The first significant record of Illinois' past is found in rocks that formed about 500 million years ago, during the Mississippian Period. These are some of the oldest rocks that are exposed on the surface. The Earth was then about 4 billion years old. Illinois was near the equator. There was land in North America, even mountain ranges. However, Illinois was part of a low-lying basin covered by a shallow, tropical sea.
About 300 million years ago, during the Pennsylvanian Period, there were upland environments in Illinois. These areas were well above sea level. They were like islands surrounded by large lowland swamps and bays. They were higher and dryer than swamps, with different kinds of plants and animals.
Explore the two sections of Tropical Illinois. The links below will lead you to more images and information.