A vertical or wedge-shaped crack in a glacier, varying in width from centimetres to tens of metres. Its maximum depth is about 40 m, as, at that depth, ice becomes plastic and any cracks merge together. Transverse crevasses occur when the ice extends down a steep slope. Longitudinal crevasses form parallel with the direction of flow as the ice extends laterally. Marginal crevasses occur across the sides of a glacier as friction occurs between the ice and the valley walls. Radial crevasses fan out when the ice spreads out into a lobe.