Driving north from Columbia, one soon enters the vast Glaciated Plain of the American Midwest. This province, which stretches from central Ohio to the eastern Dakotas, was flattened by the Pleistocene Ice Sheets and enriched by their till. Once the last glacier retreated into Canada, some 12,000 years ago, a rich, tallgrass prairie spread across the region. Periodic wildfires, intermittent drought and the grazing of huge bison herds kept the forest at bay and trees were restricted to the stream beds and river valleys.
The arrival of white settlers dramatically changed the ecosystem of this province. The prairie was plowed, the bison were killed and wildfires were suppressed. Today, the Glaciated Plain has become the great Corn Belt of North America.