While they may look like big solid masses frozen in place, glaciers are really "rivers of ice" slowly flowing downhill under the force of gravity. While streams and rivers cut narrow channels across the land, glaciers have the ability to level entire continents! The only hitch is that they take a long time to do it! Glaciers start as big piles of snow that slowly build up at the tops of mountains. As the weight of the snow builds up, the material at the bottom of the pile is turned to ice. Slowly, the ice begins to flow downhill under the weight of the entire mass, and as it flows, it picks up any loose rock in its way. Glaciers can pick up and carry sediment that ranges in size from sand grains to boulders bigger than houses. Moving more like a conveyor belt than a bulldozer, a single glacier can move millions of tons of material!