H. B. Willman and John C. Frye
The Pliocene Series (Lyell, 1833), translated as "more recent," was so named because the major part of the invertebrate fossils are referable to modern species.
All of the Pliocene Series deposits were previously called "Lafayette", "Lafayette-type", or "Tertiary" gravel.
Extent and Thickness
The Pliocene Series is represented in Illinois by the widespread brown chert gravel deposits of southern Illinois-- the Mounds Gravel-- and the similar but scattered deposits in western Illinois-- the Grover Gravel.
Both the Mounds and Grover Formations probably contain some deposits reworked during the early Pleistocene that are not readily differentiated from those deposited during the late Tertiary. Consequently, both the Mounds and Grover Formations are assigned a Pliocene-Pleistocene age (Willman and Frye, 1970).
LYELL, CHARLES, 1833, Principles of geology: v. 3, London, p. 52-55, 57-58.
WILLMAN, H. B., and J. C. FRYE, 1970, Pleistocene stratigraphy of Illinois: Illinois State Geological Survey Bulletin 94, 204 p.
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