Pennsylvanian1 rocks form the bedrock surfaqe beneath approximately 75 percent of Illinois. The study area (Fig. 1 next-frame) is generally that northwest of the Illinois River, northeast of the LaMoine Riter, but excludes Henryk Bureau, Stark, and LaSalle Counties/ Included, therefore are Rock island, Mercer, Warren, and Knox Counties (the lower units of vhich were studied by Merrill and King, 1971), plus the present coverjage of the .entire stratigraphie column in Knox, Peoria,.Fulton, and Schuyler Gounties. This study area includes the area covered by Wanless definitive works (1957,1958) on thte Beards town, Glas ford, Havana, and. Vermont quadrangles. Also included Is much of the area covered by the Avon-Canton quadrangle, report (Savage, 1922) and the Peoria quadrangle report (Udden, 1912). Knowledge about this area is therefore relatively detailed, The large surface area covered (roughly 2,650 square miles or 7,000 sq km) presents a great many possible sample sites for the several marine units.
The studied marine units that produced conodonts, in descending order (Fig. 2) are the Cramer (Trivoli of literature), Exline, Lonsdale, “Sparland”, Pokeberry, “Sheffield”, Brereton, St. David, Hanover, Oak Grove, Seahorne, “Seville” , and Seville Members. Reasons for the quotations will be explained id the discussions of the units.