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Some of the snow on a high mountaintop melts and runs off.
But much of it stays all year round.
The snow that stays becomes hard and grainy, like salt.
As new snow falls each year, the grainy snow underneath is squeezed together and becomes hard as ice.
The weight of all the snow pressing down squeezes out a stream of ice, like toothpaste is squeezed from a tube.
This gigantic stream of ice, creeping down the mountainside, is called a glacier.
Black Rapids Glacier in September 1986. (photograph by Rod March).


There are two main kinds of glaciers.
One kind is like a river of ice. It stretches from near the top of a mountain down into a valley below.
The other kind of glacier is like an enormous cake of ice and snow. This kind covers whole mountain ranges and even whole lands. All the land at the South Pole is covered by such a glacier.
Most glaciers move slowly. They travel from only a few inches (centimeters) to about forty feet (12 meters) a day.
But, slow as it is, a glacier is like a big, icy bulldozer. It scrapes, gouges, and shovels up the ground over which it moves.
It picks up everything in its path, from soil to huge boulders, and carries it along. As a glacier passes through a valley, it may dig the valley deeper and wider.
As it moves down a mountainside, it may leave long scratches and furrows.
Glaciers make valleys wider and dig out holes for lakes. Long ago, during the time that is called the Ice Age, great glaciers crept far across the land. They dug many ditches and deep holes in some of the places they passed over. Later, these holes filled up with water and became lakes.
In some places the glaciers left rich soil that they had picked up as they moved.
In other places, they left behind huge boulders that now sit far from the mountains that were once their home.

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