| Delaware Boundary Markers | |
| Nrhp Type: | hd |
| Nocat: | yes |
| Location: | State boundary lines between Delaware and Maryland, and between Delaware and Pennsylvania |
| Coordinates: | 38.46°N -75.6936°W |
| Builder: | Charles Mason |
| Added: | February 18, 1975 |
| Refnum: | 75002101 |
The Delaware Boundary Markers historic district is located along the state boundary lines between Delaware and Maryland, and between Delaware and Pennsylvania. The district includes 94 contributing sites along the Mason–Dixon line and includes the Transpeninsular Line, Post Marked West site, Tangent Line, the Arc Corner, and the Twelve-Mile Circle.[1]
The district was added to the National Register of Historic Places on February 18, 1975, for its significance in engineering and politics/government.
The markers are mainly on three parts of the Delaware boundary. First, the southern boundary with Maryland along the Transpeninsular Line from Fenwick Island to the Midpoint Marker, the first crownstone set by Mason and Dixon. Second, the western boundary with Maryland from the Transpeninsular Line Midpoint Marker to the boundary with Pennsylvania at the Tri-State Monument. Third, the northern boundary with Pennsylvania, the Twelve-Mile Circle. In addition, the starting measurement point, the Post Marked West, and the boundary from the Arc Corner Monument to the Tri-State Monument are included in the district.[1]