The only explanation I can think of is derived from the identification (in chapter 62) of Thoth with the Nile, ⁂⁂⁂⁂. From this point of view the god is both the son of the Rock and issues from the place of the Double Rock, ⁂⁂⁂, or of the two Rocks, called in the time of Herodotus Krophi and Mophi.