They have deep, rounded hollows with a steep back wall and a rock basin
Snow accumulates in hollows on hillsides, especially in hollows on hillsides with a less sunny north and east facing aspect
Snow turns into ice and then the ice moved downhill
Freeze-thaw and plucking loosened and removed material from the back of the hollow creating a steep back wall
Moraine dragged along the base of the glacier, deepened the floor of the hollow by abrasion and formed a rock basin
A rock lip was left where the rate of erosion decreased
The lip was often heightened by the deposition of moraine
When the ice melted the rock lip and moraine acted as a natural dam to meltwater
Many rock basins are occupied by a deep, round corrie lake or tarn
Aretes and pyramidal peaks
An arete is a knife-edged ridge often found at the back of a corrie or separating two glaciated valleys. Aretes are often extremely narrow features. A typical arete forms when erosion in two back-to-back corries causes the land in between to become even narrower. If three or more corries have formed on a mountain, erosion may lead to the formation of a single peak rather than a ridge. This is called a pyramidal peak.
The diagram to the left shows the changes down a river valley before and after glaciation.
Glaciers form in river valleys. These are generally v-shaped. As the glacier moves down the valley it creates a valley which is more u-shaped.
This leaves a valley which is steep-sided, wide and relatively flat-bottomed.
Abrasion is the key agent of erosion in this process. The moving glacier grinds into the base and sides of the valley over a period of many hundreds of years.
The glacier is unable to flow past the previous interlocking spurs and simple cuts through them, forming steep-edged truncated spurs.
On the side of the main valley are smaller valleys which feed into the main valley. The main valley is eroded more quickly and deeper than the tributary valleys. This leaves the tributary valleys at a much higher level than that of the main valley. The tributary valleys are then called hanging valleysand often end in spectacular waterfalls which flow into the main valley.
Erosion of the valley floor is erratic. Certain parts of the valley are more likely to eroded more deeply. This could be as a result of thicker ice or areas of softer rock. At the end of the glacial period water may occupy this deepened section to form a long narrow ribbon lake often several tens of metres deep. Loch Ness is a classic example of a ribbon lake.
A ribbon lake is a long and narrow, finger-shaped lake, usually found in a glacial trough. Its formation begins when a glacier moves over an area containing alternate bands of hard and soft bedrock. The sharp-edged boulders that are picked up by the glacier and carried at the bottom of the glacier erode the softer rock more quickly by abrasion, thus creating a hollow called a rock basin. On either side of the rock basin, the more resistant rock is eroded less and these outcrops of harder rock are known as rock bars, which act as dams between which rainwater may accumulate after the retreat of the ice age, filling up the rock basin and creating a ribbon lake.
Small areas of rock on the valley floor are not always completely removed and this leaves roches mountonnees.These have an pstream side polished by abrasion and a downstream side made jagged by plucking.
After ice retreat many glacial troughs were filled with shallow lakes which were later infilled, and their sides were modified by frost-shattering and the development of screes which altered the glacial U-shape