The Iowa, or lower Mississippian, series consists of the formations belonging to the Kinderhook, Osage, and Meramec groups. The Kinderhook group is made up almost entirely of shales with one or two thin limestones near the top. The Osage group is composed dominantly of silty shale and siltstone throughout the Illinois basin except in the extreme western and northwestern parts where the group can be separated into the typical Fern Glen, Burlington, Keokuk, and Warsaw formations. The Meramec group, consisting of the Salem, St. Louis, and Ste. Genevieve formations, is composed almost entirely of limestone and dolomite which lend themselves readily to separation by means of insoluble residues. The Iowa series reaches a maximum thickness of 2,000 feet in Gallatin County. This s probably due to thickening of the individual formations. Local variations in the thickness of the series and beveling of upper Iowa strata in Bond and Clark counties indicate some pre-Chester folding.