Glacier formation, illustration. This sequence of five images starts at top left, with persistent ice and snow (white) accumulating to form the start of a glacier at high altitude in a mountain region. At upper centre, downhill movement (blue arrow) of the ice takes place as it flows slowly under its own weight. At top right, a vertical cross-section shows the U-shaped valley being formed as friction between the ice, ice-borne debris, and the valley floor, erodes the landscape. The two images across bottom show how glaciers can merge to cover most of a landscape, with protruding mountain peaks (lower left) known as nunataks. During ice ages, or in polar regions, large ice sheets and ice caps can be many kilometres thick and overwhelm a landscape.