The description of wall of stone states:
The wall doesn't need to be vertical or rest on any firm foundation. It must, however, merge with and be solidly supported by existing stone. (PHB 287)
To me the two statements "doesn't need to rest on any firm foundation" and "must be solidly supported" seem contradictory. So does the stone the spell effect is merged to have to serve as a foundation or not? Could someone give an example that would be a shape or positioning of the effect that has no foundation, but is supported?