Geologic units in Winnebago county, Illinois

Galena Group (Trenton Limestone) (Upper Ordovician (Stage 5 to Stage 6)) at surface, covers 52 % of this area

Galena Group (Trenton Limestone), includes Decorah, Dunleith, Wise Lake and Dubuque Formations in northern Illinois.

Platteville Group (Upper Ordovician (Stage 5)) at surface, covers 24 % of this area

Platteville Group, includes Pecatonica, Mifflin, Grand Detour, Nachusa, and Quimbys Mill Formations; includes Plattin Limestone in southwestern Illinois.

Ancell Group (Middle Ordovician (Darriwillian) to Upper Ordovician (Stage 5)) at surface, covers 23 % of this area

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.

Sinnipee Group (Ordovician) at surface, covers 0.6 % of this area

Dolomite with some limestone and shale; includes Galena, Decorah, and Platteville Formations

Ancell Group (Ordovician) at surface, covers 0.4 % of this area

Orthoquartzitic sandstone with minor limestone, shale and conglomerate; includes Glenwood and St. Peter Formations

Cambrian, undivided (Cambrian) at surface, covers < 0.1 % of this area

Sandstone with some dolomite and shale, undivided; includes Trempealeau, Tunnel City, and Elk Mound Formations

Prairie du Chien Group (Ordovician) at surface, covers < 0.1 % of this area

Dolomite with some sandstone and shale; includes Shakopee and Oneota Formations