The Mason-Dixon Survey

The Mason-Dixon Survey was a land survey completed by Jeremiah Dixon and Charles Mason between 1763 and 1767, which gave the survey the Mason-Dixon Line title. King George II ordered the survey to settle a land dispute between the Calverts of Maryland and the Penns of Pennsylvania.¹ The survey established the borders between Delaware, Maryland and Pennsylvania. This so-called “Mason-Dixon Line” later became the boundary between North and South, freedom and slavery.²

The controversy between the Calvert and Penn families existed for nearly a century before the survey.³ In 1632, Charles I of England issued Charles Calvert, Lord Baltimore, a charter to establish the colony of Maryland; the allotted territory included what would become Delaware.⁴ Swedish and Dutch colonists soon founded small settlements on Lord Baltimore’s granted land and legitimized their claims by purchasing the land directly from Indigenous people.⁵ In 1681, Charles II issued William Penn the charter for Pennsylvania, which overlapped with land issued to Lord Baltimore.⁶ The charter was accompanied by a title from the Duke of York, who claimed the Delaware territory by right of conquest from the Dutch.⁷ Competing claims between the English and Dutch and between the Calvert and Penn proprietors required careful consideration of these claims.⁸ Colonial authorities did not consider the sovereign land rights of Native Americans in negotiations.

Along with the contested Delaware territory, the Calverts and Penns disputed the northern border of Maryland and the southern border of Pennsylvania.⁹ Penn’s interpretation of his charter was to place Pennsylvania’s southerly boundary at thirty-nine degrees north latitude, while Lord Baltimore’s grant stated that his northern boundary was forty degrees north latitude.¹⁰ Despite the overlap, Penn had already started developing Philadelphia on the disputed land without consulting officials.

Conflict over the borders turned violent during Cresap’s War in 1730.¹¹ The exact details of the event are unclear, but one version claims that a ferry boat operator, Thomas Cresap, who considered himself a Pennsylvanian, was attacked and nearly killed by two Pennsylvania colonists. When Cresap attempted to bring the assailants to court, the Pennsylvanian magistrate designated him a Marylander and dismissed the case as outside of Pennsylvania’s jurisdiction. The eruption of violence prompted the Crown, Penns, and Calverts to act.¹² On July 4, 1760, Lord Baltimore and the Penns signed an agreement for a survey to definitively mark the boundaries between Maryland, the Delaware territory—or Three Lower Counties —and Pennsylvania, after which George II ordered the survey.¹³

The two men chosen to conduct the survey were Charles Mason, a mathematician and astronomer, and Jeremiah Dixon, a surveyor. Both were members of the Royal Society, a British learned society formed to promote excellence in science.¹⁴ George II charged Mason and Dixon with finding the latitude measurement for the southernmost point of Philadelphia.¹⁵ From there, they were ordered to proceed thirty to thirty-five miles west, then about fifteen miles south, where they would start to survey the intercolonial boundary between Maryland, Pennsylvania, and Delaware.¹⁶ Charles Mason’s 1769 map entitled A map of that part of America where a degree of latitude was measured for the Royal Society visualized these travel instructions.

Mason and Dixon arrived in Philadelphia in 1763. To start the survey, they designated the home of Thomas Plumstead and Joseph Huddle as the southern limit of the city.¹⁷ Once they marked this boundary, the expedition used a sixty-foot chain and wooden frames to complete their measurements.¹⁸ They continued until 1766 when they reached the Proclamation Line of 1763, or the Appalachian Ridge, which barred Anglo-American westward migration.¹⁹ To ensure the survey could proceed, the governor of Maryland, Horatio Sharpe, requested assistance from Sir William Johnson, the Superintendent of Indian Affairs for the Northern Colonies, to guarantee safety for the expedition.²⁰ Johnson, who had a close relationship with the Haudenosaunee, appointed Hugh Crawford, whom Charles Mason described as an interpreter, and a Mohawk chief named Henrick to accompany the expedition. Additionally, about fourteen Haudenosaunee men joined.²¹

The new members assisted Mason and Dixon as they traveled beyond the Proclamation Line into land that was contested between the Delaware people, the Haudenosaunee, and British colonists.²² On October 9, 1767, when the expedition reached Dunkard Creek, the Haudenosaunee refused to continue, as they had reached a war zone between the Haudenosaunee and Delaware.²³ The surveyors and the Haudenosaunee, likely fearing violence from the Delaware, turned back eastward, 233 miles west of their starting point, ending the survey line.²⁴ Mason’s map entitled A plan of the west line… which is the boundary between the provinces of Maryland and Pennsylvania captures a void at the line’s western end marked where the expedition was cut thirty-one miles short of its intended path.²⁵

The phrase “Mason-Dixon Line” was not popularized until the Compromise of 1820, which admitted Missouri as a slave state and Maine as a free state.²⁶ Within this context, the Mason-Dixon Line became politically significant as the regional divide between North and South, and crossing the line came to indicate the difference between enslavement and freedom. With the passage of the Fugitive Slave Act in 1850, the borderlands on either side of the line turned into a hotbed for vigilantism and violence.²⁷ Today, conceptions of the Mason-Dixon Line remain strong, with Americans on both sides of the border emphasizing their cultural and historical differences.

Banner image credit: When Maryland almost got Philadelphia: The Remarkable Story of the Mason-Dixon Line, Maryland Center for History and Culture

Bibliography

Black, Janine, and Barry Arkles. “The Mason-Dixon Survey at 250 Years: Recent Investigations.” The Pennsylvania Magazine of History and Biography 140, no. 1 (2016): 83–101.

Cope, Thomas D. “The First Scientific Expedition of Charles Mason and Jeremiah Dixon.” Pennsylvania History: A Journal of Mid-Atlantic Studies 12, no. 1 (1945): 24–33.

Gray, Edward G. Mason-Dixon: Crucible of the Nation. Cambridge: Harvard University Press, 2023.

Luquer, Thatcher T.P. “Mason and Dixon’s Line.” The Military Engineer 23, no. 130 (1931): 375–77.

Mason, Charles, and Jeremiah Dixon. The Journal of Charles Mason and Jeremiah Dixon. Philadelphia: American Philosophical Society, 1969.

Strang, Cameron B. “The Mason-Dixon and Proclamation Lines: Land Surveying and Native Americans in Pennsylvania’s Borderlands.” The Pennsylvania Magazine of History and Biography 136, no. 1 (2012): 5–23.

Tittmann, Otto Hilgard. Report on the Resurvey of the Maryland-Pennsylvania Boundary Part of the Mason and Dixon Line. Harrisburg: Harrisburg Publishing, 1909.

Wainwright, Nicholas B. “Mason and Dixon’s Map.” The Princeton University Library Chronicle 45, no. 1 (1983): 28–32.