The Mason-Dixon Line: Four Men in the Woods for Four Years

In seventeen sixty-three, Charles Mason and Jeremiah Dixon arrived in Philadelphia with a set of precision instruments and a commission to settle a boundary dispute that had been running for nearly eighty years. The proprietors of Pennsylvania and Maryland had been fighting over the border between their colonies since the sixteen-eighties. Families had been arrested on charges of living in the wrong colony. Courts in two jurisdictions had issued conflicting rulings on the same pieces of land. It was the kind of dispute that could simmer for generations without resolution unless someone actually went and measured.

Mason was an English astronomer who had recently returned from observing the transit of Venus across the face of the sun for the Royal Observatory in Greenwich. Dixon was a surveyor from County Durham. Neither had been to America before. They spent four years in the field.

They worked from east to west, beginning at a stone set by a previous survey at the Delaware coast. They cut a line through forest, across ridges, and over streams, planting limestone markers every mile, stones carved with a P on the Pennsylvania side and an M on the Maryland side, and every five miles a larger stone with the Penn and Calvert coats of arms. They ran the line with a precision that amazed contemporaries and has held up under modern GPS verification. The final placement, established in seventeen sixty-seven, set the boundary at roughly thirty-nine degrees forty-three minutes north latitude.

The line they drew became the most consequential boundary in American history for reasons Mason and Dixon could not have predicted. By the mid-nineteenth century it had become shorthand for the division between slave states and free states, between the South and the North, between two competing visions of what the republic was. They had surveyed a property line. History turned it into a moral argument.