A mountain glacier and its features A glacier is a moving mass of ice. Some glaciers snake down mountain valleys, while others such as the ice sheets of Greenland or Antarctica are so huge and thick they almost totally cover the land. Although it is solid, ice can flow down slopes and around bends, although much more slowly than a river—often less than a metre (3 ft 3 inches) per day. Glaciers occur in very cold regions, high in mountains and in the far north and south polar regions.
How glaciers form
A glacier is fed by snow. Over many years, the snow piles up at the head of a high valley and compacts into ice. It collects in a cirque, a bowl-shaped feature. Being thick and heavy, the ice moves under the pressure of its own weight, flowing downhill as a glacier.
Most glaciers move very slowly—just a few metres a day—but they are a powerful force.The ice carries pieces of rock loosened by frost weathering and scrapes against the valley sides. It carries this loose rock along in long bands called lateral moraines, or underneath the glacier as subglacial moraine. As two glaciers merge their lateral moraines combine into a medial moraine. Where the ice runs over a steeper slope, it develops cracks known as crevasses. Lower down, the glacier melts at its snout. If it moves forward less than it melts, the snout retreats over time, leaving a pile of the rocks that it has carried as a terminal moraine.
A glacier and its moraines
Other glacial features
Where a glacier melts and "retreats" up its valley, it leaves behind some distinctive glacial features besides moraines. These include drumlins, small hills of gravel, and roche moutonnées, tear-shaped mounds of rock. Both features have been shaped and smoothed by the heavy mass of flowing ice. Eskers are winding ridges of gravel that were laid down by meltwater streams flowing beneath the glacier.
In mountainous regions, valleys have been gouged out by glaciers. They have a characteristic U-shape, making them quite distinct from river valleys, which have a V-shape. In some parts of the world, most notably Norway and New Zealand, U-shaped valleys near the coast have been “drowned” by rising sea levels. These deep inlets, known as fjords, have very steep sides. Some fjords snake inland from the sea for many kilometres.
A Norwegian fjord
Consultant: Ian Fairchild