A glacier is a mass of ice that moves slowly downhill. It is made from layers of snow that collect in basins between high mountain peaks. As the layers build up, the snow turns to ice. The ice becomes so thick and heavy, it starts to move. Glaciers move very slowly—just a few metres a day—but they are powerful forces. Some snake down mountain valleys, while others are ice sheets that cover large areas of land. Greenland and Antarctica are covered by ice sheets.

Moraines

As the glacier moves, it gouges out loose rocks and carries them downhill. These rocks collect together in “dirty” bands called moraines. Where two glaciers meet, their moraines merge together. Further down the valley where the glacier melts (at its snout), all the rocks are dumped in heaps known as end moraines. Where a glacier runs over steeper slopes, cracks, known as crevasses, form on its surface.

Mountain valleys that have been gouged out by glaciers are U-shaped. River valleys are usually V-shaped. In some parts of the world, for example the coasts of Norway and New Zealand, U-shaped valleys near the coast have been filled in by the sea. These steep-sided inlets are known as fjords.

Ice Ages

Thousands of years ago, the world’s climate was much colder than it is today. Ice sheets and mountain glaciers spread out to cover much of North America and Europe. The thick ice smoothed over hills and plains, and gouged out wide valleys in the mountains. In lowland areas, the ice scoured basins out of the ground. After the ice melted, these filled with water and became lakes.