This stems from Mike Mearls' interpretation in the answers to this question:
Mike Mearls :@Daganev @wax_eagle Correct, you'd need rocks of some sort (ruins, boulders) to support it.
The RAW for wall of stone states that:
It must, however, merge with and be solidly supported by existing stone.
Does this mean that the supporting stone that the wall of stone is merging with must be contiguous stone (such as a large boulder, or rocky outcrop)? Or can a wall of stone bear on/merge with riprap, aggregate/gravel, or ballast rock, provided that the substrate can handle the forces without displacing excessively?