S.D. Codified Laws § 7-1-52
Beginning at a point on the boundary line between the states of South Dakota and Wyoming ten miles south of its intersection with the forty-fourth parallel of north latitude, the same being the northwest corner of Custer County, thence east in a direct line and along the north boundary of Custer County to the center of the main channel of the south fork of the Cheyenne River; thence southerly and up the center of the main channel of said river to its intersection with the northerly boundary of the Pine Ridge Indian Reservation; thence east and along said boundary line of said reservation to its intersection with the center of the main channel of White River; thence easterly and down the center of the main channel of White River to the range line between ranges seventeen and eighteen east Black Hills meridian; thence north and along said range line to the center of the main channel of the Cheyenne River; thence westerly up and along the center of the main channel of said river to its junction with the Belle Fourche River; thence southerly up and along the center of the main channel of the south fork of the Cheyenne River to the boundary line between Meade and Pennington counties; thence west and along said boundary line and along the boundary line between Pennington and Lawrence counties to the boundary line between the states of South Dakota and Wyoming; thence south and along said state line to the place of beginning.
SDCL 7-1-52