The buttes are characteristically Arikaree or Gering formations topping Brule clay.
The Indians on reservations and in Indian schools include members of the Yankton, Yanktonai, Oglala, Brule, Sisseton, Wahpeton, Flandreau, Sioux, Blackfeet, Miniconjou, Sans Arc and Ute tribes, on the Standing Rock and Cheyenne River reservations in the north of the state, the Lower Brule and Crow Creek reservations in the central part, and the Pine Ridge and Rosebud reservations in the south.
By Wisconsin, and the Menominee, Montreal and Brule Rivers, which separate it in part from Wisconsin.
About 5 6, 5 60 acres of Lower Brule lands were opened for settlement in 1889, about 1,600,000 acres of Sisseton and Wahpeton lands' in 1892, 168,000 acres of the Yankton Sioux lands in 1895, 416,000 acres of the Rosebud lands in 1904, and 800,000 acres in 1908.
The superficial Miocene and Pliocene deposits in the west, above referred to, are underlaid by the White river groups of the Oligocene, whose outcrops of Brule clay and Chadron formation also have been mentioned.