Tamms Group

Tamms Group, includes Bailey Limestone, “Flat Gap Limestone”, Grassy Knob Chert, Backbone Limestone, and Clear Creek Formation.
State Illinois
Name Tamms Group
Geologic age Upper Silurian (Pridoli) to Lower Devonian (Emsian)
Lithologic constituents
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.

NGMDB product
Counties Alexander - Jackson - Pulaski - Union