Rivers of Ice: How Glaciers Shape the Land
1A glacier is a huge mass of ice that moves slowly over land. Glaciers form in places where more snow falls in winter than melts in summer, such as high mountains and the lands near the North and South Poles. Year after year, the leftover snow piles up. The weight of the new snow presses down on the old snow beneath it and squeezes out the air. Over many years, the packed snow turns into thick, solid ice.
2Once the ice grows thick and heavy enough, something surprising happens: it begins to move. Pulled by its own weight, a glacier creeps downhill or spreads outward, a bit like very thick pancake batter on a griddle. Most glaciers move too slowly for anyone to see them moving, so people sometimes call them rivers of ice.
3As a glacier moves, it changes the land beneath it. Rocks frozen into the bottom of the ice scrape the ground like the teeth of a giant file. Over thousands of years, a glacier can turn a narrow, V-shaped river valley into a wide valley shaped like the letter U. Glaciers can also grind rock into fine powder and scratch long grooves into solid rock. Many lakes, including the Great Lakes, sit in basins that glaciers helped dig out.
4Glaciers also carry rocks and soil from one place to another. A glacier can pick up stones of every size, from tiny pebbles to boulders as big as a house. When the ice finally melts, it drops its load wherever it happens to be. That is why farmers in some places find huge boulders sitting alone in flat fields, far from any rock like them. Long ridges of rock and dirt, called moraines, mark places where the edge of a glacier once stood.
5Long ago, during the last Ice Age, glaciers covered much more of the land than they do now. Thick ice spread across much of what is now Canada and the northern United States. When that ice melted, it left behind many of the valleys, lakes, hills, and boulders that people see in those places today.
6Glaciers matter today, too. Most of the fresh water on Earth is frozen in glaciers and ice sheets. In warm months, melting glacier ice feeds many rivers and streams, bringing water to plants, animals, and people who live far below. These slow rivers of ice are still at work, shaping the land and the lives around them.