Corries are armchair-shaped rock hollows with a steep-back wall and a rock basin with a rock lip. They often contain a small lake called a tarn.
How are they formed?
- Snow accumulates in hollows on the hillsides, especially on the North/East facing side where reduced insolation (the solar radiation that reaches the earth’s surface) allows more accumulation of snow.
- Nivation, which is a series of processes that operate under a patch of snow leading to the disintegration of the rock surface, acts on a shallow periglacial hollow and enlarges it into an embryo corrie.
- As the hollow grows, the snow becomes thicker and is increasingly compressed to form firn and then ice.
- The back wall becomes steeper due to plucking, which occurs when rocks and stones become frozen to the base and sides of the glacier and are plucked from the ground or rock face as the glacier moves.
- The rotational movement of the ice and the moraine supplied by the plucking and freeze-thaw weathering on the back wall deepen the hollow by abrasion, forming a rock basin.
- A rock lip is left where the rate of erosion decreased, often becoming heightened by the deposition of moraine.
- After the last ice has melted, the corrie fills with meltwater and rainwater for form a tarn, e.g. – Red Tarn on Helvellyn in The Lake District.