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
Snowflakes collect in a hollow. As more snow falls, the snow is compressed and the air is squeezed out to become firn or neve. 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 slipand 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
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.
Glaciers cut distinctive U-shaped valleys with a flat floor and steep sides. The glacier widens, steepens, deepens and smoothes V-shaped river valleys, eg Great Langdale Valley in the Lake District. The images below show the difference between a U-shaped valley and a V-shaped valley.
U-shaped glacial valley
V-shaped river valley
Just like rivers, glaciers have tributaries. As the main glacier erodes deeper into the valley, the tributary is left higher up the steep sides of the glacier. U-shaped valleys ending with a waterfall at the cliff-face are called hanging valleys.
When a river erodes the landscape, ridges of land form in its upper course which jut into the river. These are called interlocking spurs. A glacier cuts through these ridges leaving behind truncated spurs.
Arêtes and pyramidal peaks
An arête is a knife-edge ridge. It is formed when two neighbouring corries run back to back. As each glacier erodes either side of the ridge, the edge becomes steeper and the ridge becomes narrower, eg Striding Edge found on Helvellyn in the Lake District.
A pyramidal peak is formed where three or more corries and arêtes meet. The glaciers have carved away at the top of a mountain, creating a sharply pointed summit, eg Mont Blanc, The Matterhorn and Mount Everest.
A pyramidal peak
Valley floor landforms
Windermere in the Lake District
As a glacier flows over the land, it flows over hard rock and softer rock. Softer rock is less resistant, so a glacier will carve a deeper trough. When the glacier has retreated, (melted) water will collect in the deeper area and create a long, thin lake called a ribbon lake. Many of the lakes in the English Lake District are ribbon lakes, eg Windermere. The areas of harder rock left behind are calledrock steps.
If the glacier hits a particularly resistant outcrop of rock it will flow over and around it. This leaves a rock mount smoothed by abrasion from the glacier. These come in two types:
Roches moutonnée often have steep, jagged faces created by plucking on the lee (far) side and a gradual incline which is smoothed and polished by abrasion on the other (stoss) end. It may have striations on it indicating the direction of glacier movement.
Crag and tail tends to be larger than a roche moutonnée. Crag and tail is the opposite of the roche moutonnée as the ice hits the steep resistant rock outcrop first. This protects the lee (far) side of the obstacle from erosion. Edinburgh castle is built on crag and tail.