While the lower end of a glacier typically remains stationary, the ice within a glacier continually moves downhill. This motion usually ranges from a speed of just a few centimeters a day to more than ten meters a day. The ice deforms as icy crystals warp and pass each other. Most of the motion occurs within the deepest layer of the ice, for it is here where the greatest stress is placed on the ice. The ice also deforms most easily in warmer weather. In the winter when changes in stress are more abrupt, the ice near the surface is more brittle, forming crevasses. In the summer, glaciers move by sliding on their beds. Thawing and refreezing occur, and as ice moves downhill, the speed quickens. According to investigators, the overall rate of glacier movement changes hour by hour and day by day.
As shaunabannan explained, the ice within a glacier can move forward without changing the overall size of the glacier. Another really cool thing about glaciers is that they tend to move large rocks with them as they flow. Since glacier ice is more viscous than river water, glaciers can carry huge boulders. This is known as glacial till, and we can use glacial till to tell how far glaciers extended in the past.
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