Geologic units in Alexander county, Illinois

Tamms Group (Upper Silurian (Pridoli) to Lower Devonian (Emsian)) at surface, covers 42 % of this area

Tamms Group, includes Bailey Limestone, “Flat Gap Limestone”, Grassy Knob Chert, Backbone Limestone, and Clear Creek Formation.

Clayton Formation, Porters Creek Formation, Claiborne Formation, Wilcox Formation (Tertiary (Paleocene) to Tertiary (Pliocene)) at surface, covers 22 % of this area

Clayton and Porters Creek Formations; Claiborne and Wilcox Formations.

Maquoketa Formation or Group (Upper Ordovician (Stage 6 to Hirnatian)) at surface, covers 12 % 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.

Silurian System undivided (Silurian (Llandovery to Pridoli)) at surface, covers 8 % 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.

Kimmswick (Trenton) Limestone and Decorah Formation (Upper Ordovician (Stage 6)) at surface, covers 5 % of this area

Kimmswick (Trenton) Limestone and Decorah Formation in southern and western Illinois.

Ft. Payne Formation (Mississippian (Tourniaisian to Visean)) at surface, covers 3 % of this area

Ft. Payne Formation, includes Springville Shale and Chouteau Limestone in southern Illinois.

Cretaceous undivided (Upper Cretaceous (Campanian to Maastrichtian)) at surface, covers 3 % of this area

Cretaceous undivided, includes Post Creek (Tuscaloosa), McNairy, and Owl Creek Formations (southern Illinois).

Muscatatuck Group (Middle Devonian (Eifelian to Givetian)) at surface, covers 1 % 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.

Holocene series (Quaternary-Holocene Series) at surface, covers 1 % of this area

Alluvium - clay, silt, sand, and gravel.

Ullin and Salem Limestones (Mississippian (Visean)) at surface, covers 0.9 % of this area

Ullin and Salem Limestones in southern 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 0.8 % of this area

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

Tertiary and Cretaceous undivided (Upper Cretaceous (Campanian) to Tertiary (Pliocene)) at surface, covers 0.5 % of this area

Tertiary and Cretaceous undivided.

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.