S.C. Code Ann. § 4-3-380

Marion County is bounded as follows: on the north by Dillon County; on the west by the Great Pee Dee River; thence down the Great Pee Dee River, which divides it from Florence, Williamsburg and Georgetown Counties, to the mouth of the Little Pee Dee River; thence up the Little Pee Dee and Lumber Rivers, separating it on the east from Horry County, to the line of Dillon County on the Little Pee Dee River.