's true, the erosion on the Sphinx is generally attributed to water erosion -- now, because of Robert Schoch's work on it -- but the way the erosion occurred is a much-debated issue (if a deadlock is considered a debate ;').
Rainer Stadelmann concluded a couple of years ago (published paper, ref journal) that the facial features on the Sphinx are not those of Khafre, but of Khufu, builder of the Great Pyramid. Throughout my life I'd always wondered how anyone could look at the Sphinx and the statues of Khafre and consider any resemblance whatsoever. Stadelmann's work does not stem from that of Schoch, or his (former?) associate John Anthony West (JAW transformed himself into an anti-Bush fanatic, so I dropped his newsletter), and obviously doesn't suggest that the Sphinx / pyramids are 10,000 years old or whatnot.
The records cited in favor of a pre-pyramids dating of the Sphinx date from the New Kingdom, and (like a number of others) are copies of Old Kingdom originals no longer extant. One remarks that Khufu made repairs to the Sphinx, an event difficult to explain if it had not yet been built.
My own view is that the Sphinx had some sort of existence before the pyramids (not however going back 12,000 years) and follow Schoch who showed it was constructed at the same time as one or two of the oddball megalithic style temples often attributed to a New Kingdom pharaoh. How the face of the Sphinx can match that of Khufu is difficult to explain, as there was (not long ago) only one known image of his face, a small figurine that can be held in one hand. A monumental statue fragment found, hmm, circa 20 years ago has more recently been identified as Khufu's, but obviously has an uncertain attribution, and furthermore, doesn't resemble the Sphinx' face.
IMHO, the head was recarved (that is also Schoch's view), probably during the Ethiopian dynasty (best known pharaoh being Tirhaka), which explains the black African features of the face, and the fact that the head is out of scale with the body. Probably the statue had lost most of its head over the centuries, and so there was much less stone to be recarved.
Here's a video source:
Mysteries of the Sphinx (video, Charleton Heston)
http://www.bcvideo.com/ordermom13.html
Many believe the Sphinx to be a Dog and was built to reflect the alignment of the Great Pyramid with the Dog Star Thubin.
Others came and mutilated the face.