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A Complete Summary of Maryland's Border History

Maryland’s shape is one of the strangest in the United States. It’s so unintuitive that I’ve come across many central Marylanders who don’t even know the “little triangle in the west” exists. Here’s a summary of the history behind how Maryland got its shape. Scroll to the bottom for a timeline.


The first border is simple. In 1632, Cecil Calvert was granted the land between the Potomac River and a line of latitude at 40 degrees north. Therefore, Maryland’s southern border was set at the Potomac River. The Potomac splits into the North and South Branch at Oldtown in Allegheny County. Maryland tried to claim more land by asserting that the border followed the South Branch down into current-day West Virginia, but the border was set along the North Branch. Although most states separated by a river have their border in the river’s middle, Maryland actually has jurisdiction over the entire Potomac.

However, from there, things get complicated. 50 years later, Pennsylvania’s southern border was set at the 40 degree parallel as well, but an inaccurate map was used, creating an overlap. Marylanders and Pennsylvanians clashed, and both states deployed troops between 1736 and 1738 in what is now northern Baltimore County in an event called the Cresap's War. Jeremiah Dixon and Charles Mason settled the dispute between 1763 and 1767 by mapping our northern border, the Mason-Dixon Line. If Maryland had won the dispute, and the border remained at its northernmost extent, Philadelphia would be in Maryland, and this would have profound impacts on our state today.


Maryland’s westernmost border in Garrett County was straight forward, just a vertical line connecting the source of the Potomac to the Mason-Dixon. However, while this line is 36 miles long, there is a point where the distance between the Mason-Dixon and Potomac River is less than 2 miles wide, the shortest width of any point in the United States. The southern tip of the line was supposed to be the Fairfax Stone, the source spring of the North Branch of the Potomac River, but inaccurate surveying officiated the border a hair north, and the Fairfax Stone is now fully in West Virginia by just over a mile.


More complicated was our eastern border on the Delmarva Peninsula with Delaware, which was also surveyed by Mason and Dixon. For 100 years, Maryland claimed all of Delmarva. However, the British Crown denied Maryland’s claim on the basis that Maryland was a Catholic colony, and the formerly Dutch settlers in Delaware were Protestant. An agreement was made to draw a line from the Atlantic Ocean to Delmarva’s midpoint, then another line due north tangential to a circle drawn around a church in New Castle, Delaware. This vertical line slants northeast ever-so-slightly, so from its intersection with the circle, another perfectly vertical line was drawn tangential to the circle to the Maryland-Pennsylvania border. Now came the location of the horizontal line to Delmarva's midpoint. It was supposed to begin at Cape Henlopen, but another map inaccuracy, this time with labelling, placed the border about 23 miles farther south than it should’ve been, resulting in more lost land for Maryland.


The final border is the southern border with Virginia on the Eastern Shore. The original 1632 charter split the Eastern Shore between Maryland and Virginia by a horizontal line between Watkin’s Point and the Atlantic Ocean, farther south than today. However, conflict arose when Virginians settled over the poorly-defined line and refused to pay taxes to either state. To clarify the border, Maryland agreed to move it a few miles north, drawing a line from the mouth of the Potomac across the Eastern Shore to the Atlantic Ocean in 1668. However, the marshes between the Pocomoke River and the Chesapeake Bay where the city of Crisfield is today were too difficult to survey, so the surveyors began at the 38th parallel on the west bank of the Pocomoke River, and drew the remainder of the border along the middle of the Pocomoke and passing by Watkins Point. This time, Maryland gained land due to poor surveying. However, poor compasses made what was labelled the Calvert-Scarborough Line slant towards the northeast, resulting in 15,000 acres of Maryland land ending up in Virginia.

Finally, a series of arbitration decisions in the 1850s and 1870s about oyster harvesting defined the border across the Chesapeake Bay from Watkin’s Point to the mouth of the Potomac.

So, the historical takeaway is Maryland could’ve been a lot bigger. We just got screwed by poor maps.

1632: Maryland’s charter defines the southern border at the Potomac River along its North Branch to the Fairfax Stone, and the northern border at 40 degrees north. It also sets the Virginia-Maryland border on the Eastern Shore at a line south of the 38th parallel from Watkins Point to the Atlantic Ocean

1668: Maryland and Virginia agree to move their Eastern Shore border north to the 38th parallel, about equal with the mouth of the Potomac across the Chesapeake

1688: Calvert-Scarborough Line adjusts MD-VA border to follow the middle of the Pocomoke River until the 38th parallel. Virginia disputes the border over oystering interests for nearly 200 years.

1681: Pennsylvania’s southern border creates a zone of overlap due an inaccurate map.

1736-1738: Cresap’s War fought between Marylanders and Pennsylvanians over the overlap zone.

1763-1767: Jeremiah Dixon and Charles Mason settle the overlap conflict by placing the northern border 15 miles south of the southernmost house in Philadelphia. They also divide the Delmarva Peninsula by drawing two connected lines tangent to the New Castle Circle extending to a line originating at Cape Henlopen and extending to the Delmarva Peninsula’s midpoint. Inaccurate labelling of Cape Henlopen gives Delaware more land than intended.

1877: Black-Jenkins arbitration affirms Calvert-Scarborough Line, with adjustments around the Chesapeake Islands to settle oyster disputes. Black-Jenkins also sets Maryland’s border as the low-water mark of the Potomac on the Virginia shoreline, affording Virginia some rights to the river such as riparian use and construction.


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