The term “facies” is commonly used in diverse manner to denote almost any type of sedimentary deposit, with or without reference to defined stratigraphic units. Study of the problem of classifying sedimentary rocks and interpreting their historical meaning indicates the desirability of restricting application of the term “fades” to particular aspects or expressions of defined stratigraphic units. The collective physical and organic characters found in any sedimentary rock which indicate environment of deposition may be designated by using the term “lithofacies.” A sedimentary facies, then, may be defined to comprise any areally segregated part of a designated rock division in which physical-organic characters differ significantly from those of another part or parts. A facies consists of one or more lithofacies.

Examination of efforts to devise formal classification and nomenclature of sedimentary deposits in terms of facies indicates that such schemes are unnecessary.

Features of Pennsylvanian rocks of the midcontinent region are described briefly in order to demonstrate distinctions in the concepts of facies and lithofacies.