Recent stratigraphic studies of Allegheny rocks [Pennsylvanian] of western Pennsylvania seem to show that the marine Kittanning rocks are analogous with Freeport rocks of fresh-water origin. Detrital sediments dominate Allegheny rocks but nondetrital coals, ironstones, and limestones do occur and they separate detrital facies. The couplets of detrital and non-detrital rocks can be considered as records of cyclic sedimentation.

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