Read More About It - Glaciers and Sea Level
Through much of the Pennsylvanian, huge ice sheets covered the continent that was centered on the South Pole. Glaciers formed and melted many times. When the glaciers expanded, sea level fell. Illinois was a mix of low uplands, broad river floodplains, wide deltas with subdivided river channels, shallow marine bays, and swamps. Peat accumulated in the swamps.
When the glaciers retreated, water from the melting ice caused the sea level to rise and oceans covered most of the state. Sea levels rose and fell as much as 70 meters (about 230 ft.) as a result of glacial activity.