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Glacial gneiss erratic. An erratic is a boulder which has been transported from its point of origin by a glacier. Often the erratic is deposited in an area with a different geology, aiding its identification. This rock is made of gneiss, a common metamorphic rock formed by the high temperatures and pressures deep within the Earth's crust. Here the layers within the rock can be seen to have been folded by the high pressures, producing the banding seen here. Photographed in Norway. (KEYSTONE/SCIENCE PHOTO LIBRARY/DR JUERG ALEAN/SCIENCE PHOTO LIBRARY)