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Ten thousand years ago, an extensive deciduous forest covered Illinois. New varieties of plants and small animals began to replace those adapted to colder regions. Forests persisted throughout the Archaic period in southern Illinois. But in central Illinois, changes in climate transformed the landscape into patches of forest and prairie. Forests bordered ponds, streams, and rivers. Vast prairies covered the flat lands of the state. Forests also dominated the more rugged sections of northern Illinois.
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