The lower middle Mississippian (lower Greenbrier age) of southeastern West Virginia is composed of the Hillsdale and Denmar (new) formations of the Meramecian series. These formations are predominantly calcarenites and calcilutites deposited in a transgressing sea along the flank of a linear, southwestward plunging, non-volcanic miogeosyncline and are contemporaneous with purer limy sediments of the Mississippi Valley. The Hillsdale formation is thick-bedded, nearly black, fine- to medium-grained, cherty limestone which is recognizable to northwestern Pocahontas County and disappears westward as a wedge in south-central West Virginia. The Denmar formation succeeding it contrasts lithologically in having less chert, lighter color, and more sandy material. The Denmar format on includes Reger's previously defined Sinks Grove and Patton limestones. The outcrop of this formation exceeds the northern limit of the Hillsdale by 23 miles and may be seen as far north as southern Randolph County. Westward under the basin it is thought to be the equivalent of the Maxville limestone of Ohio.

Stratigraphic and faunal evidence shows the relationship of the Hillsdale with the St. Louis of southwestern Virginia, and the Denmar with the Ste. Genevieve of the same area. The presence of Lithostrotionella and other fossils in the Hillsdale is diagnostic of its similarity to the St. Louis, while Platycrinites huntsvillae in the Denmar is a good index species to show its relationship with the Ste. Genevieve elsewhere.

Numerous faunules are listed from both the Hillsdale and the Denmar as well as stratigraphic sections, at varying horizontal intervals, represented in tabular and columnar form.