Glacial erosion takes place by abrasion, gouging, plucking and sapping. Ice, itself, does not do much abrasion; but as the ice moves, it gathers boulders, rock fragments, sand, soil which become its cutting tools.
As it moves over the land, glaciers erode all loose material on its way and degrade almost all projections of the solid rocks, just like bulldozer works. Valley glaciers smooth their floors and walls, thus, reshape the ‘V’ shaped valleys to ‘U’ shaped ones.
At the foot of steep slopes, they gouge the bed rock and form depressious in form of troughs, which after melting of glaciers give rise to trough lakes. Continental glaciers modify the whole topography and smoothen the surface by abrasion. In case of valley glaciers, rain water or melt water enters the joints, cracks, fractures and crevasses present in glacial mass and freezes resulting in expansion.
This expansion and fragmentation of glacier loose boulders and are plucked up by the glaciers by the process of sapping. In this way, the valley glaciers make headward erosion giving rise to great amphitheatre like basins at the head of the glaciers.