Marcia Cobb, age 14, of Stuart, Nebraska, for her question:

 What caused the Ice Age glaciers to creep southward?

An Ice Age, of course, is a chilly spell lasting thousands of years. We would expect large glaciers to form, especially in the polar regions. But it seems odd that the Arctic glaciers then crept southward over much of North America. However, there is a reason why ice fields always move and move in certain directions.

Glaciers are children of the earth and long spells of wintery weather. They form when and where the summer sunshine fails to deliver quite enough heat to melt all the frosty snow that fell during the cold season. If our hottest summer days were in the range of 30 to 40 degrees, we would have glaciers forming all over the place. The winter precipitation would be snow and more snow. Frosty spells would freeze the snow fields solid. The weighty ice would impact into thick and massive glaciers.

Glaciers, of course, are made from frozen fresh water that falls from the clouds. And ice is a mineral. As a mineral, it follows its own set rules of behavior. It is one of the earth's most brittle minerals. And brittle minerals cannot stand up under much weight or pressure. A massive glacier is enormously heavy and hence provides its own weight and pressure. This bears on the entire ice field, causing it to move. The bulk of the ice field is a unit and the earth's gravity acts on it as a unit, causing the total mass of the icy mineral to move in certain patterns.

Many modern glaciers are on mountain slopes. Gravity constantly pulls them downward like slow rivers of solid ice. Some of the Ice Age glaciers also slid slowly down mountain slopes. But many of those immense ice fields were thick enough to be higher than the hills. Ice sheets a mile or two miles thick covered vast areas of hills and dales. And these flat ice fields moved according to orderly rules of nature.

A glacier must begin to move when it reaches a thickness of 200 to 300 feet. It may be only a mile or so wide, but at this thickness the ice is pushed out of shape by its own weight. And pressure comes from the center of the glacier, pushing the ice outward along the edges. As more snowfalls add to its thickness, it grows bigger in area. The Ice Age glaciers formed in northern pockets. From there they spread out in all directions. The northern rims pushed through the Arctic where glaciers are expected. The east, west and southern rims pushed into territories where glaciers were unknown. The vast walls of advancing ice crushed the temperate forests in their paths and drove animal life farther and farther southward.

At last the world climate changed and warm summery seasons returned to North America. Rain fell upon the icy glaciers and each summer it melted more and still more of their frozen snows. Melted ice water gathered on their surfaces and gushed down their edges. The cruel glaciers began to shrink on all sides. Their retreat was fastest and most noticeable in the south where summers were warmer and where the invading ice had done the most damage. As the southern edges of the ice field inched back, plant and animal life gradually returned to their former homes.