French Canyon

The Kankakee Torrent and to a lesser degree, later Lake Chicago and Lake Nipissing floods, removed glacial deposits and scoured out the Illinois River Valley at Starved Rock State Park in a relatively short period of time. This rapid down-cutting of the Illinois River Valley left the tributaries of the Illinois River hanging at their original height, forming the canyons we see today. Since the time of the floods, the canyons have been widened and deepened by the over-hanging streams, however, this widening and deepening of the canyons has been very slow comparatively. In the past 4,000 years the canyons have been eroded down only about ten (10) feet and have been widened very little. Hence, the canyons we see today look nearly the same today as they would have 4,000 years ago.

The layers you see on the canyon wall are not layers of different bedrock, rather, these layers represent a different constitution of cement within the St. Peter sandstone. Cement is a chemically precipitated mineral material, usually silica, carbonates, or iron oxides that binds the grains of a sedimentary rock. Not all parts of the sandstone contain the same amount of cement. Those parts with less cement are weaker and are more susceptible to erosion. Whereas those areas of the rock that contain more cement are more resistant to erosion. This type of weathering is called differential weathering. The end result is an uneven erosional surface.