conglomerate of the Ogallala/Arikaree formation
The Pawnee Buttes in the Pawnee National Grasslands, rise some 300-feet above the plains. They are capped by Miocene sandstone and conglomerate called the Ogallala/Arikaree formation, which is more resistant to weathering.
The lower portions of the buttes are composed of sedimentary rock called the Brule formation, part of the Oligocene White River Formation. The White River badlands stretch across parts of Colorado, Wyoming, Nebraska and South Dakota. Many fossils have been found in this layer as it has eroded away. Fossils identified so far include a mesohippus leg, a camel skull and numerous Stylemys tortoise shells.