|Name||Galena Group (Trenton Limestone)|
|Geologic age||Upper Ordovician (Stage 5 to Stage 6)|
Sedimentary > Carbonate > Dolostone (Bed)Dolomite can be silty, shaly or argillaceous, or cherty.
Sedimentary > Carbonate > Limestone (Bed)Limestone can be silty.
Sedimentary > Clastic > Mudstone > Claystone > Bentonite (Bed)K-bentonite
|Comments||Thickness from stratigraphic column for northern Illinois (north of 40 degrees north latitude), from oldest to youngest: Decorah Fm (0-25’), Dunleith Fm (0-135’), Wise Lake Fm (0-80’), and Dubuque Fm (0-40’). Unit is listed as the Galena - "Trenton" Group on the stratigraphic column for northern Illinois (north of 40 degrees north latitude). The Decorah Formation is comprised, from oldest to youngest, of the Spechts Ferry Shale, Millbrig K-bentonite Bed, and Guttenberg Dolomite according to the stratigraphic column on the back of the geologic map.|
|Stratigraphic units||Spechts Ferry Shale, Millbrig K-bentonite Bed, Guttenberg Dolomite|
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 - Bureau - Carroll - DeKalb - Grundy - Jersey - Jo Daviess - Kane - Kendall - La Salle - Lee - McHenry - Ogle - Stephenson - Whiteside - Winnebago|