Quantitative and semiquantitative mineralogical, chemical, and textural analyses of 158 shale samples collected laterally from three horizons and vertically from a core through the Cincinnatian Series which is exposed in Ohio, Indiana, and Kentucky provide data for an understanding of the depositional and postdepositional environment of this paleontologically classic section. Statistical treatment of the data provides no evidence for the existence of the Cincinnati arch in Ordovician time, but does reveal a strong north to south lateral variation in the Clarksville member of the Waynesville formation which is probably an indication of marine transgression in that direction and/or an east-west trending barrier. The vertical homogeneity of the shales suggests static environmental conditions and encourages the restriction of the present stratigraphic nomenclature of the Cincinnatian Series to biostratigraphic, rather than lithologic implication. The desirability of a revision of the Cincinnatian stratigraphic names is indicated.

The author recognizes two chemical environments of deposition, one a normal open circulative oxidizing marine environment from the surface to near or below the sedimentary interface, and the other a reducing environment from near or below the sedimentary interface to a limited depth within the sediment. The postdepositional P-T environment was that of “early diagenesis” associated with shallow depth of burial.

The author suggests an alternative explanation for the repeated limestone-shale lithology of the Cincinnatian Series. Spatial- rather than time-controlled fluctuations in current flow may have allowed the deposition of the limestone units, principally derived from indigenous fauna, in very low-energy environments, and the shale units in somewhat higher-energy environments where partially terrigenous, mostly silt-sized detritus was deposited.

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