The Valmeyeran Series is named for Valmeyer, Monroe County, near which much of the series is exposed. It is the middle series of the Mississippian System and includes formations assigned to two series (Osagian and Meramecan) in other areas. The Valmeyeran Series underlies most of central and southern Illinois and includes strata from the top of the Chouteau Limestone upward to the base of the Shetlerville Member of the Renault Limestone. The series is thickest, over 1800 feet, in southeastern Illinois, and it thins to 600 feet or less before being truncated by erosion in northern Illinois.