Not much. The reason that, in Egypt, the pyramids were built in that shape is that there is a region where sandstone has been sand blasted into pyramidal shapes. The ancient Egyptians no doubt copied this natural shape. Wind-blown sand can produce lots of amazing shapes over time, and we know that Mars has plenty of strong winds and exposed surface rocks. I am actually amazed that geologists looking at Mars haven't seen more strange shapes. Of course there were the famous Martian Canals earlier in this century which vanished once we got to Mars and took pictures first hand. As for the Martian Face, I am not impressed by either the image quality or by the logic behind its interpretation. In Carl Sagan's book 'The Demon-Haunted World' he makes the arguments that any astronomer would make in dismantling the notion that this 'Face' is genuine. He, after all, has a lot more at stake in finding evidence of intelligent life off the Earth, than any of the supporters of the Martian Face.