McConnells Mill State Park is situated in Southeastern Lawrence County in Western Pennsylvania. The park coincides with the southern boundary of the Laurentide Ice Sheet. The glacier reached its maximum thickness and extent approximately 20,000 years ago and then began to rapidly recede to about 10,000 years ago.
When the glacial melted and receded, the enormous meltwater deposited large boulders which remain today. The melting caused sea levels to rise by as much as 10 meters in some places. The ice sheet 20,000 years ago was as much as 2 miles thick in some spots and estimated at about 1 mile thick on the edges coinciding with the McConnells Mill State Park.
The bedrock in the park is primarily composed of sedimentary rocks, including sandstone, shale, and limestone.
The sandstone in the park is part of the Pottsville Formation, which is estimated to be around 300 million years old and was deposited during the Pennsylvanian Period. This sandstone is exposed in many of the park's geological formations, including the iconic "covered bridge" and the "kissing rocks" along Slippery Rock Creek.
The shale in the park is part of the Monongahela Group, which is also around 300 million years old and was deposited during the Pennsylvanian Period. This shale is exposed in many of the park's rock formations, including the area around the mill and along the banks of Slippery Rock Creek.
The limestone in the park is part of the Glenshaw Formation, which is around 320 million years old and was deposited during the Mississippian Period. This limestone is exposed in the park's more elevated regions, including the gorge rim and the hillsides surrounding the park.