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Glacial retreat. Image 1 of 2. Terminus of the Steigletscher glacier, Switzerland, in summer 1994. A glacier is a river of ice that flows down from mountain areas. The ice forms from compacted snowfall, and erodes the rock underneath it as it moves forward. The position of the terminal end of the glacier depends on the rate of melting at the terminus, versus the rate of snowfall further up the glacier. Since 1850, glaciers have been in retreat worldwide. It is thought that global warming has accelerated this trend in recent years. See E235/339 for this glacier in 2006. (KEYSTONE/SCIENCE PHOTO LIBRARY/CHR 366 DR JUERG ALEAN/SCIENCE PHOTO LIBRARY) === ===