The mystery of the Washington Square Park tombstone has been solved.

On Oct. 23, construction workers unearthed an 18th-century tombstone in the southwest side of the park. According to Joan Geismar, an archaeological consultant for the New York City Department of Parks and Recreation, the gravestone was found two and a half feet underground and bore the inscription, "Here lies the body of James Jackson who departed this life the 22nd day of September 1799 aged 28 years native of the county of Kildare Ireland."

According to records from the New-York Historical Society, James Jackson was a watchman and grocer who lived at 19 E. George St. (currently Market Street on the Lower East Side).

"Research has shown that Mr. Jackson was a man of some means — not wealthy, but with personal worth," Geismar said.

According to Geismar, the park site was part of a potter's field from the late 18th to early 19th century, which is usually a burial ground devoid of markers for the "indigent, the forgotten and usually the unknown."

"It was such a shock because you don't expect to find a tombstone in a potter's field," Geismar said.

She said the 210-year-old, three-foot-tall sandstone marker was in remarkable condition and "an extraordinary find."

After researching old newspapers online, Geismar discovered that at the time, the "onslaught of summertime Yellow Fever," which caused Jackson's death, "resulted in the mandate that all victims of Yellow Fever could only be buried in the Potter's Field."

The tombstone was the first and only grave marker to be found since 1890 when excavation for the park's arch began, according to parks department spokesperson Cristina DeLuca.

Workers dug seven feet below the site but found no human remains, which may have been moved when the area was converted into parade grounds. Geismar ensured that she observed the excavation of the archaeological find in order to protect it. But the search for skeletal remains ends there.

"We have never been searching for them, but merely locating those that might be in harm's way from the ongoing construction," Geismar said. "The aim is to locate and protect any intact burials where construction is planned."

For now, Geismar said, Jackson's headstone will be conserved and possibly presented above ground to honor the history of Washington Square Park.