A glacier is a river of ice that forms wherever snow accumulates faster than it melts. Over many years the buried snow is compressed into dense ice, and once the mass grows thick enough, gravity sets it slowly in motion. Most glaciers creep only a few centimeters to a few meters a day, but their power is immense. They pluck boulders from the bedrock, grind stone into fine flour, and carve broad U-shaped valleys with steep walls. When a glacier retreats it leaves behind ridges of debris called moraines and sometimes lakes filling the hollows it scooped out.
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