The first landform formed by a glacier is the corrie. This is a circular rock hollow (hence the alternative name of cirque used by some geographers), usually located high on the mountainside, with a steep and rocky backwall up to 200 metres high in the UK, but much higher in the Alps.
Although most of the corrie is ringed by steep rocks leading to sharp rocky ridges, the front is open with nothing more than a small rock lip on the surface.
The hollow is typically filled with a small round like, called a tarn, after the ice has melted.
Corries begin where snowfields (called neves), which accumulate below the mountaintops, form ice and grow.
As with many landforms, it is necessary to refer to several different processes in order to explain the formation of the corrie
FORMATION on next card...
Formation of a corrie
- Freeze- thaw weathering plays a part in its formation. Frost action on the mountaintops and slopes above supplies loose rocks (scree). Water seeps down the bergschrund crevasse onto the headwall, increasing the amount of freeze-thaw activity, cutting back the headwall and making it steeper.
- Ice sticking to the headwall pulls away blocks of rock by plucking as the glacier moves.
- Loose rocks obtained from freeze-thaw and plucking are embedded in the ice and act as tools for scraping out the bottom of the hollow by abrasion.
- As a result of the rotational slip movement of the ice, there is greater pressure from the ice at the bottom of the headwall and in the base of the hollow than near to the front where the glacier leaves the corrie hollow to flow down the valley; the rock lip forms near the exit as a result of less powerful erosion.
When all the ice has melted the corrie provides an ideal place for a tarn to form. There is a natural ice-carved hollow in which the water can accumulate. The rock lip acts as a natural dam on the side that is not surrounded by steep slopes. A location in upland areas means that precipitation is likely to be high and there will be a large amount of run-off down the steep sides of the corrie, because the corrie forms a natural catchment area. A tarn fills the floor of most well-developed corries.
Arete and pyramidal peak
An arete is a two-sided sharp-edged ridge, whereas the pyramidal peak, as its name suggests, is a three-sided slap of rock, of which the most famous example in the Matterhorn with its three near-vertical rock faces.
Both landforms are created by the cutting back of the headwalls of corries on the slopes below the peaks by the processes of freeze-thaw weathering and plucking.
For an arete, two corries, one on each side of the ridge, cut back until only a narrow piece of rock is left on the ridge top.
For a pyramidal peak, three corries cut back.
All the peaks continue to be sharpened by frost action.