Paleobotanic, sedimentologic, and lithologic analyses of 34 core samples of underclay from the New River and Kanawha Formations of the proposed Pennsylvanian System stratotype, West Virginia, and 14 samples from strip mines in the Llewellyn Formation from the anthracite fields, Pennsylvania, indicate that more than half of the coal beds studied began forming in response to changing sedimentation and edaphic conditions that were facies independent. Peak formation was a consequence of factors that favored increased organic accumulation in swamps previously dominated by clastic deposition (clastic swamps). The underclay beds grade upward from gray mudstone that is intensely rooted, but otherwise free of plant fossils, to fissile, vitrain-rich, rooted, black shale containing abundant plant fossils, to coal. The black shale flora consists of lycopod, pteridosperm, and calamite branches and large trunks. Foliage is absent because it was destroyed by rooting and decay. The trunks are lying in situ. They are unidirectionally oriented in places, and usually associated with Stigmaria rooted hummocks. Trunk accumulations are never associated with flood-transported or lacustrine sediments. Trunks were preserved when a rising water table led to a reduced oxidation potential in the underlying mud. The change from clastic sedimentation to peat formation was rapid in most swamps, as evidenced by a transition zone only a few millimeters thick. However, evidence of longer transition stages is present for some swamps where as much as 0.5 m of black shale was deposited. Occasionally, several cycles of underclay-black shale formation were repeated before peat accumulation began. The transitional process could have been arrested at any time before ideal peat-forming conditions were attained. The gradual transitions from clastic to organic sedimentation suggest that swamp types form a continuum between clastic and peat swamp end members.