Lower Devonian (ILDl;0)

State	Illinois
Name	Lower Devonian
Geologic age	Devonian
Original map label	6170
Comments	The lower Devonian includes the following formations: Clear Creek Chert, Backbone Limestone, Grassy Knob Chert, Bailey Limestone. Based on the statigraphic column on the 1967 paper map, these units collectively consist of about 60% limestone, 30% chert, and 10% dolomite.
Primary rock type	limestone
Secondary rock type	chert
Other rock types	dolostone (dolomite)
Lithologic constituents	Major Sedimentary > Carbonate > LimestoneThe lower Devonian includes the following formations: Clear Creek Chert, Backbone Limestone, Grassy Knob Chert, Bailey Limestone. Based on the statigraphic column on the 1967 paper map, these units collectively consist of about 60% limestone, 30% chert, and 10% dolomite. Minor Sedimentary > Chemical > ChertThe lower Devonian includes the following formations: Clear Creek Chert, Backbone Limestone, Grassy Knob Chert, Bailey Limestone. Based on the statigraphic column on the 1967 paper map, these units collectively consist of about 60% limestone, 30% chert, and 10% dolomite. Sedimentary > Carbonate > DolostoneThe lower Devonian includes the following formations: Clear Creek Chert, Backbone Limestone, Grassy Knob Chert, Bailey Limestone. Based on the statigraphic column on the 1967 paper map, these units collectively consist of about 60% limestone, 30% chert, and 10% dolomite.
Map references	Willman, H.B., and others, (compilers), 1967, Geologic Map of Illinois: Illinois State Geological Survey, scale= 1:500,000, paper.
Unit references	Willman, H.B., and others, (compilers), 1967, Geologic Map of Illinois: Illinois State Geological Survey, scale= 1:500,000, paper.
Geographic coverage	Alexander - Jackson - Pulaski - Union