We can try to date the Great Sphinx by following two methods. The first one consists in considering the civilization which is accredited by Egyptologists for having built it; they assume that the construction of the Great Sphinx dates back to the IV Dynasty, circa 2,500 B.C..
The second method, on the contrary, consists in seeking and analysing some revealing time markers: either from a geological point of view, as Robert Schoch suggests, or from an astronomical perspective, as it is proposed by Graham Hancock. These two similar approaches, however, lead us to different conclusions: the construction of the Great Sphinx might be dated between 7,000 and 5,000 B.C., according to Schoch’s view, or earlier circa 10,500 B.C., as Hancock says.
Evaluating in depth the effects of atmospheric factors would probably be the most objective approach, provided that we were in possession of missing information, like: the original shape and size of the statue, the finishing materials, if any, the exact alternation and duration of climate conditions (dry and humid eras), the wind intensity to which the statue was exposed to, and so forth. But even if we knew all these missing pieces of information, the elaboration of an erosion model would be extremely difficult and the outcomes of simulations using such model would probably leave a great degree of incertitude.
Having said that, we believe worth it anyhow to do an exercise in this direction without claiming, however, to formulate an hypothesis scientifically unassailable. First of all, we must highlight that we were not able to find sufficient studies on dating of stone age artefacts. A comparative method in analysing the erosive effects of meteorological agents on different, and possibly better known, archaeological sites could be of some help to our purposes; although we should consider the influence that the latitude and seasonal shifts have on winds average speed (Seasonal variation in wind speed and sea state from global satellite measurements, D.T.Sandwell, Russel W.Agreen, Journal of Gephysical Research, March 20, 1984, http://topex.ucsd.edu/sandwell/publications/9.pdf), so that at 30°N (Giza), it was measured an average wind speed less than 6 m/s, which was approximately 30% lower than the measurement obtained at 42°N (Rome). Measurements taken at different latitudes, even if they do not take in due account local orographic factors, still do represent an interesting general principle.
The shape of the Great Sphinx is rather surprising since, looking at its proportions, it seems to be the result of a significant deflation of its materials due to an extremely long exposure to atmospheric factors. This point was already highlighted by Schoch (official site) in his theory, but let’s see it more in details. Based on the data in our possession, the statue is approximately 74 meters long; it has a maximum width of 18 meters at shoulders and ilium and of just 10 meters at flank; the highest point of its head is at 20 meters height, which is about 12 meters above the base level of its back. From these dimensions, the volumetric development is of about 12,000 cubic meters. Estimating the volume eroded by the atmospheric agents over the time, however, would require to know the original shape and dimensions of the statue that we ignore of course. We have thus decided to adopt as a reference model the bronze lions in Trafalgar square, London, having a length of about one twelfth of the Lion of Giza. If we do recalculate the volume of the Great Sphinx, using the same proportions as those of the bronze lions, we can see that the statue could have had in origin a volume twice that of its current one. By overlaying the two side silhouettes, we notice also that the eroded portion of the statue is mainly concentrated over the back and the head characterizing so its abnormal profile.
According to our reasoning, we still don’t know how much time has elapsed from its construction but we have here an interesting point of interpolation: assuming that the statue dates back to 12,500 years ago, as Hancock says, and that the Pharaoh’s head was carved (or juxtaposed) over the lion’s body a bit more than four thousand years ago, the statue should have lost linearly since its construction more than four thousand cubic meters of limestone and about three meters in thickness over its back. This process of course would have affected the stability of the head which surmounts the body.
Although we cannot determine with accuracy an average deflation ratio allowing the exact dating of the Great Sphinx, if we assume as an average value of ten, twenty or forty centimetres per one hundred thousand years (the fluctuations from the average value may be substantial if we consider that the statue was almost completely buried in the sand for a long period of time during the last centuries), then we should assume that the Lion of Giza was not carved just some thousands but presumably millions of years ago. This is certainly an original and interesting theory as we can further investigate later on.