Sedimentary structures, textures, plant fossils, and the continental character of associated Pennsylvanian strata suggest that basal Pennsylvanian quartzarenite in eastern Kentucky and southwestern Virginia was deposited in a fluvial environment. Along the Kentucky-Virginia border, a southwest-trending paleovalley as much as 400 ft deep and 7 mi wide is cut into Mississippian rocks; it contains the basal quartzarenite of the Middlesboro Member (Pennsylvanian) of the Lee Formation. The arenite is mineralogically and texturally mature, and previous studies have interpreted it as a beach, barrier bar, or barrier island deposit. Those studies resulted in an interpretation of the geologic history of the area that included uplift and erosion of the northwestern part of the basal Pennsylvanian Pocahontas Formation followed by deposition of the Middlesboro arenite by a southeastward-transgressing sea.
Another interpretation of the same subsurface data used to support the coastal marine depositional model is that the Pocahontas was deposited in an interior valley southeast of and subparallel to the Middlesboro paleovalley. In this interpretation, the two paleovalleys are part of the same southwest-trending drainage system, and the Mississippian-Pennsylvanian unconformity, contrary to most presently accepted ideas, probably extends completely across the Appalachian basin. Furthermore, filling of the Middlesboro paleovalley by coarse sand and gravel may have caused damming of the Pocahontas paleovalley at a confluence southeast of the present eastern limit of the Pennsylvanian in southwestern Virginia. If Pocahontas sediments were deposited in a subsiding interior valley blocked by the basal beds of the Middlesboro, then the Pocahontas and Lee Formations are in part lateral equivalents.