Alexandria commisioned a sculpture/memorial for the long-neglected Freedman's Cemetery. Mario Chiodo created the sculpture named The Path of Thorns and Roses. (Marvin Joseph/The Washington Post)

The minister’s voice reverberated through the oaks and hollies just southwest of George Washington’s tomb, calling on visitors to remember those “unnumbered trail blazers who rest beneath this hallowed space.”

He was referring to scores, and perhaps hundreds, of slaves and their families whose remains lie somewhere beneath the butterscotch-colored soil on a ridge above the Potomac River .

William “Billy” Lee, Washington’s manservant throughout the Revolutionary War, was buried here about 1828, researchers say. West Ford, a longtime servant of the Washington family, is thought to be the last person buried here, in 1863.

The rest of the names are unknown. But the details of their final resting places may soon be identified through a multi-year archaeological survey to be launched May 30. Before beginning the project, the plantation organized Thursday’s blessing ceremony.

The survey at Mount Vernon is one of several local efforts to learn more about the burial sites of African Americans, both enslaved and freed, during the early years of the United States.

Ten miles north, at the border of Alexandria, the long-neglected Contrabands and Freedmen Cemetery has been restored, and workers are preparing for a September dedication. And at Fort Ward, on Alexandria’s west side, residents and city employees are working to identify and commemorate the graves of African American residents buried there.

The primary goal of the Mount Vernon project, said Curtis G. Viebranz, president of the historic site, “is to create a map to show exactly where the interred are” and how many were buried there. No graves will be excavated, he told the few dozen people who came Thursday to pray, sing, place flowers and sprinkle soil on a commemorative wreath.

Eleanor Breen, Mount Vernon’s deputy director for archaeology, said that about 316 slaves lived on the plantation at the time of Washington’s death in 1799. A visitor’s account from 1833 described 150 slaves buried there in unmarked graves, and researchers said Thursday that the number of graves may be significantly higher.

In 1928, the Mount Vernon Ladies’ Association placed a simple marker on the cemetery site to bring attention to the unmarked burial places. It was replaced decades later with a brick archway and a path that leads to a sunken area of three concentric circles with a partially constructed column at the center.

Investigators using ground-penetrating radar have identified nearly 60 possible grave sites in limited explorations of the area over the decades, Breen said. The archaeological team involved in the new effort will dig by hand to identify likely graves, she said, being careful not to disturb the ones they find. The search will cover 3,500 square feet and could involve the use of radar in later stages.

Alexandria’s Contrabands and Freedmen Cemetery is believed to hold many more remains. Archaeologists think at least 1,700 people of African descent are buried there, many of them children.

The cemetery was created from a pasture that belonged to a Confederate sympathizer. It was seized by the federal government to create a burial place for the remains of escaped and newly freed slaves who had moved to Alexandria during and after the Civil War.

A newspaper article in the late 19th century described neighbors digging clay from the land, and bones visible in the soil after it rained. In the mid-20th century, an expanded Washington Street and its sidewalks ate into the cemetery land. An office building went up on the site, then a gas station, and a segment of Interstate 95 was constructed nearby. The cemetery was all but forgotten until city historian T. Michael Miller discovered a 19th-century newspaper reference to it in the 1980s.

In 1995, research historian Wesley E. Pippenger found something called “the Gladwin records” in the Virginia Library in Richmond. It listed every single burial — including name, age, residence and date of death — at the Alexandria site.

In archaeological circles, the records were a treasure, allowing the newly formed Friends of Freedmen’s Cemetery to identify people who too often existed in census or other records only by first name and the word “slave.”

When the Virginia Department of Transportation found graves in its right of way during the reconstruction of the Woodrow Wilson Bridge, it began marking and commemorating the sites.

Today, bronze panels list the name of every person believed buried at the cemetery, with bas-relief etchings that show slaves escaping the South, and, after Emancipation, studying.

Wooden grave markers that had deteriorated have been replaced. Plans are afoot to indicate that some graves probably lie beneath nearby sidewalks or roads as well. A sculpture by Mario Chiodo, “The Path of Thorn and Roses,” sits squarely on the old gas station’s concrete pad. The memorial will be dedicated in September.

“African Americans are so used to their story not being told, they’re excited when it is,” said Audrey Davis, acting director of the Alexandria Black History Museum.

Fran Bromberg, Alexandria’s assistant city archaeologist, called the cemetery “an incredible project. I’m pleased that archaeology was used for justice.”