I have always thought the Sphinx was a yarddang. Probably everybody who thought about it at all thought so.I've thought about it, and it doesn't make sense, because the Sphinx was literally mined out of what is now the enclosure around the Sphinx. As Robert Schoch has said, it's possible that the head of the Sphinx began as a yardang (though now it's impossible to tell), but the body of the Sphinx could not have been.
Of course the head (the original lion head) and neck were the yarddang. The body was quarried out, not mined and the blocks were used to construct the temple complex next to the Sphinx.