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Geography

Glacial erosion landforms

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Glaciers have a huge impact on landscapes. They exert colossal forces on the land and are responsible for dramatic changes caused by erosion.

Corries, cwms or cirques

Corries, also known as cwms or cirques, are often the starting point of a glacier. The diagram below shows the formation of a corrie, cwm or cirque.

Formation of a corrie

Formation of a corrie

Snowflakes collect in a hollow. As more snow falls, the snow is compressed and the air is squeezed out to become firn [firn: A dense type of snow which has thawed and then been compacted and recrystallised. Older and denser than neve snow. ] or neve [neve: A dense type of snow which has thawed and then been compacted and recrystallised. ]. With the pressure of more layers of snow, the firn will, over thousands of years, become glacier ice. Erosion and weathering by abrasion, plucking and freeze-thaw action will gradually make the hollow bigger.

Even though the ice is trapped in a hollow and unable to move down hill, gravity will still encourage it to move. This circular motion is known as rotational slip and can cause the ice to pull away from the back wall creating a crevasse or bergschrund. Plucked debris from the back wall causes further erosion through abrasion which deepens the corrie.

Red Tarn in the Lake District

Red Tarn in the Lake District

Some of this debris is deposited at the edge of the corrie, building up the lip.

These processes create a characteristic rounded, armchair shaped hollow with a steep back wall.

When ice in a corrie melts, a circular lake is often formed at the bottom of the hollow. This is known as a tarn, eg Red Tarn on the eastern flank of Helvellyn.

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