S.D. Codified Laws § 7-1-17
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, thence due east and on a direct line to the center of the main channel of the south fork of the Cheyenne River; thence southwesterly and up the center of the main channel of said south fork of said river to the one hundred third meridian of west longitude; thence south and along said meridian of longitude to the township line between townships six and seven, south of the Black Hills base line; thence west and along said township line to the boundary line between the states of South Dakota and Wyoming; thence north and along said boundary line to the place of beginning.
SDCL 7-1-17