Illustration of the distribution of native prairie prior to widespread cultivation. Beginning perhaps 8,000 years ago, prairie vegetation expanded while forested areas decreased in size during a period of warmer and drier climate. (After Transeau 1935).

The average annual temperature was cooler and there was more precipitation during the Early Archaic when compared to today. However, temperatures began to rise and the amount of precipitation decreased about 8,000 years ago. The growing season may have been one to two weeks longer, but annual precipitation may have been 10% to 20% less than today. By 5,000 years ago, the average annual temperature decreased and the amount of precipitation increased to present-day levels.

The long period of warmer and drier weather between 8,000 and 5,000 years ago created major changes in Illinois' vegetation.