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Masses of ice moving as sheets over the land or as linear flows down the slopes of mountains in broad trough-like valleysare called glaciers. A glacier in its valley is slow unlike water flow. The movement could be a few centimetres to a few metres a day or even less or more. Glaciers move basically because of the force of gravity.
Erosion by glaciers is tremendous because of friction caused by sheer weight of the ice. The material plucked from the land by glaciers get dragged along the floors or sides of the valleys and cause great damage through abrasion and plucking. Glaciers can cause significant damage to even unweathered rocks and can reduce high mountains into low hills and plains.
As glaciers continue to move, debris gets removed, divides get lowered and eventually the slope is reduced to such an extent that, glaciers will stop moving leaving only a mass of low hills and vast outwash plains along with other depositional features.