonia, Wales. A corrie begins its formation as an accumulation of snow on the mountain side. Over time, as more snow builds up on the mountain, pressure is increased, compacting snow into ice and eventually forming a glacier. The glacier gradually erodes the surface through abrasion and plucking as the glacier migrates down the mountain. Abrasion occurs where debris and material within the glacier grind against the rock below, causing erosion. In addition to abrasion, plucking erodes the developing corrie through frozen ice attached to debris being pulled away due to movement of the glacier. The strength of erosion is aided by freeze-thaw weathering which weakens debris through expansion and contraction of ice within cracks, making it easier for material to be transported within the glacier. Due to gravity, the glacier moves in a circular motion called a rotational slip, creating a steep back wall, bowl shape and a rock lip. These features over time become more apparent after the ice has melted within the corrie. Occasionally water remains within the corrie, forming a tarn which is a small lake, such as Red Tarn in the Lake District.