Faults in Illinois
Most faults in Illinois are the result of stresses that were applied to the margins of North America during mountain building or rifting episodes. Faults have developed at many times during earth's history in Illinois and are still developing today.
Specifically, there was widespread normal faulting in southernmost Illinois as a result of rifting, or incomplete breakup of the continent, in the early part of the Cambrian Period. Local, small fault movements probably took place during middle to late Ordovician time in connection with the Taconian orogeny in the eastern U.S. Renewed faulting occurred during the Devonian Period in response to the Acadian orogeny in the east. The greatest amount of deformation in Illinois, both folding and faulting, began near the end of the Mississippian Period and continued throughout the Pennsylvanian into the Permian Period. This deformation took place in response to plate collisions that produced the Appalachian and Ouachita Mountains.