 After Pennsylvania abolished slavery in 1781, the western part of this line and the Ohio River became a border between slave and free states, with Delaware retaining slavery until the 13th Amendment was ratified in 1865.Mason and Dixon's actual survey line began to the south of Philadelphia, Pennsylvania, and extended from a benchmark east to the Delaware River and west to what was then the boundary with western Virginia.The surveyors also fixed the boundary between Delaware and Pennsylvania and the approximately north–south portion of the boundary between Delaware and Maryland.