Who Needs the Mason-Dixon Line?

Tilberg, Frederick. 1933. Mason-Dixon Line Marker, Zora, Adams County, PA. Historic American Buildings Survey. Image from the Library of Congress.

In reading about Pennsylvania, as I have been doing a lot lately, I have come across many references to the Mason-Dixon line. We’re all familiar with the Mason-Dixon line, of course. It has served as a powerful symbol of the division between the free North and the slaveholding South, the line enslaved people had to cross to escape to freedom.[1] Despite that knowledge, it had never before occurred to me to wonder why the Mason-Dixon line exists at all. It’s quite a story, involving royal patronage, feuding families, a minor war, a famous lawsuit, and some cutting-edge science.

The story of the Mason-Dixon line begins in 1681 with the grant of land made to William Penn by King Charles II of England to repay a debt the crown owed to Penn’s father. This grant gave Penn land between the 40th and 43rd parallels north, extending to Delaware Bay 12 miles north of New Castle (Robinson 1902; Veech 1857). On the face of things, this grant seems straightforward, but in fact, portions of this land were already claimed by other English colonies, including Maryland, Virginia, New York, and Connecticut, and the borders of Pennsylvania as we know it today were not finalized until 1792, more than 100 years after the original land grant.

Musser, K. 2007. Province of Pennsylvania 1681-1776. Image from Wikimedia Commons.

The most problematic area of Penn’s grant was the border between the colonies of Pennsylvania and Maryland. Uncultivated land west of Delaware Bay and south of the 40th parallel had, fifty years earlier, been granted to Cecil Calvert, the second Lord Baltimore, to establish a colony (Robinson 1902; Veech 1857). The problem was that no one knew exactly where the 40th parallel north was located. Penn, at one point, believed it to be south of Baltimore, based on an inaccurate map (Walker 2014). The situation was made even more complicated by the fact that the land that now makes up the state of Delaware, while claimed by Maryland, had been granted by the crown to the Duke of York in 1674, following his defeat of the Dutch in New Amsterdam, and the Duke, soon to become King James II, had in turn assigned that land to William Penn to administer (Robinson 1902; Veech 1857).

The overlap in lands claimed by Pennsylvania and Maryland was substantial, about 2,000 square miles, and included the area, south of what we now know to be the 40th parallel north, on which Penn had established Philadelphia as the capital of Pennsylvania (Robinson 1902). Philadelphia’s location on the Delaware River, with access to the Delaware Bay, made it immensely valuable as a port city in an era where sea travel was of critical importance to settlement and trade. And the issue of which colony controlled the contested land was not an abstract one, as both proprietors continued to make land grants in this area, as well as sending out competing tax collectors to collect revenues for their colonies. Similarly, when disputes between settlers arose, it was unclear which colony’s courts had jurisdiction, and competing courts ending up overruling each other on a number of occasions (Black & Arkles 2016).

Several meetings between Penn and Charles Calvert, the third Lord Baltimore, who by then was governor of Maryland, failed to reach any agreement about the border between the two colonies. In 1684, both Penn and Calvert returned to England to try to settle the dispute in the British courts to little avail. The conflict between the two colonies continued on for years, even after the deaths of both Calvert and Penn. Finally, in 1732, Penn’s sons and the fifth Lord Baltimore reached an agreement that divided the Delmarva Peninsula in half and set the boundary between the Pennsylvania and Maryland along the parallel of latitude 15 miles south of Philadelphia. That agreement should have resolved the problem but did not, in large part because there was still no clear line of demarcation between the two colonies and no effort was made to create one (Robinson 1902; Veech 1857). As a result, colonists in the disputed area still did not know which colony they lived in, owed taxes to, or were subject to the laws of.

Place, Francis. 1695. William Penn. Image from Wikimedia Commons. Closterman, John. c1700. Charles Calvert. Image from Wikimedia Commons.

Tensions abounded in the disputed area, coming to a boiling point later in the 1730s with Cresap’s War. Thomas Cresap and his family lived about 80 miles west of Philadelphia and considered themselves to be Marylanders. They repeatedly ran afoul of their Pennsylvanian neighbors in disputes that started with stray horses, progressed to establishing competing ferry services and recruiting new colonists, and culminated with Cresap shooting a Pennsylvanian who was trying to break into his house to arrest him. A Pennsylvania jury acquitted Cresap of murder, believing his argument of self-defense, but his Pennsylvanian neighbors then accused him and other Marylanders of raiding their crops. Another armed group attempted to arrest him, succeeding only after setting his house on fire. Cresap was eventually released and moved away from the disputed area, but bad feelings between the colonists remained, and both Pennsylvania and Maryland were forced to send militia units to the region to keep the peace (Doutrich 1986; Walker 2014).

King George II eventually had enough of the situation and ordered that the colonies cease fighting. Subsequently, in 1738, he forbade the proprietors from making land grants in the disputed area and established a temporary boundary that would apply until the Court of Chancery could make a final ruling. That ruling finally came in 1750, setting the boundaries as agreed upon in 1732, in a legal case that continues to be cited as a precedent in disputes over land rights (Veech 1857).

After almost 70 years of conflict, the boundary between Pennsylvania and Maryland was settled, but it still needed to be surveyed so that everyone knew where the boundary actually was, and it took another decade before this work began. Early efforts to conduct the survey were largely unsuccessful, facing problems ranging from swampy land and wild animals to inadequate technology and mathematical knowledge. It can’t have helped that the whole project was overseen by a group of commissioners appointed by the two colonies, few of whom had surveying experience, whose role was largely limited to holding meetings in local inns with the costs billed, of course, to the colonial governments (Walker 2014). As a result, by 1763, after years of work, the surveyors had succeeded only in establishing a horizontal border halfway across the Delmarva Peninsula (Robinson 1902).

Recognizing that the remaining work—surveying an 80-mile line dividing Delmarva vertically and a 233-mile line marking the border between Pennsylvania and Maryland—would be much more challenging, the colonial proprietors sought expert help from England in the form of Charles Mason and his assistant Jeremiah Dixon, who had recently gained considerable acclaim from their observations of the transit of Venus under commission from The Royal Society. Both Mason and Dixon had studied astronomy and mathematics, and Dixon had also trained as a surveyor, making them well equipped to undertake this complex project (Black & Arkles 2016; Robinson 1902).

Montgomery, D.H. 1910. “Illustration of Charles Mason and Jeremiah Dixon surveying the Mason–Dixon line.” The Leading Facts of American History. Boston, MA: Ginn and Company. Image from Wikimedia Commons.

Mason and Dixon arrived in Philadelphia in November 1769 and began what eventually became a five-year undertaking. Using astronomical observations, specialized surveying equipment, and extensive mathematical calculations, they and their crew of some 40 wagon drivers, axemen, cooks, tent keepers, messengers, and chain carriers established the Mason-Dixon line, marking it with stone pillars for every mile and carved crown stones with the arms of the Penns and the Calverts every five miles (Veech 1857). Their final bill for the work, presented in September 1768, was for £3,256, equivalent to well over half a million dollars today (Walker 2014). The boundary line soon became known by the names of the men who surveyed it, a fitting tribute to a remarkable achievement.

Mason, Charles, Jeremiah Dixon, Robert Kennedy, and James Smither. 1768. A plan of the west line or parallel of latitude, which is the boundary between the provinces of Maryland and Pensylvania. Philadelphia: Robert Kennedy. Image from the Library of Congress.

 [1] As is usually the case, reality is more complex than myth. In fact, Delaware, although north of the Mason-Dixon line, allowed the enslavement of human beings until 1865, and both Maryland and West Virginia, south of the line, were on the Union side in the Civil War, with West Virginia breaking off from Virginia in 1863 to rejoin the Union (Robinson 1902).

Works Cited

Black, Janine, and Barry Arkles. 2016. “The Mason-Dixon Survey at 250 Years: Recent Investigations.” The Pennsylvania Magazine of History and Biography 140(1): 83-101.

Doutrich, Paul. 1986. “Cresap's War: Expansion and Conflict in the Susquehanna Valley.” Pennsylvania History 53(2): 89-104.

Robinson, Morgan Poitiaux. 1902. The Evolution of the Mason and Dixon Line. Richmond, VA: Oracle Publishing Company.

Veech, James 1857. Mason and Dixon’s Line: A History. Pittsburgh: W.S. Haven.

Walker, Sally M. 2014. Boundaries: How the Mason-Dixon Line Settled a Family Feud and Divided a Nation. Somerville, MA: Candlewick Press.