Environmental mapping in Pennsylvanian and Mississippian rocks from Oklahoma to Pennsylvania has shown that most lenticular masses of sandstone and shale are parts of deltaic complexes. In Late Mississippian and Pennsylvanian rocks, deltaic expansion commonly follows brief marine transgressions. Widespread marine limestones may terminate against broad arcs of prodeltas composed of evenly laminated gray shales with ironstone nodules. The prodelta deposits become more sandy upward and are succeeded by conformable sheet sands or unconformable lenticular sandstones. Thicknesses of the combined delta and prodelta deposits in eastern and central United States are as much as 150 feet, composed entirely of shale, or sandstone or both.

Source areas for delta sands are north, east, and southeast of the Appalachian basin; northeast and north of the Illinois basin and northern Mid-continent; and south, southeast, and southwest of Oklahoma. The Ozark uplift, Nemaha ridge, and central Kansas uplift were unimportant sources; the Canadian shield, northern Appalachians, Transcontinental arch, and Ouachita and Arbuckle uplifts were principal sources. Deltaic growth from different directions was not contemporary.

Detailed mapping of minor features of these deltas, now in progress, shows intricate patterns of sand and shale and indicates that surface configuration of a delta is an important determinant of distribution and thickness of Pennsylvanian coals. Four deltas have been studied in the Lower Mississippian (Pepper and Dewitt), twelve in the Upper Mississippian (Swann and Potter), and twenty in the Pennsylvanian. Examples of entire deltas and details of portions of deltas are illustrated.

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