S.D. Codified Laws § 7-1-4

Beginning on the boundary line between the states of South Dakota and Nebraska where the east boundary line of the Pine Ridge Indian Reservation intersects the same, thence north and along the east boundary line of the Pine Ridge Indian Reservation to the township line separating townships thirty-nine and forty, east Black Hills meridian; thence west and along said township line to the fifth guide meridian west of the sixth principal meridian; thence south and along said fifth guide meridian to the boundary line between the states of South Dakota and Nebraska; thence east and along said state line to the place of beginning.