During the last ice age, glaciers covered the entire northern part of our continent, shaping mountains and carving valleys. Today, most of the earth's glaciers are found in Antarctica and Greenland, but there are glaciers on every continent, including Africa, most commonly above the snow line. Tim Creyts, a glaciologist with the Lamont-Doherty Earth Observatory, Earth Institute, Columbia University, explains what glaciers are, how they move and sculpt the landscape, and how climate change is affecting glaciers around the world.