How do corries and tarns form?

Corries and tarns form through glacial erosion and deposition in mountainous regions during periods of glaciation.

Corries, also known as cirques, are armchair-shaped hollows found on the sides of mountains. They form when snow accumulates in a depression and compacts into ice, creating a small glacier. Over time, the glacier erodes the landscape through processes like plucking and abrasion. Plucking occurs when the glacier freezes onto rocks and pulls them away as it moves, while abrasion happens as the glacier grinds against the rock beneath it, deepening the hollow. The back wall of the corrie becomes steep due to freeze-thaw weathering, where water enters cracks, freezes, and expands, breaking the rock apart. The base of the corrie is often smoothed and deepened into a bowl shape by the glacier's movement.

Tarns are small mountain lakes that form in the basins of corries after the glacier has melted. As the glacier retreats, it leaves behind a depression that can fill with meltwater, rainwater, or groundwater, creating a tarn. The water in tarns is typically clear and cold, and these lakes are often surrounded by steep rock walls and moraines, which are accumulations of debris left by the glacier. The presence of a tarn indicates that the glacier was once powerful enough to carve out a significant depression in the landscape.

Both corries and tarns are key features in glaciated landscapes and provide evidence of past glacial activity. They are important for understanding the processes of glacial erosion and deposition, and they contribute to the dramatic scenery often found in mountainous regions.