A wave-dominated delta and river system then spread over the area, creating the locally 205 to 385 feet (62 to 117 m) thick cliff-forming Ferron Sandstone. It is composed of brown fine-grained sandstone along with white cross-bedded sandstone with interbedded carbonate-rich gray shale. The marine bivalve "Inoceramus" and trace fossils of "Ophiomorpha" are found in the lower part of this member. Ferron Sandstone north of the area contains seams of coal in its upper part, prompting some petroleum geologists to study this member to model oil-bearing regions.