The 40th Parallel: An Invisible Line That Sparked a War

Stand at this quiet roadside sign in Lancaster County, and you’re standing on what was once one of the most bitterly contested boundaries in colonial America. To the casual traveler, the 40th parallel is just an abstract latitude line. But in the 1700s, it was the spark that ignited Cresap’s War. An eight-year conflict that pitted Maryland and Pennsylvania settlers against each other, drew in militias, and nearly pushed two colonies into civil war.

 The story begins not with muskets or militias, but with royal paperwork.

In 1632, King Charles I granted Lord Baltimore a charter for Maryland, its northern edge defined at the 40th parallel. Nearly fifty years later, Charles II granted William Penn a charter for Pennsylvania, which described its southern border as beginning “at the beginning of the fortieth degree of Northern Latitude.”

It sounds clear enough, but 17th-century geography was anything but precise. Later surveys showed New Castle, the town used as a measuring point in Penn’s charter, was located about 25 miles farther south than assumed. This created a 28-mile-wide strip of disputed territory.

The stakes were high. If Maryland’s claim held, the line would cut north of Philadelphia, placing the city itself inside Maryland. If Pennsylvania’s argument prevailed, the border would drop south to roughly where it lies today.

For decades, surveyors, governors, and lawyers wrangled over maps and measurements. But on the frontier, where charters meant less than musket barrels, it was settlers and strongmen who drew the line.

At the center of this storm stood Thomas Cresap, a wiry, combative frontiersman from Maryland. In 1730, Cresap established a ferry at “Blew Rock” (today’s Blue Rock) on the Susquehanna River near present-day Washington Boro. From his “Pleasant Garden” plantation, he ferried settlers, wagons, and livestock across the broad, dangerous river.

But Cresap was more than an entrepreneur. He was Lord Baltimore’s enforcer, charged with holding Maryland’s claim on the disputed lands.

Pennsylvanians viewed him as a thug, Marylanders as a loyal champion. His enemies nicknamed him the “Maryland Monster,” and he earned the title with gusto—fighting, intimidating, and even killing in the name of his colony.

Life in the disputed zone was tense. Imagine wagon caravans lined up for Cresap’s ferry, settlers arguing over land deeds, and sheriffs from both colonies riding in to arrest the same men. Each side had its own courts, its own taxes, its own militia.

Cresap became a one-man flashpoint. When Pennsylvania authorities tried to arrest him, he fought back ferociously, driving off sheriffs and intimidating rival settlers. Skirmishes escalated into raids, barns burned, and settlers fled in fear.

By 1736, Pennsylvania had had enough. Governor Patrick Gordon raised a militia and ordered them into the disputed zone. Maryland responded in kind. What began as a paper dispute between charters now looked dangerously like war between colonies.

The turning point came in late 1736. Pennsylvania sheriff Samuel Smith led a posse of 24 men to arrest Cresap at his fortified home. The clash was bloody. Cresap fought like a cornered wolf, wounding several attackers before being overwhelmed.

Bound in irons, he was hauled across the Susquehanna and paraded through the streets of Philadelphia. For Pennsylvanians, it was a triumphant display: the Maryland Monster finally brought to justice. For Marylanders, it was an outrage.

And yet, even in chains, Cresap refused to bow. As he was marched through Philadelphia, he famously sneered, “Damn it, this is one of the prettiest towns in Maryland!”

By 1737, the border war, now openly called the Conojocular War or Cresap’s War, was spiraling out of control. Farmers were abandoning their fields, militia companies drilled on both sides of the line, and petitions flooded London.

Finally, in May 1738, King George II ordered a ceasefire. Both colonies were told to pull back and maintain the status quo until the dispute could be legally resolved. The truce calmed open violence, but the underlying boundary problem festered.

It would take another quarter-century before the Crown commissioned two men—an English astronomer named Charles Mason and a surveyor named Jeremiah Dixon—to settle the matter once and for all. Between 1763 and 1767, they surveyed the line we know today: the Mason-Dixon Line.

Standing at the 40th Parallel marker today, it’s easy to miss the drama once tied to this latitude. The grassy roadside verge seems a poor stage for the clashes of settlers, sheriffs, and militias. Yet this invisible line nearly redrew the map of America.

Had Maryland’s claim triumphed, Philadelphia, the birthplace of independence, might have flown Lord Baltimore’s banner instead of William Penn’s. The Revolution itself could have unfolded very differently.

Instead, thanks to Cresap’s stubborn defiance, a royal ceasefire, and ultimately Mason and Dixon’s patient surveying, the border was fixed where it lies today—39° 43′ north latitude—15 miles south of Philadelphia.

Did You Know?

The Mason-Dixon Line is often remembered as the symbolic boundary between North and South in the Civil War. But its origins lie right here in Lancaster and York Counties, in a messy 18th-century feud over charters, ferries, and a hotheaded frontiersman named Thomas Cresap.

Planning Your Visit

The 40th Parallel historical marker stands along a busy stretch of Pennsylvania Route 441 at these GPS coordinates 40.000000, -76.474000 in Washington Boro. There is no sidewalk, so use extreme caution when stopping to view it. The sign makes for a quick but meaningful visit, especially when paired with nearby Blue Rock to the south, where Cresap ran his ferry and fanned the flames of a colonial border war.

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Learn More

Learn more about Cresap’s War and the Pennsylvania-Maryland border dispute in this video from LNP.

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Further Reading

• 40th Parallel
• 40th Parallel Historical Site