The Mason-Dixon Line, which separates Pennsylvania from Maryland, became emblematic of the divide between slave and free states prior to the Civil War (thus is it sometimes erroneously called the “Mason-Dixie Line”). It was surveyed between 1763 and 1767 by Charles Mason and Jeremiah Dixon to end a territorial dispute between the Province of Maryland and the Province of Pennsylvania.
What I did not know is that this dispute actually broke out into violence in the 1730s. From the Wikipedia entry on Cresap’s War:
Hostilities erupted in 1730 with a series of violent incidents prompted by disputes over property rights and law enforcement, and escalated through the first half of the decade, culminating in the deployment of military forces by Maryland in 1736 and by Pennsylvania in 1737. The armed phase of the conflict ended in May 1738 with the intervention of King George II, who compelled the negotiation of a cease-fire.
I do not know how many people actually died as a result of Cresap’s War (which is also gloriously known as the Conojocular War, after the Conejohela Valley where it was fought).
The American Heraldry Society posted some pictures of the demarcation stones of the Mason-Dixon Line to Facebook:
These feature the arms of the respective colonial proprietors: William Penn on the left, and Lord Baltimore on the right.