At high latitudes and high elevations, the climate is cold. Snow accumulates and recrystallizes throughout the year, and its density increases gradually to form glacial ice. Glacial ice has a degree of plasticity. Under its own gravity, it moves along the ground as a plastic flow and a block-like slide and becomes a glacier. Glaciers are classified as continental ice caps and mountain glaciers. A continental ice sheet is a shield that rises from its edge to its centre. The thickness of the ice often exceeds 1,000 m. In coastal areas, a large ice tongue often protrudes and breaks when reaching the sea to become a floating iceberg. Mountain glaciers exist at the snow lines of high mountains at middle and low latitudes. China has some of the most well-developed mountain glaciers.