Nivation is a type of mechanical weathering of rocks that results from alternate freezing and thawing. At night, when temperatures fall, water laying in cracks in the rock freezes and expands. The following morning the ice melts and contracts, only to freeze again after nightfall. The continual expansion and contraction causes stresses that eventually break the rock along the crack or other line of weakness. Accumulated fragments of rock produced in this way form scree on hill slopes.