On the crest of a ridge projecting into Rock Creek Park is a Freedmen's Cemetery for ex-slaves. A tiny sliver of the Freedmen's cemetery remains, and local archaeologists have identified many graves, but no markers remain. The adjacent Quaker cemetery did not fare so well - it is now paved over.
To expand upon the story in the caption, this cemetery provided final rest to freed slaves and US Colored Troops Civil War Veterans. Some 8,400 people were interred in the Freedmen's Cemetery and the adjacent Quaker cemetery, both of which closed in the 1890s. Most of the burials were not removed and remain under the Walter C. Pierce Park to this day, but no markers remain for any of the graves, Quaker or Freedmen.
On an off-topic note, you can see my RZ-67 sitting on the park bench if you look carefully.