Ancell Group, includes St. Peter Sandstone, Dutchtown Limestone, Joachim Dolomite, and Glenwood Formation; includes Prairie du Chien Group in Jo Daviess County of northwestern Illinois.
Shelburn-Patoka Formations undivided.
Prairie du Chien Group, includes Gunter Sandstone, Oneota Dolomite, New Richmond Sandstone, and Shakopee Dolomite in northern Illinois.
Platteville Group, includes Pecatonica, Mifflin, Grand Detour, Nachusa, and Quimbys Mill Formations; includes Plattin Limestone in southwestern Illinois.
Galena Group (Trenton Limestone), includes Decorah, Dunleith, Wise Lake and Dubuque Formations in northern Illinois.
Cambrian System, includes Mt. Simon Sandstone, Eau Claire Formation, Ironton-Galesville Sandstone, Franconia Formation, Potosi Dolomite, and Eminence Dolomite.
Maquoketa Formation or Group, includes Cape Limestone, Cape La Croix Shale, Thebes Sandstone, Orchard Creek Shale, Girardeau Limestone, and Leemon Formation in southern Illinois; includes Scales Shale, Fort Atkinson Limestone, Brainard Shale, and Neda Formation in northern Illinois; includes Noix Oolite in western Illinois.