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Geologic units in Lee county, Illinois

Galena Group (Trenton Limestone) (Upper Ordovician (Stage 5 to Stage 6)) at surface, covers 21 % of this area

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

Ancell Group (Middle Ordovician (Darriwillian) to Upper Ordovician (Stage 5)) at surface, covers 17 % 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.

Prairie du Chien Group (Early Ordovician (Tremadocian to Darriwillian)) at surface, covers 16 % of this area

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

Silurian System undivided (Silurian (Llandovery to Pridoli)) at surface, covers 14 % of this area

Silurian System undivided, includes Sexton Creek Limestone, St. Clair Limestone, and Moccasin Springs Formation in southern Illinois; includes Wilhelmi Formation, Elwood Dolomite, Kankakee Dolomite, Joliet Dolomite, Sugar Run Dolomite, and Racine Dolomite in northeastern Illinois; includes Mosalem, Tete des Morts, Blanding, Sweeney, Marcus, and Racine Dolomites in northwestern Illinois.

Cambrian System (Middle Cambrian? to Upper Cambrian) at surface, covers 11 % of this area

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

Platteville Group (Upper Ordovician (Stage 5)) at surface, covers 10 % 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 7 % 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.

Tradewater Formation (Pennsylvanian (Bashkirian to Moscovian)) at surface, covers 4 % of this area

Tradewater Formation