Most of the earth is covered with water, and most of that is salty and in the oceans. In fact, about 97% of Earth's water is ocean water. The remaining 3% is fresh water, and two thirds of that is frozen in glaciers. Glaciers are large masses of ice that can take many forms, from huge sheets to jagged blocks of slow-moving ice and rock. For billions of years, glaciers have carved the earth's surface and fed rivers and oceans with water. Their presence is still felt even millions of years after melting away to create valleys, lakes and even mountains.