The huge masses of ice flowing slowly over land are what we call glaciers. They form in high mountains and in polar regions, where large amounts of snow build up and freeze into ice.
Snow falls in these places during the winter, but not all of it melts in the summer. The remaining snow builds up year after year in layers. As these layers pile up, their weight increases and causes the snow crystals below the surface to compact, or press together, to form pellets.
When the layers of compacted snow reach a depth of about 50 feet, the pellets compact even more from the increased weight, and then form ice crystals. These ice crystals join together to form a glacier, or river of ice, which can range from 300 to 10,000 feet in thickness.
When the glacier is this thick and this heavy, the ice crystals deep inside it begin to slide over each other, and the glacier begins to move. This movement is in a downward direction, either down a mountain or down to the sea, because of the force of gravity.
The largest glacier in the world, the Lambert Glacier in Australian Antarctic Territory, is 40 miles wide and 320 miles long!