A well-formed, upland soil profile, preserved beneath the Lower Pennsylvanian, Lykens Valley #2 coal in the Southern Anthracite Field of northeastern Pennsylvania, is geologically the oldest example of an Ultisol yet identified. The Lykens Valley paleosol is a complete soil profile that exhibits characteristics consistent with a sustained period of soil formation in a highly oxidizing and leaching upland environment. This paleosol provides evidence that an active and complex forest ecosystem occupied tropical uplands at a time when coal formation dominated the adjacent lowland environments. In addition, it provides insights for understanding the evolution of land plants, the development of oligotrophic, terrestrial ecological interactions, and global carbon cycling. The distribution of kaolinite, illite, chlorite, and aluminum and iron hydroxides within the profile shows that this paleosol formed in a warm, moist, oxidizing environment. Further, on the basis of modern analogs, this profile suggests that a stable substrate may have persisted for as long as a hundred thousand years. The formation of Ultisols during the Lower Pennsylvanian demonstrates that plant communities had complex rooting systems that were capable of collecting nutrients and surviving in a base-poor and well-drained environment at a much earlier time than previous Upper Pennsylvanian estimates. The existence of a viable, upland-forest plant community greatly increases the land area available for terrestrial primary productivity and carbon pumping in equatorial Pangea and should thus have a major impact on global climate during the Lower Pennsylvanian.