They are drained and in large part enclosed by the North (or Belle Fourche) and South forks of the Cheyenne river (at whose junction a fur-trading post was established about 1830); and are surrounded by semi-arid, alkaline plains lying 3000 to 3500 ft.
Of these the Cheyenne is the most important, being formed by two branches, the Belle Fourche and the South Fork, which, after almost completely encircling the Black Hills, unite at a point nearly 350 m.
Under the Reclamation Act passed by Congress in 1902 the irrigation of 10o,000 acres in the Belle Fourche Valley adjacent to the Black Hills region was provided for.
Wide on top, and for two main canals from this distributing centre, one the north canal supplying water for the irrigation of 66,857 acres north of the Belle Fourche river and east of Owl Creek, and the other the south canal for the irrigation of 28,240 acres south of the Belle Fourche.
Oil has been found in eighteen different districts, the fields being known as follows: - The Carter, Hilliard, Spring Valley and Twin Creek in Uinta county; the Popo Agie, Lander, Shoshone, Beaver and a part of Dutton in Fremont county; the Rattlesnake, Arrago, Oil Mountain and a part of Dutton, Powder river and Salt Creek in Natrona county; part of Powder river and Salt Creek in Johnson county; Newcastle in Weston county; Belle Fourche in Crook county; Douglas in Converse county and Bonanza in Bighorn county.
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