Located within McConnells Mill State Park, along the valley wall of the Slippery Rock Gorge, is an interesting feature of the Pennsylvanian Homewood sandstone.
This feature is created by massive blocks of the sandstone separating from the main outcrop. Separation of these blocks is caused by erosion along vertical fractures in the rock. As these blocks separate, they slump and rotate opening up walkways and creating a beautiful “rock city” (Image 1).
The Homewood sandstone is underlain by the much softer Mercer shale. Differential weathering between these two formations causes some of the sandstone blocks to be undercut. This undercutting can cause some blocks, upon separating from the outcrop, to slide all the way down the valley wall into the water of Slippery Rock Creek.
Image 1. Rock city along Rim Road
Image 2. Blocks of Homewood sandstone along the banks of Slippery Rock Creek
ION SIMONIDES, Pennsylvania Geological Survey
April 2016