A small wedge of detrital sedimentary rocks (deposits of the outer part of a deltaic plain) was investigated to delineate subenvironments. The rock unit (Middle Pennsylvanian age) is as thick as 40 ft and occupies an area of about 300 sq mi in western Pennsylvania, where intense strip mining provides excellent continuity of outcrop. Criteria for defining rock subfacies are vertical sequences of bed-thickness properties of sandstone and siltstone, together with minor sedimentary structures and fossil content--in brief, those properties observable in smaller exposures or subsurface records.

Lateral relations of the vertically defined rock subfacies, exposed in continuous cut faces, delineate

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five distinct lithologic facies. The composite properties of these facies imply deposition in subenvironments associated with an outer, predominantly subaqueous deltaic plain--distributary-channel, distributary-mouth-bar, crevasse, reworked and probably drifted bar-sand, and open-bay subenvironments.

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