The covering of the earth's surface with glacial ice. The term also includes the alteration of the surface of the earth by glacial erosion or deposition. Due to the passage of time, ice erosion can be almost unidentifiable; the weathering of hard rock surfaces often eliminates minor scratches and other evidence of such glacial activities as the carving of deep valleys. The evidence of deposition, known as depositional imprints, can vary. It may consist of specialized features a few meters above the surrounding terrain, or it may consist of ground materials several meters in thickness covering wide areas of the landscape.

Only 10% of the earth's surface is currently covered with glacial ice, but it is estimated that 30% had been covered with glacial ice at some time. During the last major glacial period, most of Europe and more than half of the North American continent were covered with ice...