S.C. Code Ann. § 4-3-390

Marlboro County is bounded as follows: on the west by the Great Pee Dee River, which separates it from the counties of Chesterfield, Darlington and Florence; and on the southeast by Dillon County, from which it is divided by a line drawn from a dead pine on the North Carolina line S. 22.5° W. 24.75 miles until it intersects the Great Pee Dee River.