Washington DC’s hidden boundary stones

The most famous African American in colonial America is largely forgotten today, but his longest-lasting contributions are hidden in plain sight in Washington DC.

"Are you sitting down? I have news for you." Gwen Marable's cousin from the US state of Ohio called her at home in Maryland about 27 years ago. "We are descended from the sister of Benjamin Banneker, Jemima."

The Banneker family, which numbers over 5,000 known descendants today, only learned about this astonishing connection to their ground-breaking but little-known ancestor through the wonders of DNA testing. As such, no personal stories about him, no artifacts, were handed down through the generations.

"Remember, he was born one year apart from George Washington," said Marable, who went on to help launch and serve as president of the Benjamin Banneker Historical Park and Museum in Baltimore County, Maryland. "People were not writing about black people at that time."

What we do know is that this self-taught, free African American tobacco planter was, among other things, a mathematician, scientist and astronomer who helped establish the boundaries of Washington DC. To this day, nearly all 40 of those boundary stones – which mark the original 10-mile-by-10-mile diamond shape of the US capital city – remain in plain sight. They are the oldest set of federally placed monuments in the United States.

In 1790, the brand-new president George Washington was authorised to delineate the site of the new capital city along the Potomac River. It was swampland overgrown with forests, vines and weeds, sparsely settled with tobacco farms. He needed a survey team to establish its boundaries.

Enter the foremost surveyor at the time, Andrew Ellicott, who had made a name for himself charting 11 states (and making the first topographical study of the Niagara River and Niagara Falls). But he needed an assistant who could read the stars, to make the placement of the markers as accurate as possible.

His cousin recommended a neighbour friend, Benjamin Banneker, but according to historian Silvio Bedini (who wrote what is considered the definitive biography of Banneker), Ellicott had some concerns. "Andrew Ellicott questioned the potential competence of a black man without schooling or scientific training," Bedini explained in a 1991 article for the magazine of the DC History Center. "The prospect of relying upon an untrained amateur astronomer for the precise astronomical data upon which his survey would be based was not a cheerful one."

But with no other prospects, the two set to work.

Lying on his back for six nights, 61-year-old Banneker recorded the precise times of the stars' transits. With these calculations, the first stone marker was installed at Jones Point (on the Potomac River just south of modern-day Old Town Alexandria, Virginia) on April 15, 1791, amid great fanfare.

Today, that marker is visible through a window in the seawall of the 19th-Century Jones Point Lighthouse. One foot square and three feet high, it – like all of the markers – was made from the same sandstone from Virginia's Aquia Creek that was used to construct the White House, US Capitol and Supreme Court Building.

BBC