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
Several Concretion Zones Image Courtesy of Stephen LeMay
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 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.