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Glacial retreat. Image 1 of 2. People standing outside of an ice cave on Boulder Glacier, Glacier National Park, Montana, USA, in 1932. Glaciers are large, slow-moving expanses of ice, formed from compacted layers of snow. They are sensitive to climate change and in recent years most have retreated dramatically. See image E235/379 for an image taken in 1988 which shows the extent of glacial retreat in 50 years. (KEYSTONE/SCIENCE PHOTO LIBRARY/US GEOLOGICAL SURVEY)