Geologic units in Cook county, Illinois

Silurian System undivided (Silurian (Llandovery to Pridoli)) at surface, covers 93 % 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.

Muscatatuck Group (Middle Devonian (Eifelian to Givetian)) at surface, covers 3 % of this area

Muscatatuck Group, includes Grand Tower Limestone and St. Laurent (Alto and Lingle) Formation in southern Illinois; includes Detroit River Formation, Wapsipinicon Limestone, and Cedar Valley Limestone in northern and western Illinois.

Paleozoic undifferentiated (Cambrian to Pennsylvanian) at surface, covers 2 % of this area

Paleozoic undifferentiated, includes Cambrian, Ordovician, Silurian, Devonian, Mississippian, and Pennsylvanian strata within the Des Plaines Structure in northern Cook County.

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.

Wabash Formation (Silurian) at surface, covers < 0.1 % of this area

Limestone, dolomite, and argillaceous dolomite