Delaware’s 12-Mile Arc

A portrait of James the Duke of York from 1651.

Courtesy of the Metropolitan Museum of Art, Harris Brisbane Dick Fund.

A Unique Border

Delaware’s northern border is unique among U.S. state boundaries. Rather than following a river or mountain range, it traces a distinct arc, or curve, known as the Twelve-Mile Arc. This unusual boundary dates to the colonial era when European powers established colonies in the New World. Over time, the arc was surveyed multiple times, and the line shown on modern maps reflects a smoothed combination of these early measurements.

Colonial Conflict and Seizure of New Castle

The origins of the Twelve-Mile Arc can be traced to the mid-17th century, when England and the Netherlands fought a series of wars in Europe and North America. In 1664, during a brief pause in hostilities, English forces seized the Dutch colonial city of New Amsterdam and renamed it New York. The English Duke of York, who now controlled the region, sent ships south to capture the Dutch settlement of New Amstel along the Delaware River. The town was renamed New Castle and became a key center of English authority in the region.

Establishing the Twelve-Mile Circle

The Duke of York retained control of the lands along the western shore of the Delaware River and Bay until 1681, when King Charles II granted William Penn a charter for the colony of Pennsylvania. Concerned about protecting his remaining territory from Penn’s new colony, the Duke of York directed that a boundary be established using a circle with a twelve-mile radius centered on New Castle. The original center point was located at the end of New Castle’s Broad Dike, near present-day 3rd and Chestnut Streets, where a historical marker stands today.

The Duke of York later transferred the land within the circle, and all land south to Cape Henlopen, to William Penn forming the united colony of Pennsylvania. The unique arc shape created a natural boundary line between the “Upper Counties” and “Lower Counties” of Pennsylvania with The Lower Three Counties being what today is known as Delaware.

Map of Delaware from 1795.

Courtesy of the Library of Congress, Geography and Map Division.

The Lower Counties and Separate Assembly

In 1704, despite remaining legally part of Pennsylvania, The Three Lower Counties (New Castle, Kent and Sussex) established their own colonial assembly, which met in New Castle. Residents south of the arc often had different economic and political priorities than those in Pennsylvania’s northern counties which justified their desire to hold their own assembly. This assembly still answered to the governor of Pennsylvania.

The Mason-Dixon Survey and Boundary Disputes

By the mid-18th century, the center point of the Twelve-Mile Circle was formally relocated to the cupola of the New Castle Court House, reflecting legal decisions made during ongoing boundary disputes. While the arc defined the division between the Upper Counties and the Lower Counties, colonial officials were especially concerned with accurately determining the border between Pennsylvania and Maryland, as outlined in their original charters. This included clarifying where the Twelve-Mile Circle intersected Maryland’s eastern boundary.

In 1763, surveyors Charles Mason and Jeremiah Dixon completed a new survey using the New Castle Court House as the center point. Their work helped resolve long-standing disputes between Pennsylvania and Maryland and established what became known as the Mason-Dixon Line. The Twelve-Mile Arc remained intact and continued to define Delaware’s northern boundary.

From Colonial Boundary to State Line

When Delaware declared independence from both Great Britain and Pennsylvania in 1776, the arc took on new significance as a state boundary. Additional surveys were conducted in the years that followed to ensure its accuracy. Today, stone markers placed at half-mile intervals trace the arc, preserving one of the most distinctive borders in the United States.

Historic Benchmarks

This article is part of a series titled Historic Benchmarks: New Castle and the Making of a Nation which was created in partnership with the Delaware250, New Castle Historical Society, Old New Castle Outdoor Recreation Coalition, the City of New Castle, and The Trustees of the New Common.

References:

Benson, B. E., & Hoffecker, C. E. (2021). New Castle, Delaware: A Walk Through Time. New Castle, Delaware: Oak Knoll Press.

Higgins, A. (Ed.). (1973). New Castle on the Delaware (Third ed.). New Castle, Delaware: New Castle Historical Society.

National Park Service. (2021, October 8). Delaware-Pennsylvania Border Marker: First State National Historical Park. Retrieved from https://www.nps.gov/places/delaware-pennsylvania-border-marker.htm