A glacier is an accumulation of ice, air, water, and rock debris or sediment. Under the pressure of their own weight, they flow with gravity outward and downward. Glaciers flow very slowly, from tens of feet to thousands of feet per year. The ice can be as large as a continent, such as the ice sheet covering Antarctica or those that advanced throughout Wisconsin; a continental glacier. Or it can fill a small valley between two mountains; a valley glacier.