For more than 60 years, formations of Late Ordovician age in southeastern Indiana have been identified on the basis of the contained fossils. Unpredictable variability of the rocks has been given as the reason that lithologic criteria were not used. It is true that individual beds are not persistent laterally, but groups of beds can be traced by using electric and other geophysical logs, quantitative insoluble-residue logs, and

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lithologic-summary logs compiled from detailed measured sections. These techniques have the common advantage of objective automatic generalization, so that meaningless details are minimized and important lithologic trends can be seen. Many of the systematic variations in rock content thus revealed can be traced with confidence across the outcrop and into the subsurface.

Shale comprises about 80 per cent of this 900-foot-thick body of rocks; limestone of several distinctive types makes up the remainder. Formations recognized include, in ascending order, the Lexington Limestone of recent Kentucky usage and the Kope, Dillsboro, Saluda, and Whitewater Formations. The recently defined Tanners Creek Formation can not be distinguished on a strictly lithologic basis, and it is therefore included in the Dillsboro. The Lexington and the Saluda are dominantly carbonate units; the Dillsboro and Whitewater are principally shale but include significant quantities of limestone; and the Kope is almost entirely shale. The boundaries of these formations can be rather closely identified on the several types of logs mentioned above, and refinements usually are possible here original detailed data are available. Intermediate beds and horizons also can be identified and traced for considerable distances.

In dealing with apparently disorganized rock sequences of this type, it is essential to utilize or devise techniques to elicit generalizations from the confusing maze of data.

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