There are two ways in which the glacial erosion is accomplished: (i) by plucking, and (ii) by abrasion. A well-jointed rock is removed by the movement of glaciers. The mechanism of plucking involves vertical pressure by the overlying ice, and downhill drag resulting from flow pressure in the direction of glacier movement.
Stresses are created in the rock whereby existing joints are enlarged. According to W.V. Lewis, pressure-release is the most important way of facilitating the plucking of rocks.
The second method is by abrasion, when the small pieces of rock frozen into the basal ice of the glacier are dragged over the rock floor.
This is how the rock floor is scraped, polished and a notch line is scratched thereon. In doing so the load itself is ground down. Erosion not only produces ground-rocks known as rock flour, but it also produces rock pieces of different shapes and sizes.
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Through mechanical abrasion the valley glaciers smooth their floors and walls, thus they reshape the valleys. V-shaped zigzag valleys are converted into U-shaped valleys which are broad and straight. At the base of steep slopes, the glacier produces deep, rather very deep basins. After the ice melts away, these basins become sites of lakes and swamps.
Transportation:
The glaciers are characterized by their tremendous power of carrying the load of different size and shapes. The load is transported on and in the ice.
The valley glaciers derive their load from erosion of the floor and sides of their valleys, and also from the slopes above them. The surface of the glaciers receives different materials brought by avalanches and streams from the mountains sides.
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Deposition:
The entire load carried by the moving ice-mass is finally deposited. Such load is a mixture of unsorted coarse and fine debris which is without stratification. This type of glacial deposits is called till.
Rock fragments so deposited are not smooth; rather they are angular and sub-angular because they are carried by the glacier in relatively fixed position.
They are unlike the rounded pebbles and boulders carried by the streams. However, some of the rock fragments are dragged along the valley floor, so they are smoothed and bear marks of striation on one or more sides.
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Glacio-fluvial deposits:
The glacio-fluvial transport of material and the related mechanism of erosion and deposition are similar to those of a river when the action occurring belongs to the snout of the glacier.
The glacio-fluvial deposition generally forms extensive outwash plains. It is to be remembered that the melt-water deposits are stratified unlike the deposition made by the ice.
Besides, the debris carried long distances by glacial streams are rounded. In some glaciers there are tunnels beneath the ice formed by running water.