In an interview for Harper and Row, on December 21, 1980, (not, I believe, available on the Internet), Bradley explained that his story was based on local oral history, much of it confirmed with the physical evidence he found. Bradley grew up in southwestern Pennsylvania, where the story takes place, and he heard stories from people in his town. This led him to investigate and find the actual graves of the slaves in the story.
When I read the novel, I was quite intrigued because we live a few hours from Bedford. I decided that I was going to find the graves for myself, and I did so, based solely on the material provided in the book. My husband was quite skeptical about our "field trip," but book in hand, I led him through cornfields, across a creek, straight to the graves. There was, at that time, a small plaque. It was an incredibly moving experience. Of course, I should also point out that we trespassed over private property, which was probably not the best thing to do.