Mountain Building and Minerals
The bedrock in the extreme southeastern part of the state is broken by mineralized fractures and faults. The rock is limestone that formed during the Mississippian Period. The fractures are the result of a long history of tectonic activity associated with the continental plates breaking apart and rejoining. The same forces that fractured the Illinois bedrock also formed the Appalachian Mountains along the East Coast and the Ouachita Mountains in Arkansas and Oklahoma.
Heat generated by compression of the continental plates caused organic materials in the sedimentary rock to turn to oil. As a result, petroleum occurs throughout the Illinois Basin.
Southern Illinois Faults, Almost a Volcano
At Hicks Dome, lava that rose through the fractured rocks came close to erupting as a volcano. When the lava cooled, a meshwork of vertical (dike) and horizontal (sill) seams of igneous rock cut through the sedimentary layers.