In very old glacier ice, crystals can reach several inches in length. For most glaciers, this process takes over a hundred years. Surface generally moves faster than the base. Middle moves faster than the sides. When a glacier encounters an increase in the slope of its bed, crevasses form Form where the surface is in tension and close where it is in compression.
Crevasses extend only down to the zone of plastic deformation where they close up.
When a glacier encounters a very steep slope in its bed, flow may become chaotic as in an icefall. The irregular ice blocks are called seracs and may be extremely unstable. Ice will not hold a vertical wall more than about 40 m high. At the bottom of an icefall the surface may be in strong compression and periodic waves called ogives may form on the surface. The crevasse at the head of a glacier that separates the moving ice from the stationary ice is called a bergschrund.