If the ice at the base of a glacier is much below the temperature of freezing, as happens in winter, or perhaps even in summer at high altitude or in polar regions, the ice bonds tightly to the rock below. Then, the entire motion is within the glacier. The ice itself deforms as individual ice crystals warp and creep past each other. Most of the deformational creep takes place in the deepest layer of the ice, near to the basal layer. It is here that the greatest stress is placed upon the ice because the stress is proportional to the thickness of ice above.
Ice deforms most easily when it is warm.
Especially in winter, the ice near the glacier's surface can be comparatively brittle, a fact that helps explain why crevasses form. These tensional cracks or other types of breakage occur where or when changes in stress are too abrupt or rapid for deformational creep to keep up with. In summer, when the basal layer of the glacier ice warms up to near its melting point, a glacier begins to move by sliding on its bed. Thawing and refreezing take place, and water lubricates the rock surface. The pace quickens as both sliding and creep act in unison to move ice downhill.