| Ancell Group |
| Baylis Formation (western Illinois) |
| Bond Fm |
| Bordon siltstone (includes Chouteau Ls. in east-central IL |
| CRETACEOUS SYSTEM (undivided) |
| Cambrian System |
| Carbondale Fm |
| Caseyville Fm |
| Ft. Payne Fm |
| Galena Group |
| Glen Park, Hannibal, Chouteau, McCraney, Prospect Hill, Sta* |
| Kimmswick Limestone and Decorah Formation |
| Lower Pope Group |
| Maquoketa Shale Group |
| Mattoon Fm |
| Meppen Ls., Fren Glen Fm., Burlington-Keokuk Ls. |
| Muscatatuck Group |
| New Albany Shale |
| Paleozoic undifferentiated |
| Permian ultramafic intrusions |
| Platteville Group |
| Prairie du Chien Group |
| Shelburn-Patoka |
| Silurian System |
| St. Louis Limestone |
| Ste Genevieve Ls |
| TERTIARY AND CRETACEOUS (IL) |
| TERTIARY SYSTEM |
| Tamms Group |
| Tradewater Fm |
| Ullin and Salem Limestones (southern Illinois) |
| Upper Pope Group |
| Warsaw Fm. and Salem Ls. in western IL |
| not a geologic unit |