Completion of the Mason–Dixon Line survey

Background. The Penns and Calverts commissioned the English team of astronomer Charles Mason and surveyor Jeremiah Dixon to survey the newly established boundaries between the Province of Pennsylvania, the Province of Maryland, and Delaware Colony, as part of the resolution to Cresap's War, a colonial border conflict.

Course of the Survey. Mason and Dixon surveyed the line between 1763 and 1767. In April 1765 they began the Maryland–Pennsylvania line, and their survey extended from a benchmark east of the Delaware River westward toward the then boundary of western Virginia. They also confirmed the earlier survey of Delaware's southern boundary.

Completion and Halt. In October 1767, at Dunkard Creek near Mount Morris, Pennsylvania, their Iroquois guides refused to go further, having reached the border of their lands with the Lenape, with whom they were engaged in hostilities. The survey was finished about 31 miles east of what is now Pennsylvania's southwest corner, and the endpoint became known as the Mason and Dixon Survey Terminal Point.

Aftermath. The boundary dispute was peacefully resolved in 1767 when the boundary was fixed. The official report on the survey, issued in 1768, did not mention Mason's and Dixon's names. Following the survey, the two stayed on to measure a degree of Earth's meridian on the Delmarva Peninsula for the Royal Society, and before returning to England in 1768 they were admitted to the American Society for Promoting Useful Knowledge in Philadelphia.