Glaciers would be less effective at eroding the landscape without the help of freeze-thaw weathering. Before the ice advanced, freeze-thaw left many frost-shattered rocks which were easily removed by the glacier and then used as tools for abrasion. Even when the ice is present, freeze-thaw action affects rocks which outcrop above the surface of the ice because, in a cold climate, there are likely to be many changes of temperature above and below freezing point.
Of the two types of glacier, valley glaciers are considered to be more effective agents of erosion than ice sheets. Confined in a valley, the ice touches the the floor and the sides so that there is more contact between the ice and the rock and therefore more erosion. Also, valley glaciers flow more quickly, partly because of steeper gradients and partly because more meltwater is present to lubricate their flow. There is a plentiful supply of rock fragments from the frost-shattered peaks above, so these glaciers are well supplied with tools for abrasion. However, ice sheets cover and therefore erode a much greater area, so although they erode more slowly, they can still remove a large total amount of rock. As with the other agents of erosion, rocks that are soft or have weaknesses, such as many joints, are eroded more quickly, both by valley glaciers and ice sheets.
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