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. [Illustration: _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 [Illustration: _Figure 9.—Big Horn coal strip mine in Fort Union Formation at Acme, Wyo. Photograph by F. W. Osterwald, U.S. Geological Survey._]