|Geologic age||Upper Silurian (Pridoli) to Lower Devonian (Emsian)|
Sedimentary > Carbonate > Limestone (Bed)Limestone can be cherty, dolomitic, silty, or shaly or argillaceous.
Sedimentary > Chemical > Chert (Bed)
|Comments||Thickness from stratigraphic column for southern Illinois (south of 40 degrees north latitude), from oldest to youngest: Bailey Ls (0-500’), Flat Gap Ls (0-100’), Grassy Knob Chert (0-300’), Backbone Ls (0-200’), and Clear Creek Fm (0-600’). [“Flat Gap Limestone" is oil well drillers' term.] Unit label in original GIS (Esri shapefile IL_Geologic_Units_500K_2005_Py) is 'Dt'; label on Legend of geologic map is 'DSt'.|
Kolata, D.R., comp., 2005, Bedrock Geology of Illinois: Champaign, Ill., Illinois State Geological Survey, Illinois Map Series 14, scale 1:500,000.
|Counties||Alexander - Jackson - Pulaski - Union|