Introduction:
Origin of the Prairie

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Henry Gleason (1923) suggested that the prairies developed during an early post-glacial time of greater aridity. We know from fossil-pollen data that prairie began to develop in the western part of the Prairie Peninsula as early as 10,000 years ago but did not reach its full extent in the eastern part of the Peninsula until about 6000 years ago.
Illinois in the Ice Age
Illinois in the Ice Age
Ice from the Wisconsin glaciation, the last great Ice Age, began to retreat from Illinois about 24,000 years ago. As the ice retreated northward, a tundra-like vegetation containing sedges and scattered spruce trees developed on the newly exposed landscape. This vegetation community persisted until about 16,000 years ago, when spruce became more abundant. About 12,000 years ago, the climate became much warmer, with the change from a glacial to an interglacial climate. Deciduous forest, composed especially of birch, elm, ironwood, and oak, rapidly replaced the spruce forest. Conditions became gradually drier, and oak became more abundant.


Analyses of preserved pollen from the eastern part of the Prairie Peninsula show that some prairie developed in the region about 9000 years ago, but the major development of the modern prairie occurred about 5000 to 6000 years ago, when elm and other trees became much more restricted. Some sites indicate that conditions became somewhat wetter after 3000 years ago, with an expansion of prairie oak groves.


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