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How does a corrie form over time?

A corrie is a hollow in the mountain side, facing North or North-East so recieves less sunshine. Over time as snow turns to ice, air is pushed out and the resultant mass is compressed and more dense. This process repeats itself over winter seasons as the snow does not melt away completely as it is largely sheltered from warming sun.
Over time, this corrie glacier moves downhill via rotational sliding, and as it does processes of plucking and freeze-thaw weathering form a steep back-wall whilst the base of the corrie becomes deeper from abrasion - whereby moraine is dragged along the base of the glacier. At the lip of the corrie where rotational slip does not exert such powerful erosion, a rock lip develops which is the product of moraine and debris being carried en- supra- and sub-glacially. In Cwm Idwal in North Wales, it is clear that the rock-lip acted as a dam to melt water, with a tarn now in the corrie glacier's place.