St. Louis

The St. Louis Limestone is named for St. Louis, Missouri, where it is extensively exposed. No type section was designated. The St. Louis is typically exposed in Illinois in the Mississippi River bluffs at Alton, Madison County. It is also well exposed in the Mississippi and Illinois Valleys in western and southern Illinois and along the Ohio River in Hardin County. The St. Louis is 500 feet thick in southeastern Illinois and thins north-westward to less than 200 feet before being truncated by pre-Pennsylvanian erosion. The St. Louis Limestone in Illinois is characterized by fine-grained, micritic to lithographic, cherty limestone, but it contains beds of dolomite, crystalline limestone, fossiliferous limestone, and evaporites.