The Mason–Dixon line, sometimes referred to as Mason and Dixon's Line, is a demarcation line separating four U.S. states: Pennsylvania, Maryland, Delaware and West Virginia. It was surveyed between 1763 and 1767 by English surveyors and astronomers Charles Mason and Jeremiah Dixon as part of the resolution to Cresap's War, a border conflict involving Maryland, Pennsylvania, and Delaware (then a part of Pennsylvania) in colonial United States.
The largest portion of the Mason–Dixon line, along the southern Pennsylvanian border, later became informally known as the boundary between the Southern slave states and Northern free states. This usage came to prominence during the debate around the Missouri Compromise of 1820, when drawing boundaries between slave and free territory,"The Missouri Compromise of 1820 designated Mason and Dixon’s west line as the national divide between the 'free' and 'slave' states east of the Ohio River, and the line suddenly acquired new significance." and resurfaced during the American Civil War, with border states also coming into play. The Confederate States of America claimed the Virginian (now West Virginia) portion of the line as part of its northern border, although it never exercised meaningful control that far northespecially after West Virginia separated from Virginia and joined the Union as a separate state in 1863. It is still used today in the figurative sense of a line that separates the Northeast and South regionally, politically, and socially .
Background
Maryland's charter of 1632 granted Cecil Calvert land north of the entire length of the Potomac River up to the 40th parallel. See section "History of the Boundary Dispute Between the Baltimores and Penns Resulting in the Original Mason and Dixon Line" by Edward Bennett Matthews A problem arose when Charles II granted a charter for Pennsylvania in 1681. The grant defined Pennsylvania's southern border as identical to Maryland's northern border, but described it differently, as Charles relied on an inaccurate map. The terms of the grant clearly indicate that Charles II and William Penn believed the 40th parallel would intersect the Twelve-Mile Circle around New Castle, Delaware, when in fact it falls north of the original boundaries of the City of Philadelphia, the site of which Penn had already selected for his colony's capital city. Negotiations ensued after the problem was discovered in 1681. A compromise proposed by Charles II in 1682, which might have resolved the issue, was undermined by Penn receiving the additional grant of the "Three Lower Counties" along Delaware Bay, which later became the Delaware Colony, a satellite of Pennsylvania. Maryland considered these lands part of its original grant.
The conflict became more of an issue when settlement extended into the interior of the colonies. In 1732, the Proprietary Governor of Maryland, Charles Calvert, 5th Baron Baltimore, signed a provisional agreement with William Penn's sons, which drew a line somewhere in between and renounced the Calvert claim to Delaware. But later, Lord Baltimore claimed that the document he had signed did not contain the terms he had agreed to, and refused to put the agreement into effect. Beginning in the mid-1730s, violence erupted between settlers claiming various loyalties to Maryland and Pennsylvania. The border conflict would be known as Cresap's War.
Progress was made after a Court of Chancery ruling affirming the 1732 agreement, but the issue remained unresolved until Frederick Calvert, 6th Baron Baltimore ceased contesting the claims on the Maryland side and accepted the earlier agreements. Maryland's border with Delaware was to be based on the Transpeninsular Line and the Twelve-Mile Circle around New Castle. The Pennsylvania–Maryland border was defined as the line of latitude south of the southernmost house in Philadelphia (on what is today South Street). As part of the settlement, the Penns and Calverts commissioned the English team of Charles Mason and Jeremiah Dixon to survey the newly established boundaries between the Province of Pennsylvania, the Province of Maryland, and Delaware Colony.
In 1779, Pennsylvania and Virginia agreed "To extend Mason's and Dixon's line, due west, five degrees of longitude, to be computed from the river Delaware, for the southern boundary of Pennsylvania, and that a meridian, drawn from the western extremity thereof to the northern limit of the said state, be the western boundary of Pennsylvania for ever."
After Pennsylvania abolished slavery in 1781, the east–west 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.
Geography of the line
Mason and Dixon's actual survey line began to the south of Philadelphia, 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. Most of the Delaware–Pennsylvania boundary is an arc, and the Delaware–Maryland boundary does not run truly north–south because it was intended to bisect the Delmarva Peninsula rather than follow a meridian.
Mason and Dixon also confirmed the earlier survey delineating Delaware's southern boundary from the Atlantic Ocean to the "Middle Point" stone (along what is today known as the Transpeninsular Line). They proceeded nearly due north from this to the Pennsylvania border.
The Maryland–Pennsylvania boundary is an east–west line with an approximate mean latitude of 39°43′20″ N (Datum WGS 84). In reality, the east-west Mason–Dixon line is not a true straight line in the geometric sense, but is instead a polygonal chain, a series of many adjoining line segments, following a path between latitude 39°43′15″ N and 39°43′23″ N.
The surveyors also extended the boundary line west of Maryland's western boundary, into territory that was still in dispute between Pennsylvania and Virginia, though this was contrary to their original charter. Mason and Dixon's survey was finished on October 19, 1767, about east of what is now Pennsylvania's southwest corner. Where the surveyors finished their survey became known as the Mason and Dixon Survey Terminal Point.
In 1774, commissioners from Pennsylvania and Virginia met to negotiate their boundary, which at the time involved Pennsylvania's southern border west of Maryland and its entire western border. Both sides agreed that Pennsylvania's grant made its western border a tracing of the course of the Delaware River, displaced five degrees (approximately 265 miles) to the west. And both sides thought this would place Fort Pitt in Virginia territory (in fact it would not have). With that in mind, the governor of Pennsylvania argued that, despite the agreement reached with Maryland, Pennsylvania's southern border west of Maryland was still the 39th parallel, about south of the Mason–Dixon line. Negotiations continued for five years, with a series of proposed lines. In the end, a compromise was reached: the Mason–Dixon line would be extended west to a point five degrees west of the Delaware River. To compensate Pennsylvania for the claimed territory lost, its western boundary would be run due north rather than copying the course of the Delaware River.
The Mason–Dixon line was marked by stones every mile and "crownstones" every , using stone shipped from England. The Maryland side says "(M)" and the Delaware and Pennsylvania sides say "(P)". Crownstones included both coats of arms.
Many of the original stones are still visible, resting on public land and protected by iron cages; a number have gone missing or were buried. There exists an interactive map of each currently found stone.
The actual locations of the stones may differ a few hundred feet east or west from the exact positions where Mason and Dixon intended to place them, still, the line drawn from stone to stone forms the legal boundary. Page 21.
The lines have been resurveyed several times over the centuries without substantive changes to Mason's and Dixon's work, and additional benchmarks and survey markers were placed where necessary.
History
The line was established to end a boundary dispute between the British colonies of Maryland and Pennsylvania/Delaware. Maryland had been granted the territory north of the Potomac River up to the 40th parallel. Pennsylvania's grant defined the colony's southern boundary as following a 12-mile (radius) circle () counter-clockwise from the Delaware River until it hit "the beginning of the fortieth degree of Northern latitude." From there the boundary was to follow the 40th parallel due west for five degrees of longitude. But the 40th parallel does not, in fact, intersect the 12-mile circle, instead lying significantly farther north. Thus Pennsylvania's southern boundary as defined in its charter was contradictory and unclear. The most serious problem was that the Maryland claim would put Philadelphia, the largest city in Pennsylvania, in Maryland.
The dispute was peacefully resolved in 1767 when the boundary was fixed as follows: Between Pennsylvania and Maryland: The parallel (latitude line) south of the then southernmost point in Philadelphia, measured to be at about 39°43′ N and agreed upon as the Maryland–Pennsylvania line. Between Delaware and Maryland: The existing east–west transpeninsular line from the Atlantic Ocean to the Chesapeake Bay, as far as its midpoint from the Atlantic. A 12-mile (radius) circle () around the city of New Castle, Delaware. A "tangent line" connecting the midpoint of the transpeninsular line to the western side of the 12-mile circle. A "north line" along the meridian (line of longitude) from the tangent point to the Maryland-Pennsylvania border. Should any land within the 12-mile circle fall west of the north line, it would remain part of Delaware. (This was indeed the case, and this border is the "arc line".)
The disputants engaged an expert British team, astronomer Charles Mason and surveyor Jeremiah Dixon, to survey what became known as the Mason–Dixon line. It cost the Calverts of Maryland and the Penns of Pennsylvania £3,512 9/ – () to have surveyed with such accuracy. To them the money was well spent, for in a new country there was no other way of establishing ownership. The Mason–Dixon Trail stretches on or near Pennsylvania's border with Delaware and Maryland and is a popular attraction to tourists.
The Mason–Dixon line is made up of four segments corresponding to the terms of the settlement: the tangent line the north line the arc line the 39°43′ N parallel
The most difficult task was fixing the tangent line, as they had to confirm the accuracy of the transpeninsular line midpoint and the 12-mile circle, determine the tangent point along the circle, and then actually survey and monument the border. They then surveyed the north and arc lines. They did this work between 1763 and 1767. This actually left a small wedge of land in dispute between Delaware and Pennsylvania until 1921.
In April 1765, Mason and Dixon began their survey of the more famous Maryland–Pennsylvania line. They were commissioned to run it for a distance of five degrees of longitude west from the Delaware River, fixing the western boundary of Pennsylvania . However, in October 1767, at Dunkard Creek near Mount Morris, Pennsylvania, nearly west of the Delaware, their Iroquois guides refused to go any further, having reached the border of their lands with the Lenape, with whom they were engaged in hostilities. As a result, the group was forced to quit, and on October 11, they made their final observations, from their starting point.