Formation and movement of glacial ice: glaciers form in areas where more snow falls in winter than melts during the summer. Snow above the snowline does not melt in the summer: glacial ice formation. Snow recrystallized into a mush denser mass of small grains called firn. Once the thickness of the ice and snow exceeds 50m, firn fuses into a solid mass of interlocking ice crystals glacial ice: movement of a glacier. Plastic flow involves movement within the ice: under pressure, ice behaves as a plastic within the ice. Along the ground, the entire ice mass slides along the ground as basal slip: meltwater acts as lubricant. Ice in the zone of fracture is carried along piggyback style. Below a depth of about 50m ice behaves plastically (deforms without breaking) and gradually flows. Ice in contact with valley floor remains fixed as stress builds to the point that the glacier lurches forward.