AN OVERVIEW OF THE FORMATION PROCESSES OF ILLINOIS WATERFALLS

Most waterfalls in Illinois have been initiated by one of three events:

Glacial meltwater flooding and bluff erosion along the Illinois River and its tributaries:

1. These waterfalls are restricted to the Ordovician St. Peter Sandstone unit with retreat dominated by seepage processes. They are well developed in Starved Rock and Matthiessen State Parks.
2. These waterfalls are restricted to Silurian dolostone units, with retreat dominated by fluvial erosion and dissolution of blocks bounded by joints and bedding surfaces. Examples include Rock Creek Falls in Kankakee River State Park and Bourbonnais Creek Falls.

Glacial meltwater flooding and bluff erosion along the Mississippi River:

1. The waterfalls on siliciclastics retreat by Niagara-type turbulent undercutting and/or seepage processes. Fountain Bluff Station Falls is an example.
2. The waterfalls on carbonates retreat by fluvial erosion and dissolution of blocks bounded by joints and bedding surfaces. An example is the upper waterfall at Falls Creek Overlook Park.

Stream incision through resistant Pennsylvanian sandstones underlain by weak siliciclastics with retreat processes shifting through time from Niagara-type turbulent undercutting to seepage processes once erosion is restricted to a resistant sandstone unit. Most of these waterfalls occur south of the Illinoian glacial boundary. Many are in Ferne Clyffe Falls State Park and Shawnee National Forest.