Ice Age

   The earth is very old. During its long history there have been many changes in climate. Between one and two million years ago one of these changes took place —the great Ice Age began.
   It grew colder in the Far North. More snow fell in the winter than could melt away in the summer. The snow grew deeper and deeper. It changed to solid ice. As more snow fell, the ice grew thicker. It began to move. Great sheets of moving ice, or glaciers, were formed.
   The edges of the sheets were pushed outward. At last the ice covered most of what is now Canada. And it spread southward into what is now the United States.
   The ice was probably a mile deep in places. It moved over hills and valleys, rivers and forests. It moved slowly—perhaps only a foot a day. Millions of trees and other plants were buried by the ice. Many animals moved south. Among the an­imáis able to stand the cold near the edge of the ice were mammoths, mastodons, and woolly rhinoceroses.

   For thousands of years the ice moved southward. But at last the summers grew longer and warmer, and the ice began to melt back. It melted back so that most of North America was out from under it.
   But again it grew colder and the ice sheets moved southward. Again it grew warmer and they melted back. Twice more the ice moved slowly southward and twice more it melted back. During the great Ice Age much of Europe, too, was covered with ice. The ice sheets left many calling cards behind them. They gouged deep scratches in the rock they moved over. They cut off the tops of hills and widened valleys. They pushed along great piles of soil, which remained behind when the ice melted back. They left huge boulders perched in places where only ice could have put them.
   Scientists now know that the great Ice Age was not the only ice age in the earth's history. Millions of years earlier there were other ice ages. The ice may possibly come again some day, too.
   In fact, scientists are not sure that the great Ice Age is really over. There have not been very many thousand years since the last retreat of the ice. But there are no signs that it is getting colder in the Far North. The glaciers that are still there are not spreading.