North America

For reasons that are not yet fully understood, Earth periodically enters a time of planet-wide cooling. Large areas of the land and seas are covered in ice sheets thousands of feet thick, that remain unmelted for thousands or hundreds of thousands of years. Today, only Greenland and Antarctica lie beneath continent-sized glaciers. But in the very recent geologic past, North America's northern regions, including the entire landmass of Canada, were ground and polished by an oceanic amount of water frozen into a single mass of ice. This ice began to accumulate as the planet'sweather cooled, and began to stay frozen all year round. As it built up higher and higher, it began to move out from the piled-high center, flowing while still solid.

Vast amounts of Canadian soil and rock, called glacial till, rode on the ice sheets as they moved, or surfed slowly before the front of the ice wall. Some of the richest farmland in the United States midwest and northeast arrived in its present location in this way-as well as boulders that must be removed from fields before plowing. In the unusual geographic conditions following the retreat of the ice sheets, barren soil lay on the landscape, no longer held down by the glacier. Windstorms moved tremendous amounts of this soil far from where the glacier left it, to settle out of the sky as a layer of fertile soil, called loess in German and English. Loess soils settled in the Mississippi and Missouri valleys, and also Washington, Oregon, Oklahoma, and Texas.

This continental glaciation happened seven times over the last 2.2 million years. Warm intervals, some of them hundreds of thousands of years long, stretched between these planetary deep-freezes. Geologists do not agree whether the ice will return or not. Even if the present day is in a warm period between glaciations, tens or hundreds of thousands of years may elapse before the next advance of the ice sheets.