|Geologic age||Middle Ordovician (Darriwillian) to Upper Ordovician (Stage 5)|
Sedimentary > Clastic > Sandstone (Bed)Sandstone can contain dunes, also can be shaly or argillaceous and conglomeratic at base.
Sedimentary > Carbonate > Dolostone (Bed)Dolomite can be cherty, sandy, or shaly or argillaceous.
Sedimentary > Carbonate > Limestone (Bed)Limestone can be cherty, sandy, or shaly or argillaceous.
Sedimentary > Chemical > Evaporite > Anhydrite (Bed)Gypsum or anhydrite.
Sedimentary > Chemical > Evaporite > Gypsum (Bed)Gypsum or anhydrite.
|Comments||Thickness from stratigraphic columns (from oldest to youngest): St. Peter Ss (75-350’), Dutchtown Ls (0-300’), and Joachim Dol (0-450’) in southern Illinois; St. Peter Ss (0-600’), Joachim Dol (0-50’), and Glenwood Fm (0-75’) in northern Illinois. St. Peter Ss is composed, from oldest to youngest, of the Kress Mbr, Tonti Mbr, and Starved Rock Mbr in northern Illinois (from strat. col.).|
|Stratigraphic units||Kress Member, Tonti Member, and Starved Rock Member of the St. Peter Sandstone.|
Kolata, D.R., comp., 2005, Bedrock Geology of Illinois: Champaign, Ill., Illinois State Geological Survey, Illinois Map Series 14, scale 1:500,000.
|Counties||Boone - Calhoun - Carroll - DeKalb - Jo Daviess - Kendall - La Salle - Lee - Monroe - Ogle - Stephenson - Winnebago|