Conflicting claims of ownership between Maryland and Delaware kept
Delaware's boundaries uncertain during most of the colonial period and led to highly
geometric dividing lines on the north, south, and west.
Northern Boundary: The Twelve-Mile Circle
The northern boundary marks the division between the two separate land
grants to William Penn. It is a circle with a twelve-mile radius, with the center of
the circle in the center of the town of New Castle. In 1750, the center of the circle was
fixed at the coupola of the courthouse in New Castle.
Southern Boundary: The Transpeninsular Line
In 1751, a line was surveyed straight across the Delmarva Peninsula
beginning at what they called Cape Henelopen, which was to be the southern boundary
of Delaware. The surveyors, however, actually began at Fenwick Island, fifteen miles
south of what is today Cape Henelopen.
Western Boundary: Survey by Mason & Dixon
In 1760, the midpoint of the tranpeninsular line was accepted as the
beginning of Delaware's western boundary with Maryland. In 1764, Charles Mason and
Jeremiah Dixon surveyed the boundary line north from that point. Mason and Dixon
also verified the transpeninsular line.
The Wedge: A tiny piece of land in northwestern Delaware bordering
on Maryland and Pennsylvania.
When Mason & Dixon surveyed the western boundary of Delaware the line
they ran northward from the midpoint of the transpeninsular line did not quite meet the
twelve-mile circle. It ended up a little to the west. Ownership of this land
was in dispute until 1921, when it was granted to Delaware.
Eastern Boundary: The Delaware River & Bay
The twelve-mile circle continues into the Delaware River. Below it, the
center of the main ship channel in the Delaware River and Bay is considered to be the
boundary with New Jersey.
The Mason Dixon Line: Is Delaware North or South of the
For the most part, Delaware is east of that famous line that traditionally
divides the North from the South. Mason and Dixon surveyed the Pennsylvania-Maryland
boundary westward from a point on the twelve-mile circle. Only the small portion of
the Mason-Dixon Line that is the top of the wedge is part of Delaware's boundaries.