Cyclothems: Change Through Time
Repetition in the Rocks
Nearly all rocks of Pennsylvanian age in North America and Europe show layers of marine and non-marine deposits in a repeating pattern. Shale and limestone layers formed when sea level was high. Sandstone and coal layers formed when sea level was low.
Each group of layers is called a cyclothem and represents one cycle of rising and falling sea level (although some layers may be missing because of erosion). The changing sea levels were caused by tectonic activity and glaciation in the southern hemisphere.
About 50 cyclothems have been found in the Pennsylvanian rocks of Illinois. Pennsylvanian rocks extend well below the ground surface and are around 300 meters (about 985 ft.) thick in some parts of the state.