Glacial retreat. Image 1 of 2. Terminus of the Triftgletscher glacier, Switzerland, in August 2002. 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. The melting forms the glacial lake seen here, which first started to form in 2001. Since 1850, glaciers have been in retreat worldwide. It is thought that global warming has accelerated this trend in recent years. See E235/341 for this glacier in 2003.