Approximately 300 million years ago (during a time geologists call the Pennsylvanian Period) Illinois looked nothing like it does today. Much of it was not even dry land. Much of the area that we now call Illinois was a mixture of swampy lowlands and shallow marine bays.
From the northeast flowed at least one major river system. The river(s) built large deltas through the low swamps and into the shallow bays. The mud that the river(s) carried was deposited in these deltas and bays. This mud turned into a rock called the Francis Creek Shale.
In some ways the area might have been similar to southern Louisiana (USA) and the adjacent Gulf of Mexico. However, the plants and animals would have been very different from today. They were different for at least two reasons. First, many of the plants and animals that are common today had not yet evolved at that time. Second, the climate would have been tropical. The tropical climate was a result of continental drift; 300 million years ago the area was just a few degrees north of the equator.