Studies of distribution and character of Pennsylvanian coals in the eastern and central United States have shown that their accumulation resulted from several environmental patterns, as follows: (1) distribution controlled by the building of a widespread delta (Illinois 5, 5a, and 6 coals in southern Illinois basin); (2) unfilled channels, either of alluvial streams (western Illinois No. 4 coal) or unfilled delta distributary channels (No. 5 coal, southeastern Illinois) – thinner coal is found away from the unfilled channels; (3) coal accumulation in an estuary prior to drowning (No. 1 coal of Illinois); (4) deposition on a narrow coastal strip comparable with Atlantic coastal marshes of today (Mulberry coal of Missouri and Kansas); (5) local accumulation in a lagoon behind an offshore barrier (Ogan, Winters, Clarion, and Scrubgrass coals of southern Ohio); (6) local accumulation in a cut-off meander of a stream, with clastic partings and no underclay (local coals in Pleasantview Sandstone of western Illinois); (7) a plain exposed following abrupt marine regression (Lexington coal of Missouri and Iowa); (8) a level depositional plain formed by burial of pre-Pennsylvanian topographic irregularities (No. 2 coal of Illinois; Croweburg coal of midcontinent).

Some coals formed on delta plains or flood plains are separated from channels by natural levees, and an example has been found in the substrata of the No. 5 coal in southeastern Illinois. Where a fluviatile or deltaic channel was filled, because of greater compaction of shales a way from the channel, a coal may thin or wedge out over the channel sandstone.

A study of the environments in which Pennsylvanian coals have formed contributes also to the understanding of the genesis of underclays. It is suggested that an understanding of composition and geometry of coal substrata should guide exploration for future coal reserves.