Geologic units in Monroe county, Illinois

St. Louis Limestone (Mississippian (Visean)) at surface, covers 26 % of this area

St. Louis Limestone.

Lower Pope Group (Aux Vases Sandstone through Glen Dean Limestone) (Mississippian (Visean)) at surface, covers 22 % of this area

Lower Pope Group (Aux Vases Sandstone through Glen Dean Limestone).

Warsaw Formation and Salem Limestone (Mississippian (Visean)) at surface, covers 14 % of this area

Warsaw Formation and Salem Limestone in western Illinois.

Tradewater Formation (Pennsylvanian (Bashkirian to Moscovian)) at surface, covers 11 % of this area

Tradewater Formation

Kimmswick (Trenton) Limestone and Decorah Formation (Upper Ordovician (Stage 6)) at surface, covers 9 % of this area

Kimmswick (Trenton) Limestone and Decorah Formation in southern and western Illinois.

Meppen Limestone, Fern Glen Formation, and Burlington-Keokuk Limestone (Mississippian (Tourniaisian to Visean)) at surface, covers 6 % of this area

Meppen Limestone, Fern Glen Formation, and Burlington-Keokuk Limestone.

Ste. Genevieve Limestone (Mississippian (Visean)) at surface, covers 5 % of this area

Ste. Genevieve Limestone.

Platteville Group (Upper Ordovician (Stage 5)) at surface, covers 3 % of this area

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

Maquoketa Formation or Group (Upper Ordovician (Stage 6 to Hirnatian)) at surface, covers 1 % of this area

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.

Ancell Group (Middle Ordovician (Darriwillian) to Upper Ordovician (Stage 5)) at surface, covers 1 % 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.

Holocene series (Quaternary-Holocene Series) at surface, covers 0.4 % of this area

Alluvium - clay, silt, sand, and gravel.