Muir and Riggs Glaciers, Muir Inlet, Alaska
As this picture of Muir and Riggs Glaceris in Alaska shows, glaciers are really rivers, but rivers of solid ice instead of liquid water. Just because they are solid does not mean they don't move, though. Glaciers do flow downhill, just very, very slowly.
Glaciers begin life as snowflakes. When the snowfall in an area far exceeds the melting that occurs during summer, glaciers start to form. The weight of the accumulated snow compresses the fallen snow into ice. These "rivers" of ice are tremendously heavy, and if they are on land that has a downhill slope the whole ice patch starts to slowly grind its way downhill. These glaciers can vary greatly in size, from a football-field sized patch to a river a hundred miles (161 kilometers) long.
>> Find out more about glaciers and icecaps.