By William Ecenbarger

STEWARTSTOWN, Pa. - Under mouse-gray skies promising rain at any minute, some 100 people gathered over the weekend in a muddy farm field along the Mason-Dixon Line in York County, Pa., and Harford County, Md., and formed a circle around a shallow, elliptical enclosure.

Inside was the remaining six-inch stub of a limestone boundary marker placed there by Charles Mason and Jeremiah Dixon 21/2 centuries ago, and next to it was a new four-foot replica of that stone placed there this year by a group of preservationists.

The assembly, which included land surveyors, local history buffs, Boy Scouts, and a handful of state and local officials, applauded when the two stones were unveiled simultaneously at the 45-minute ceremony. The marker is designated No. 40 because it's 40 miles from the beginning of the east-west line near Newark, Del.

The original stone had been buried a foot underground for more than a century until it was located using modern GPS technology in 2014 by the Mason-Dixon Line Preservation Partnership. Since it was formed in 1990, the partnership has located and inventoried most of the 233 markers placed along the line between 1763 and 1767.

The mood of the gathering was upbeat, but among the preservationists there was an undercurrent of concern.

Next year marks the 250th anniversary of the completion of the Mason-Dixon Line, one of the most important boundaries and one of the most important symbols in American history. It's an imaginary line, but it has one tangible aspect: those limestone markers, which were quarried in England, shipped to America, and placed at one-mile intervals by the Mason-Dixon party.

However, today, many of the original boundary stones are weathered, have been damaged or vandalized - or are missing altogether. There is no government agency assigned to protect them.

Todd Babcock, the leader of the preservation partnership, said the group has replaced and repaired some of these stones. But he's concerned about the future. "If someone steals a stone to decorate their backyard," he said, "you've lost a tangible part of Mason's and Dixon's historic achievement."

He said determining the precise locations of the stones was critical: "Several years ago, we were reliably informed that a stone had been stolen by a local resident. We got the police involved and tracked it down. The thief claimed he was trying to protect the stone from vandals. Fortunately, we had an accurate surveyed position of the stone and were able to put it back in the correct position."

Babcock said most of the damage is caused by vehicles and equipment. "At one point in our survey, I photographed an intact stone, but three months later, a snow plow hit it and pushed it into a farmer's field. It had been on the line for over 200 years. We repaired it and put it back in place."

Vandals also play a role. "A lot of times, we've seen places where people actually shoot the markers with rifles. You can see marks on the sides of them."

Most of the milestones bear a "P" on the north face and an "M" on the southern face, but every fifth mile along the line was marked by the "crown stones" engraved with the Penn family coat of arms on one side and the Calvert family coat of arms on the other. The rectangular markers were quarried on the Isle of Portland in the English Channel. The milestones vary in height from 31/2 feet to five feet.

Mason and Dixon were summoned from England in 1763 to settle a dispute between the Penn and Calvert families over where each province began and ended. Because of inept royal geography, the king's grants to the Penns and the Calverts overlapped. No one noticed for a long time, but then sea captains arriving in Philadelphia with the latest navigational instruments began informing the Penns that their city was in Maryland. The Quaker Penns were not about to give up their famous city to the Catholic Calverts.

The Line was a stunning achievement of skill and courage. Mason and Dixon - constantly fighting against accidents, hostile Indians, snow-covered mountains, flooded rivers, wild animals, and nitpicking bureaucrats - used instruments that were crude by today's standards to plot a border that is still accepted as the official boundary today.

It was an important scientific achievement. When Mason and Dixon completed their four years of work in 1767, it was the most ambitious geodetic survey ever conducted - a 325-mile boundary between Pennsylvania and Maryland.

On March 1, 1780, it also achieved symbolic status because Pennsylvania abolished slavery and the 12-year-old line became the boundary not just between Pennsylvania and Maryland, but between freedom and slavery - and the goal of every runaway slave.

Babcock said the preservation partnership hopes to protect the stones by educating the public about them - especially homeowners who have them on their property. But he would like to see stronger steps taken, perhaps by state historic groups, to preserve this unique part of American history.

"I feel a kinship with Mason and Dixon, and every time I come out, I learn something new about the methods that they used and the difficulties they overcame," he said. "I think their story has been lost in the association of the line dividing North vs. South, and I still find most people don't know about the men whose name the line bears."

William Ecenbarger is a former Inquirer reporter and the author of "Walkin' the Line," a travel-history book about the Mason-Dixon Line.