This is the same principle used to determine relative age in the trash pits discussed previously.
In fact, sedimentary rocks are, in a sense, trash from the Earth's surface deposited in basins.
The exposed bones to his left at head level are part of Bone Bed 3 where the burning stops short of the shelter wall.
The Fiber Layer is visible above Bone Bed 3 as a dark lens in the excavation wall. This section contains a fairly detailed description of Bonfire Shelter's deposits as well as information about the site's radiocarbon dates.
Mott Davis examines Bone Bed 2 near rear of shelter at south end.