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Delaware - Maryland - Pennsylvania Tri-state

Coordinates (NAD27) N3943'26.3" W7547'19.9"
UTM Coordinates (NAD27) 18S 432415 4397212
UTM Coordinates (WGS84) 18S 432391 4397420
Elevation 240 feet (73.1 meters)


This tri- state is marked by a monument inscribed M-M-P-P (representing Maryland and Pennsylvania) and 1849. An explanation of this and other interesting nearby monuments follows:

The northern boundary of Delaware is formed in part by a circular arc with a radius of 12 miles, whose vertex is the dome of the courthouse in New Castle. The southern boundary is a line starting at Cape Henlopen on the Atlantic Ocean running west to a point midway between the Atlantic Ocean and Chesapeake Bay. The western boundary is the Mason-Dixon line, which was supposed to run from the "midway" point to the western tangent on the 12 mile circle and then north to the Pennsylvania line. As a practical matter it was impossible to run the line to the tangent point exactly, and it intersescted the circle slightly southeast of the intended location, so the Mason-Dixon line follows the arc for a short interval and then runs north to the Pennsylvania line, at which point it turns west. No line was run between this turn point and the circular arc to the east, creating a wedge-shaped area claimed by both Delaware and Pennsylvania. The area was awarded to Delaware in 1921. Much of this interaction is fossilized by various nearby monuments, as follows:

About a mile east of the tri-state is the Arc Corner Monument, which marks the intersection of the circular arc with the phantom east-west extension of the Mason Dixon Line.

About a mile and a half south of the present-day tri-state is the tri-state point originally claimed by Pennsylvania. It is monumented by a triangular prism actually marked D-M-P and shown on the USGS map as "Prism". See photo below.

Slightly south and west of the prism are a series of "arc monuments". These mark the portion of the Mason-Dixon line that follows the arc. Arc Stone 3 is the westernmost point in Delaware.

Photos (click to enlarge)

Author at the tri-state monument.
Photo by Brian J. Butler
Disc on top of tri-state monument. To add confusion, this disc points to the US Coast and Geodetic Survey mark, which is named MDP Corner, whereas the actual tri-point is not inscribed with a D for Delaware.
Photo by Brian J. Butler
Author at the MDP Corner marker about 200 feet north of the tri- point. The significance of this monument is not clear, but it was placed by the US Coast and Geodetic Survey in 1935.
Photo by Brian J. Butler
Disc on top of the MDP Corner monument. Note that it says "MDP", leading to much confusion as to whether this is the tri-point. It is not.
Photo by Brian J. Butler
Author at the Arc Corner monument.
Photo by Brian J. Butler
Top of the Arc Corner monument showing turn point at the end of the Delaware Arc. Also shows why DuPont chose Delaware for chemical and fertilizer empire.
Photo by Brian J. Butler
Sign on road through corner of Maryland explaining the "wedge".
Photo by Brian J. Butler
The "Prismatic Stone" marking the original DE-MD-PA tri-point. This stone is on the property of the Dupont Stine-Haskell Research Center, which straddles the Delaware-Maryland line. Thanks to Geoffrey J. Backus, Fire Chief at the Dupont facility, for this photo.
Photo by Geoffrey J. Backus
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