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 cover almost all the upper half of North America, and then melted away. Each glacier was a thick sheet of ice 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 ever-growing sheet built up to a thickness of two miles at its center.    As all glaciers do, these great glaciers slid. They pushed down giant trees in their paths and scraped the earth bare of soil. Many animals moved farther south to escape. Others stayed and were destroyed.     When winters of little snow came, the summer cut into the edges of the ice sheets. 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 melting 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 systems was also influenced by the glaciers. This is the Mississippi-Missouri-Ohio system. These rivers were miles wide at first. Through the years they settled into  _ present channels. In the last sentence, the word "their" refers to   _  .
lakes rivers glaciers systems