How Glaciers Shape the Land

Imagine a river made of ice. It creeps so slowly that you could never see it move, yet it is strong enough to carve a mountain. That is a glacier. A glacier begins in a cold place where more snow falls each winter than melts each summer. Year after year, the snow piles up, and its weight presses the bottom layers into thick, solid ice. When the ice becomes heavy enough, gravity pulls it slowly downhill.

Many glaciers creep along only a few inches or a few feet in a day. As a glacier slides, it works like a giant sheet of sandpaper. Rocks frozen into its bottom scrape and scratch the ground beneath it. The ice also freezes onto loose rock and plucks out chunks as it moves, the way you might pull a loose tooth. Over thousands of years, a glacier can change the shape of a whole valley. A narrow river valley shaped like the letter V becomes a wide valley shaped like the letter U.

A glacier carries away the rock it breaks loose, from tiny grains of sand to boulders as big as a house. When the front of the glacier melts, it drops its load wherever the ice ends. Long, low ridges of rock and soil left behind are called moraines. Sometimes a single giant boulder is left sitting in a flat field, far from the mountain it came from.

Glaciers have shaped many places where there is no ice today. The last ice age ended more than ten thousand years ago. Back then, enormous sheets of ice covered much of Canada and the northern United States. As the ice scraped across the land, it dug out deep hollows, and when it melted, water filled them. The Great Lakes were carved partly by those ice sheets, and many of Minnesota's thousands of lakes formed the same way. Even now, glaciers cover about one tenth of all the land on Earth.