A thin layer of glacial till covers this region and several distinctive plant species, remnants of the native forest and prairie, still exist. The park is one of the largest significant natural areas in the northern part of Illinois. Most of the outstanding natural features are located in the dedicated Nature Preserve which is protected by state law. Use of the 710-acre preserve will be restricted to scientific study and limited interpretation activities. The planned development of Castle Rock centers around the Nature Preserve.
All development is limited by soil types.
Soil types consist of silt loams and fine, sandy loams which are connected with the general high degree of slopes which limit use and development at Castle Rock. These soils are subject to severe erosion under heavy use. Castle Rock is a large sandstone butte situated between the Rock River and Highway 2. The rock is made up of St. Peter sandstone. There are only a few places in the state where St. Peter sandstone comes to the surface, even though it underlies practically the entire state of Illinois.