As a public service, let me note that I have personally never found that the wall of an old cemetery marked the actual limit of the burials; there are always some forgotten graves outside. So be careful digging anywhere near an old graveyard.
article about this discovery in the most recent University of Virginia Magazine has some fascinating information about those slaves, including this account of Sally Cottrell Cole (above):
Sally Cottrell Cole was born enslaved around 1800 and worked at Monticello as a maid to Thomas Jefferson’s granddaughter Ellen Randolph. In 1825, the Jefferson family hired Cottrell out to U.Va.’s first professor of mathematics, an Englishman named Thomas H. Key, whose wife required a nurse and maid. Professor Key abruptly resigned in July 1827 and returned to England. Before leaving, he purchased Cottrell for $400 on the condition that she be immediately freed. Virginia law at the time required freed slaves to leave the state within 12 months. Rather than leave, she worked for U.Va. chemistry professor John P. Emmet, then on her own as a seamstress. Cottrell was baptized at First Baptist Church in 1841 and, five years later, married Reuben Cole, a free black man.