Geologic units in Iroquois county, Illinois

Silurian System undivided (Silurian (Llandovery to Pridoli)) at surface, covers 34 % 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.

New Albany Shale, Blocher Shale, Sylamore Sandstone, Selmier Shale, Sweetland Creek Shale, Grassy Creek Shale, Saverton Shale, and Louisiana Limestone (Middle Devonian (Givetian) to Upper Devonian (Fammenian)) at surface, covers 21 % of this area

New Albany Shale, Blocher Shale, Sylamore Sandstone, Selmier Shale, Sweetland Creek Shale, Grassy Creek Shale, Saverton Shale, and Louisiana Limestone.

Borden Siltstone (Mississippian (Tourniaisian to Visean)) at surface, covers 18 % of this area

Borden Siltstone, includes Chouteau Limestone in east-central Illinois.

Tradewater Formation (Pennsylvanian (Bashkirian to Moscovian)) at surface, covers 14 % of this area

Tradewater Formation

Muscatatuck Group (Middle Devonian (Eifelian to Givetian)) at surface, covers 11 % 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.

Carbondale Formation (Pennsylvanian (Moscovian to Kasimovian)) at surface, covers 3 % of this area

Carbondale Formation.

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.

Borden Group (Mississippian) at surface, covers < 0.1 % of this area

Mostly siltstone; lenses of crinodial limestone in upper part. Much cherty and silty limestone and dolomite in northwest. NP, top of New Providence Shale