A study of the conodont assemblages from the Maple Mill, Grassy Creek, and Louisiana formations discloses that these assemblages are closely related and also that these assemblages are more closely related to conodonts from unquestionably Devonian rocks than to an assemblage from rocks of undoubted Mississippian age. I have assigned the Maple Mill shale to the upper Devonian because of these reasons; therefore, the Maple Mill shale must be redefined to indicate its Devonian age. The Maple Mill and Grassy Creek shales, of Iowa and Missouri respectively, are time equivalents, because subsurface studies show that the 2 shales grade laterally into each other.
The methods employed to determine the age and correlation of the Maple Mill shale were applied to the English River and Prospect Hill siltstones. These methods indicate that these siltstones are lower Mississippian, and that they are time equivalents of the Bushberg and Hannibal formations of Missouri.
The Louisiana limestone of Missouri has been correlated with the McCraney limestone of Iowa because of similarity of a few macro-fossils and lithologic characteristics. Conodont studies suggest that the Louisiana is related to the Grassy Creek, whereas the McCraney is related to the Hannibal. This difference precludes the possibility of the two limestones being correlated as time equivalents.