In this topic you will learn about the forces that change Earth's surface.

A glacier is a large mass of snow and ice that moves downward and outward over the land. Glaciers form when more snow falls in the winter than melts in the summer. The snow collects in layers. The weight of the upper layers turns the snow to ice. Glaciers move or flow downhill. Heat from friction and Earth melts some of the bottom layer of ice. The thin layer of water helps the glaciers move.

Rock debris collects at the terminus of a glacier. The terminus is the end, or outer margin, of a glacier. When a glacier melts, the rock debris is left behind. This rock debris which is carried and deposited by a glacier is called moraine. Moraines are made up of glacial till, an unsorted mixture of rock materials deposited as a glacier melts.

The rock fragments in glaciers grind away bedrock. They may carve deep scratches and can smooth and polish the rock below them. Generally, glaciers flatten the land and round sharp peaks. Some glaciers carve valleys, making them deeper and U-shaped. Isolated boulders left behind by a glacier are called erratics. Gravel, sand, and clay carried from glaciers by melting water and streams are outwash plains.

In the past, glaciers covered large parts of the world. Periods of very cold temperature, called ice ages, produced many glaciers. Periods of warmer weather existed between ice ages.

Glaciers are not the only things that change Earth's surface. Wind, waves, running water, and gravity also change the surface. As wind blows, it picks up small particles of dust, soil, and sand. Rock in the path of these winds is worn down as if rubbed by sandpaper. Ocean waves break rock into smaller pieces. Waves also pull materials away from the shore.

Running water causes the greatest change to Earth's surface. Water flows across the land and picks up particles of rock and soil. The water drops the particles in places far away from where they were picked up. Gravity causes rocks, boulders, soil, and mud to move from a higher place to a lower place. Landslides and mudflows are caused by gravity.

Knowing about glaciers helps us understand how they change Earth's landscape.