Photograph 1 DEM images. (a) Field of drumlins in the Vale of Eden: note the steep stoss and gently sloping lee sides which allow the ice-flow direction to be calculated.
Ice sheets are large masses of glacier ice over 50,000 km2 in area. They form when snow accumulates over thousands of years, and can be several kilometres thick. Gravity causes ice sheets to slide and deform away from the upland areas where the snow accumulated.
There are only two places on Earth now which have ice sheets: Greenland and Antarctica. At the last glacial maximum, about 21,000 years ago, vast swathes of Canada, Britain and Scandinavia were also covered by ice sheets.
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