in the Cretaceous. The up to 150 foot (45 m) thick formation consists of fine-grained tan to brownish-gray colored quartz-rich sandstone that is interbedded with thin layers of carbon-rich shale, coal, and conglomerate. Petrified wood is found in the lower part of the formation while fossilized marine bivalves such as Corbula and Pycnodonte newberryi are in the upper layers. This fossil progression shows a record of flooding that created the seaway. Dakota erode into small cliffs and hogbacks that can be seen in the southern section of the park. Mancos Shale and Mesaverde Formation (Cretaceous) Approximately 94 to 85 million years ago, the seaway advanced onto and retreated from land as it laid down the Mancos Shale. The Mancos is composed mostly of shale but two of its members, the Ferron and Muley Canyon, are sandstone that were laid down when relative sea level temporarily dropped. The five Mancos members from oldest to youngest are: Tununk Shale, Ferron Sandstone, Blue Gate Shale, Muley Canyon, and Masuk. Parts of this formation are found in some mesas and buttes in the southernmost part of the park and in badlands east of the park. Open marine conditions created the locally 40 to 720 feet (12 to 220 m) thick gullied slope-forming Tununk Shale Member. It is made of bluish-gray shale with interbedded mudstone, fine-grained sandstone and siltstone. The Tununk erodes into a slope and is locally fossil-rich. It is most prominently exposed in the Blue Desert immediately southeast of Cathedral Valley and contains fossilized examples of cephalopods, bivalves, and fish scales. 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. Open marine conditions returned in the Late Cretaceous, forming the locally 1,200 to 1,500 foot (365 to 460 m) thick slope-forming Blue Gate Shale. This member is composed of bentonite rich clays, siltstone and some sandstone. It erodes into gullied slopes similar in appearance to the Tununk Shale. The presence of two species of planktonic