Alison Ward, age 13, of Lowell, Mass., for her question:
WHEN WAS THE ICE AGE?
The time known as the great Ice Age is believed to have started between 1 million and 2 million years ago, during the Pleistocene epoch. In North America, only a few thousand years have passed since its end. The glacial period is still going on, however, in Antarctica and Greenland.
Great sheets of ice formed during the Ice Age and the Earth became very cold. The amount of dry land increased because the level of the oceans were lowered. There was dry land, for example, between Alaska and Siberia, where the Bering Strait is now located.
Four million square miles of lvorth America were buried under a great sheet of creeping ice that reached as far south as Long Island and the southernmost parts of the Ohio and Missouri rivers.
This continental glacier or ice mass may have been 10,000 feet thick. And there is evidence to prove that this glacier formed and then melted away at least four times during the Ice Age.
Three continental glaciers spread southward from three centers: one section was in Labrador, one just west of the Hudson Bay and the third was in western Canada.
Most of Canada was covered with ice. An area of about 10,000 square miles, mainly in southwestern Wisconsin called the Driftless Area, was never covered by ice, although it was within the glacial limit.
The glacier changed many land surfaces. Former river valleys were blocked with glacial deposits, or moraines, forming new lakes, and rivers were forced to find new courses elsewhere. The Great Lakes occupy former river valleys, scooped deeper by glacial erosion and built up on the sides with deposits in the form of moraines.
Niagara Gorge and Falls are part of the new drainage route, which was a result of glaciation, of the waters from the upper Great Lakes.
Many changes in plant and animal life occured as a result of the invasion of the continental glacier. Arctic animals had to migrate southward in front of the glaciers.
Many of the native animals that are now found in North America are descendants of the many varieties of animals known to have lived there before the Ice Age.
Some animals, including man, were able to survive the change in climate. Animals that could not adapt simply died and became extinct.
Plant life moved south in front of the glacier and then moved back again as the glacier melted. Many species dies out and others were left outside the area in which they normally grew.