| Geologic age Millions of years ago | Missouri Plateau—Black Hills | High Plains—Plains Border—Colorado Piedmont | Pecos Valley—Edwards Plateau—Central Texas |
|---|---|---|---|
| Quaternary | |||
| Pleistocene | Glacial deposits, alluvium, and terrace deposits | Alluvium, sand dunes, and loess | Piedmont, terrace, and bolson deposits |
| 2 | erosional surface | ||
| Tertiary | |||
| Pliocene | EROSION | ||
| 5 | Flaxville Gravel and Ogallala Formation | 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 Formation | Denver, Poison Canyon, and Raton Formations | |
| 65 | |||
| Cretaceous | Hell Creek and Lance Formations | Vermejo and Laramie Formations | |
| Fox Hills Sandstone | Trinidad and Fox Hills Sandstones | ||
| Shales, sandstones, and limestones deposited in Late Cretaceous sea | |||
| Dakota Sandstone and Lakota Formation | Dakota Sandstone | ||
| Glen Rose and Edwards Limestones | |||
| 136 | |||
| Jurassic | Sundance Formation, Ellis Group, and Unkpapa Sandstone | Morrison Formation | Jurassic rocks not present |
| 190-195 | |||
| Triassic | Dominantly red rocks | ||
| 225 | |||
| PALEOZOIC | Paleozoic rocks, undivided | ||
| 570 | |||
| PRECAMBRIAN | Precambrian rocks, undivided | ||
As the mountains continued to rise, the eroding streams cut into the old core rocks of the mountains, and that debris too was carried to the flanks and onto the adjoining plains. The mountainous belt continued to rise intermittently, and volcanoes began to appear about 50 million years ago. Together, the mountains and volcanoes provided huge quantities of sediment, which the streams transported to the plains and deposited. The areas nearest the mountains were covered by sediments of Late Cretaceous and Paleocene age ([table 1])—the Poison Canyon Formation to the south, the Dawson and Denver Formations in the Denver area, and the Fort Union Formation to the north ([fig. 8]). Vegetation continued to flourish, especially in the northern part of the Great Plains, and was buried to form the thick lignite and subbituminous coal beds of the Fort Union Formation ([fig. 9]). The earliest mammals, most of whose remains come from the Paleocene Fort Union Formation, have few modern survivors.
Beginning about 45 million years ago, in Eocene time, there was a long period of stability lasting perhaps 10 million years, when there was little uplift of the mountains and, therefore, little deposition on the plains. A widespread and strongly developed soil formed over much of the Great Plains during this period of stability. With renewed uplift and volcanism in the mountains at the end of this period, great quantities of sediment again were carried to the plains by streams and spread over the northern Great Plains and southeastward to the arch or divide separating the Williston and Anadarko basins ([fig. 8]). Those sediments form the White River Group, in which the South Dakota Badlands are carved. In addition to the Titanotheres, huge beasts with large, long horns on their snouts who lived only during the Oligocene (37 to 22 million years ago), vast herds of camels, rhinoceroses, horses, and tapirs—animals now found native only on other continents—grazed those Oligocene semiarid grassland plains.
Figure 8.—Progressive southeastward expansion of areas covered by Paleocene, Oligocene, and Miocene-Pliocene sedimentary deposits.
Powder River basin Denver basin Raton basin PLAINS Margin of Oligocene deposition Margin of Miocene-Pliocene deposition
Figure 9.—Big Horn coal strip mine in Fort Union Formation at Acme, Wyo. Photograph by F. W. Osterwald, U.S. Geological Survey.
Sometime between 20 and 30 million years ago the streams began depositing sand and gravel beyond the divide, and, for another 10 million years or more, stream sediments of the Arikaree and Ogallala Formations spread over the entire Great Plains from Canada to Texas, except where mountainous areas such as the Black Hills stood above the plains. Between 5 and 10 million years ago, then, the entire Great Plains was an eastward-sloping depositional plain surmounted only by a few mountain masses. Horses, camels, rhinoceroses, and a strange horselike creature with clawed feet (called Moropus) lived on this plain.