 The river canyon is part of the Driftless Area of Illinois, a region that was bypassed by the last ice age; "the glacial sweep which ironed out hills and filled valleys in other parts of the state left this area unscratched".During the dramatic events that surrounded the end of the last Ice Age, the Apple River, which once flowed eastward into the Pecatonica River, reversed its original course and began to flow southwest through an erosion gorge to the Mississippi.The result is a deep canyon, part of which is preserved in the Park.