Even though you've probably never seen a glacier, they are a big item when we talk about the world's water supply. Almost 10 percent of the world's land mass is currently covered with glaciers, mostly in places like Greenland and Antarctica. In a way they are.
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 glaciers can vary greatly in size, from a football-field sized patch to a river a hundred miles long. You can imagine how a billion-ton icecube can rearrange the landscape as it slowly grinds its way overland. Many lakes, such as the Great Lakes, and valleys have been carved out by ancient glaciers.