A glacier is a river or great moving sheet of ice. Rivers of ice are always in valleys. They are often called valley glaciers. Great moving sheets of ice are often called icecaps.
   A glacier begins as a snowfield. Little by little the snow becomes solid ice. The ice grows thicker and begins to move. The snowfield has become a glacier.
   A valley glacier moves down its valley. It may move less than a foot a day. It may move more than a hundred. All valley glaciers move, but some of them do not seem to be doing so. A glacier looks as if it were standing still if the ice at the end melts as fast as more ice is pushed down. An icecap moves outward in all directions. But it may move quite a bit faster at some places than at others.
As glaciers move, great cracks appear in them. These cracks are called crevasses. Crevasses make the exploring of glaciers exciting and dangerous.
   Glaciers scratch the rocks they move over. They grind up rocks, too. As a rule a glacier pushes along great piles of rock rubbish. These piles are called moraines.
   Valley glaciers are common in the Alps and Himalayas. They are common, too, in Norway, Alaska, and western Canada. Glacier National Park, which has several glaciers, is in northwestern United States. Antarctica and Greenland are almost com
pletely covered with big sheets of ice.
   The climate of the world has changed many times. Suppose it should get so warm that all the glaciers melted. Then the water in the oceans would rise 100 feet. A great deal of what is now dry land would then be under the sea.