About one million years ago, the Ice Age began. The Ice Age was a long period of time in which four great glaciers pushed southward to____almost all the upper half of North America, and then melted away. Each glacier was a thick sheet of ice land and snow that spread out from a center near what is now Hudson Bay in Canada. The winters were long, and the cool summers were too short to melt much of the ice and snow. The____sheet built up to a thickness of two miles at its____.
As all glaciers do, these great glaciers slid. They pushed down giant trees in their____and scraped the earth bare of soil. Many animals moved farther south to escape. Others stayed and were destroyed. When____of little snow came, the summer suns cut into the edge of the ice____ As the glaciers melted, rocks, soil and other things that had mixed with the ice and snow were left. New hills, lakes and rivers were formed.
The last of the great glaciers began its____about 11,000 years ago. Its melting formed the Great Lakes. These lakes today are little changed from their early sizes and shapes. The largest of the North American river____was also influenced by the glaciers. This is the Mississippi-Missouri-Ohio system.
