Geologic units in La Salle county, Illinois

Carbondale Formation (Pennsylvanian (Moscovian to Kasimovian)) at surface, covers 28 % of this area

Carbondale Formation.

Ancell Group (Middle Ordovician (Darriwillian) to Upper Ordovician (Stage 5)) at surface, covers 25 % 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.

Shelburn-Patoka Formations undivided (Pennsylvanian (Kasimovian)) at surface, covers 18 % of this area

Shelburn-Patoka Formations undivided.

Prairie du Chien Group (Early Ordovician (Tremadocian to Darriwillian)) at surface, covers 10 % of this area

Prairie du Chien Group, includes Gunter Sandstone, Oneota Dolomite, New Richmond Sandstone, and Shakopee Dolomite in northern Illinois.

Mattoon Formation (Pennsylvanian (Kasimovian)) at surface, covers 6 % of this area

Mattoon Formation.

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

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

Tradewater Formation (Pennsylvanian (Bashkirian to Moscovian)) at surface, covers 3 % of this area

Tradewater Formation

Bond Formation (Pennsylvanian (Kasimovian)) at surface, covers 3 % of this area

Bond Formation

Galena Group (Trenton Limestone) (Upper Ordovician (Stage 5 to Stage 6)) at surface, covers 2 % of this area

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

Cambrian System (Middle Cambrian? to Upper Cambrian) at surface, covers 0.7 % of this area

Cambrian System, includes Mt. Simon Sandstone, Eau Claire Formation, Ironton-Galesville Sandstone, Franconia Formation, Potosi Dolomite, and Eminence Dolomite.

Maquoketa Formation or Group (Upper Ordovician (Stage 6 to Hirnatian)) at surface, covers 0.1 % 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.