According to Rootsweb on Ancestry the following graves are slave graves.
The slaves were allowed to be buried in the cemetery as long as their monuments were not like those of the whites that were buried here. They accomplished the difference by cutting sandstone slabs and creating above ground monuments with sloped sides. At the time they were placed at the back of the cemetery furthest away from the main road. As the years have progressed the cemetery has grown and they are now pretty much in the center of the cemetery. There are only a few of these graves in the cemetery but they are quite interesting.
Located at Washburn Prairie Cemetery in Washburn, MO.
Frankly I find it said that the recorded record of these people are listed as "slaves". They had names and should have had them on their monuments and in the records.