Extensive ice cover on land in the polar regions.
A land-based ice mass large enough to cover the majority of the topography of the underlying stony substrate, such that its form is mainly determined by internal dynamics (by internal deformations and/or sliding on the substrate). An ice sheet flows outward from an elevated central plateau with low mean surface slope. The edges of the sheet tend to drop off more steeply and the majority of the ice is transported by rapidly flowing ice streams or outflow glaciers, in some cases out to sea or to an ice shelf floating on the water. Today, there are only three major ice sheets in the world – one on Greenland and two in the Antarctic: the East and West Antarctic Ice Sheets, which are separated by the Transantarctic Mountains. During past ice ages, there were additional ice sheets.