The Tesnus [Pennsylvanian] has interbeds of dark dull hornstone which contain Radiolaria. The only other known fossils are macerated plants, and a fossil stump in the basal conglomerate. The Caballos Novaculite, which underlies the Tesnus unconformably, was originally suggested to be of deep water origin. It is also radiolarian. The formation is structurally weak, suggesting it is thinned out by stretching in the NW. and thickened by flowage in its SE. exposures.