Glacier, mass of ice that flows outward from ice caps or down from above the snow line. Glaciers cover about one-tenth of the earth's land area. They are classified as continental glaciers or ice caps, valley glaciers, and piedmont glaciers. The largest ice caps occur in Antarctica and Greenland. Almost all of Antarctica and about 85% of Greenland are buried by ice. Smaller ice caps occur in the islands of northern Canada and in Iceland and Norway. Valley glaciers occur in mountain ranges on every continent. Piedmont glaciers form when glaciers flow out from their valleys to form an ice sheet at the foot of a mountain range. Glaciers originate in areas above the permanent snow line, that is, above the level where snow does not melt completely during the summer. In Antarctica the permanent snow line is at sea level, but around the equator it is about 17,000–18,000 ft (5,200–5,500 m) above sea level. Snow that accumulates on gentle slopes forms snowfields, in which the snow is compacted into a white, spongelike substance called firn or névé. Under pressure, the névé is gradually transformed into hard, clear, blue ice, which consists of interlocking crystals. Pressure causes molecules of water to be released in the ice, lubricating the crystals, allowing them to glide over each other. Most valley glaciers move a few feet a day. They may produce rounded hollows called cirques, or turn the valleys into U-shaped troughs with steep sides. Glaciers cease to flow at a point where melting, evaporation, or the breaking away of icebergs balances the rate of accumulation of ice at the source.
The surfaces of glaciers are pitted with crevasses. Lines of rock fragment called moraines are also trapped within the ice, and some rocks are torn from the ground by the ice and frozen into the base and sides of the glacier. As the ice moves, this debris acts like sandpaper and wears away the bedrock over which it passes. It produces smooth land surfaces and may turn valleys into U-shaped troughs with steep sides. A moraine is deposited at the snout of the glacier as the ice melts. Sometimes streams form and transport eroded debris beyond the end of the glacier and deposit it over a wide area. Such deposits are called glacial drift. Among the types of glacial drift are drumlins, which are oval-shaped hills, and eskers, which are long ridges of sand and gravel.