Glaciers are large bodies of ice that move over Earth’s surface. A glacier is formed as snow accumulates over time and turns to ice, a process that can take more than a hundred years. Once a glacier has formed, it moves very slowly, at a rate of years, or even decades; some glaciers are frozen solid and do not move at all. The world’s fastest glacier, Jakobshavn Glacier in Greenland, moves at a rate of about 40 meters (130 feet) per day. When a glacier does move, it is often downhill due to gravity. As the glacier slides over Earth’s surface, it erodes its surface by polishing bedrock, pushing soil, grinding up rock, and digging into the ground. Through this process, called glaciation, glaciers carve out unique landscapes like the U-shaped valley of Lake McDonald Valley in Glacier National Park, Montana, United States.

There are two types of glaciers, and each creates different landscape features through glaciation. The first type is the alpine glacier, which form on mountains. Alpine glaciers can form bowl-shaped dents in the ground, which are called cirques. If the cirque fills with water from the melted glacier, that lake is called a tarn. When three or more cirques chisel out the mountain and form a pyramid-like peak, that is called a horn. The Matterhorn in the Swiss Alps is a famous example of a horn. An arête is a very sharp mountain ridge formed when two alpine glaciers meet. An example is the Garden Wall in Glacier National Park.

The second type of glacier is the continental glacier. Continental glaciers are large ice sheets that cover a vast area. Earth’s only two ice sheets can be found in Greenland and Antarctica. V-shaped valleys were carved into U-shaped valleys from the slow and steady movement of continental glaciers.

While most glaciation is cause by erosion, glaciers also pick up things like rocks and move them in a process called deposition. As a glacier moves, it scratches Earth’s surface, picking up everything from small rocks to boulders the size of a house. These rocks and boulders become stuck in the glacier and make their mark on the landscape. Once the glacier melts, the rocks and boulders are left behind in a new location, often far from where the glacier first picked them up.