[Illustration: _Figure 7.—Structural setting of the Great Plains. Williston basin and Anadarko basin are separated by a midcontinental arch._] [Illustration: Table 1.—Generalized chart of rocks of the Great Plains] Geologic age Missouri High Plains—Plains Pecos Millions of Plateau—Black Border—Colorado Valley—Edwards years ago Hills Piedmont Plateau—Central Texas Quaternary Pleistocene Glacial deposits, Alluvium, sand Piedmont, terrace, alluvium, and dunes, and loess and bolson terrace deposits deposits 2 erosional surface Tertiary Pliocene EROSION 5 Flaxville Gravel Ogallala formation and Ogallala Formation Miocene Arikaree Formation Arikaree Formation 22-24 erosional surface Oligocene White River Group White River Group Mostly missing because of erosion or nondeposition 37-38 erosional surface Eocene Wasatch and Golden Valley Formations 53-54 Dawson Arkose Paleocene Fort Union Denver, Poison Formation Canyon, and Raton Formations 65 Cretaceous Hell Creek and Vermejo and Laramie Lance Formations Formations Fox Hills Sandstone Trinidad and Fox Hills Sandstones Shales, sandstones, and limestones deposited in Late Cretaceous sea Dakota Sandstone Dakota Sandstone and Lakota Formation Glen Rose and Edwards Limestones 136 Jurassic Sundance Morrison Formation Jurassic rocks not Formation, Ellis present Group, and Unkpapa Sandstone 190-195 Triassic Dominantly red rocks 225 PALEOZOIC Paleozoic rocks, undivided 570 PRECAMBRIAN Precambrian rocks, undivided