The eastern two-thirds of the state park is a listed Illinois Nature Preserve noted for its old-growth forest grove of white oak and hickory.

The woods are filled with squirrels, who eat the acorns and other mast shed by the old-growth hardwoods.

The preserve includes many tree species, such as white oak (Quercus alba), chinkapin oak (Q. muehlenbergii), bur oak (Q. macrocarpa), black oak (Q. velutina), red oak (Q. rubra), sugar maple (Acer saccharum), shagbark hickory (Carya ovata), bitternut hickory (C. cordiformis), mockernut hickory (C. tomentosa), black walnut (Juglans nigra), basswood (Tilia americana), American elm (Ulmus americana), slippery elm (U. rubra), Ohio buckeye (Aesculus glabra), sycamore (Platanus occidentalis), hophornbeam (Ostrya virginiana), black cherry (Prunus serotina), and white ash (Fraxinus americana).

[1] Other woody plants include bladdernut (Staphylea trifolia), spicebush (Lindera benzoin), pawpaw (Asimina triloba), blackhaw (Viburnum prunifolium), eastern redbud (Cercis canadensis), hawthorn (Crataegus spp.

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