A 60 ft sequence of rocks overlying the Lower Kittanning coal bed in western Pennsylvania records a transgressive-regressive sequence of marine deposits that intertongue with nonmarine rocks. The transgressive phase was initiated by subsidence of an area the shape and position of which seems to have been controlled by the character of the underlying deposits. The regressive phase seems to have begun when rate of influx of detrital sediment exceeded the prevailing subsidence. These determining factors are similar to those operating in the present delta of the Mississippi River. Subfacies of the marine and nonmarine facies in the rocks of Pennsylvanian age and their identifying characteristics are described.