Illinois State Museum

Hardwood Forest - Climate and Geology

In 1673, the climate of eastern Illinois was temperate and similar to today. Our scientists estimate an average annual high temperature of 17ºC (about 63ºF), average low temperature of 6ºC (about 43ºF) and an average annual rainfall of 104 cm (about 41 inches). The number of frost-free days was 170–180.

The bedrock here is Pennsylvanian limestone, sandstone, coal, and shale. The gently rolling hills are the result of a thin layer of wind-blown loess that overlies loamy till deposited by Ice Age glaciers.