A glacier is a large, thick mass of ice that forms on land when fallen snow gets compressed into ice over many centuries. Glaciers are important features in the hydrologic cycle and affect the volume, variability, and water quality of runoff. They are classified into three main groups: ice sheets, mountain glaciers, and piedmont glaciers or ice shelves. Alpine glaciers are frozen rivers of ice that slowly flow under their own weight down mountainsides and into valleys. Glaciers also exist on the fringes of ice sheets. Unlike alpine glaciers, ice sheets cover entire continents. An ice cap is a dome-shaped glacier with an area less than 50,000 square kilometers that flows in all directions and covers much of the underlying topography. Glaciers occur in all parts of the world and at almost all latitudes. They slowly move downwards, changing the shape of the topography as they go. Glaciers that move downwards carve out U-shaped valleys as they slide, and bits of rocks carried by glaciers collect to form ridge-like mounds called moraines.