Geologic units in Douglas county, Illinois

Shelburn-Patoka Formations undivided (Pennsylvanian (Kasimovian)) at surface, covers 39 % of this area

Shelburn-Patoka Formations undivided.

Mattoon Formation (Pennsylvanian (Kasimovian)) at surface, covers 29 % of this area

Mattoon Formation.

Carbondale Formation (Pennsylvanian (Moscovian to Kasimovian)) at surface, covers 11 % of this area

Carbondale Formation.

Tradewater Formation (Pennsylvanian (Bashkirian to Moscovian)) at surface, covers 6 % of this area

Tradewater Formation

Borden Siltstone (Mississippian (Tourniaisian to Visean)) at surface, covers 5 % of this area

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

Bond Formation (Pennsylvanian (Kasimovian)) at surface, covers 4 % of this area

Bond Formation

Muscatatuck Group (Middle Devonian (Eifelian to Givetian)) at surface, covers 4 % 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.

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 3 % of this area

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

Silurian System undivided (Silurian (Llandovery to Pridoli)) at surface, covers 0.1 % 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.

Meppen Limestone, Fern Glen Formation, and Burlington-Keokuk Limestone (Mississippian (Tourniaisian to Visean)) at surface, covers < 0.1 % of this area

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