Source: National Atlas
In 1681, the western boundary of Pennsylvania was established in William Penn's charter - "The said Lands to extend westwards five degrees in longitude, to bee computed from the said Easterne Bounds..."1 At that time, European explorers were just beginning to penetrate the Shenandoah Valley and no one knew where the edge of Pennsylvania was located.
The French did not accept the English claim to the Ohio River Valley. The Treaty of Aix-la-Chapelle of 1748 settled one of the many French/English conflicts in Europe, but did not resolve Penn's claims to lands that the French also claimed in North America.
Between 1755-61, John Mitchell produced "A map of the British and French dominions in North America, with the roads, distances, limits, and extent of the settlements." It suggested the Forks of the Ohio, including the site of modern-day Pittsburgh that the French called "Fort duQuesne," was located in Pennsylvania:
However, the Virginia and Maryland colonies did not accept the Pennsylvania colony's definition of its boundary. Joshua Fry and Peter Jefferson (Thomas Jefferson's father) produced a map of Virginia in the early 1750's showing that Virginia claimed the land along the Monongahela River and Forks of the Ohio, where the Allegheny and Monongahela rivers join and Pittsburgh is now located.
 Fry-Jefferson map showing western Pennsylvania border located far to the east of the final boundary line "A map of the most inhabited part of Virginia containing the whole province of Maryland with part of Pensilvania, New Jersey and North Carolina. Drawn by Joshua Fry & Peter Jefferson in 1751." Source: Library of Congress |
In 1754, at the start of the French and Indian War, Fry initially led the Virginia Regiment on a military expedition to the forks to protect Virginians who were building a fort at that strategic location. Fry died after falling off his horse, and young George Washington assumed command of the Virginia Regiment. The Virginia Regiment expedition in 1754 was a major failure. The French seized the site that later developed into Pittsburgh and erected Fort Duquesne (see how fort is labelled on map, above). Instead of attacking Fort Duquesne, Washington ended up erecting his own defensive Fort Necessity - and then had to surrender to the French and their Indian allies.
Washington experienced defeat again a year later in 1755, while accompanying a British army led by General Braddock. When the British under John Forbes finally captured Fort Duquesne in 1758 and renamed it Fort Pitt, his army had been organized in Pennsylvania. Much to the frustration of the land speculators in Virginia, that expedition established good roads that connected Philadelphia to the Ohio River, undercutting the Virginia claim in the region.
The east-west border between Virginia and Pennsylvania was resolved partially by the Mason and Dixon line, which was surveyed in the 1760's after the Treaty of Paris in 1763 clarified that the French had surrendered their claims to land along the Ohio River.
 Lewis Evans map showing Mason-Dixon line extended as the western Virginia/Pennsylvania border, "A general map of the middle British colonies in America, viz. Virginia, Maryland, Delaware, Pensilvania, New-Jersey, New York, Connecticut & Rhode-Island: Of Aquanishuonigy the country of the confederate Indians comprehending Aquanishuonigy proper, their places of residence, Ohio & Thuchsochruntie their deer hunting countries, Couchsachrage & Skaniadarade their beaver hunting countries, of the Lakes Erie, Ontario and Champlain. Wherein is also shewn the antient & present seats of the Indian nations." Source: Library of Congress |
The location of the "Easterne Bounds" of Pennsylvania had to be resolved before surveyors could locate a line "five degrees in longitude, to bee computed from the said Easterne Bounds..." The southern border of Pernnsylvania was also complicated, because the colonial charters for Pennsylvania and Maryland granted the same land (along the 40 degree parallel of latitude) to both colonies.
The Calverts of Maryland and the Penns agreed on a boundary in 1760, and then hired Charles Mason and Jeremiah Dixon ("neutral" surveyors from England) to survey it. They spent four years between 1763-1767 marking the location of the boundary with monuments on the ground. They started with a limestone marker 15 miles south of Philadelphia. From that point of beginning, they surveyed east to Delaware and south to define the Delaware/Pennsylvania border.
When Mason and Dixon started surveying west of the headwaters of the Potomac River, Mason and Dixon were defining the boundary between Virginia and Pennsylvania. Marlyland's western border was the "headspring" of the Potomac, so its western bundary was defined by that natural feature rather than a line of longitude.
However, due to hostility of the Native Americans in the mountains, Mason and Dixon were unable to complete their east-west line. Their survey stopped 36 miles east of the line that would be "five degrees in longitude... computed from the said Easterne Bounds."
The last colonial governor of Virginia, Lord Dunmore, took advantage of this confusion to sell "Augusta county" land in the disputed area, at prices far lower than those being charged by the Pensylvanians. This may have been a ploy on his part to stimulate hostility between the leaders of the two colonies, most of whom were land speculators. Dunmore issued a proclamation affirming the Virginia claim to the territory, and even had Pennsylvania officials arrested and sent to Virginia for trial:
One option proposed for resolving the territorial dispute was to create a new state of Westsylvania. In the end, commissioners were appointed by the legislatures of Virginia and Pennsylvania to define the boundary between Virginia and Pennsylvania. The western edge of Pennsylvania, "five degrees in longitude... from the said Easterne Bounds" was finally surveyed in 1784 - one year before the Continental Congress passed the Land Ordinance of 1785, which defined how the lands across the Ohio would be surveyed and sold to settlers.
The Pennsylvania-Virginia border lasted until 1863, when the western counties of Virginia became the separate state of West Virginia.