THE AGE OF THE SPHINX

20 Jan 2000

Groundbreaking paper by geological engineer Colin Reader entitled Khufu Knew the Sphinx.

20 Jan 2000

IL's synopsis of Colin Reader's paper, including a fresh look at detective Frank Domingo's facial reconstructions.

11 Feb 2000

11 Feb 2000

2 Mar 2000

Paper by geologist Jim Harrell, including a response to Robert Schoch and Colin Reader.

17 Mar 2000

Paper by Colin Reader, responding to Jim Harrell, Robert Schoch and David Billington.

24 Mar 2000

Paper by IL responding to Robert Schoch and David Billington.

28 Mar 2000

24 Jun 2000

23 Feb 2001

Email to IL from geologist Alex Bourdeau suggesting new support for the Guari 'salt exfoliation' argument by reference to the effect of the Sun on the different aspects of the Sphinx and its enclosure

I just recently wrapped up an intense discussion with Colin Reader on the subject of "geo-dating" the Sphinx. Of course, Colin and I still disagree, but a few points from our "exchange of hand grenades" may be enlightening for those interested in this topic.

Robert Schoch gets credit for opening up this particular subject with his suggestion that the Sphinx must be thousands of years older than posited by most Egyptologists. He based his findings on the badly weathered condition of the Sphinx and its enclosure. Soon after he made this suggestion, Dr. Jim Harrel and Dr. K. L. Gauri came up with alternative explanations. In 2000, Reader reopened the debate with the suggestion that, although it's highly unlikely that Schoch's proposed date of 5-7 thousand years is tenable for archaeological reasons, the eroded condition of the enclosure's west wall might be explained by sheetwash originating on the higher, western part of the Plateau.

The debate, in a nutshell, hinges on the principal weathering agent acting on the Sphinx and its enclosure. (Early on, Colin and I agreed to exclude the critter itself from the discussion. It has been "rehabilitated" so many times, both modern and ancient, that controlling for the potential influence of these efforts is impossible. As a result, the debate focused on the condition of the south and west walls of the enclosure and the differences between the two.)

Schoch and Reader both infer that the "rounded" condition of the west wall is due primarily to the action of running water. They contend that under prevailing climatic conditions since the IV Dynasty, there simply hasn't been adequate rainfall to wash the limestone into the subrounded blocks which are so apparent in photographs of the enclosure.

Reader diverges from Schoch on one important point. He feels the rounded condition of the west wall was the result of several hundred years of sheetwash pouring into the enclosure. He further suggests that quarrying, after the Sphinx was carved, for the construction of Khufu's pyramid prevented this sheetwash from reaching the enclosure. As a result he concludes that Khufu knew the Sphinx, because the enclosure rocks were already seriously rounded before Khufu's pyramid was built. In general, Reader's approach is on much firmer ground than Schoch's, but from my perspective, it still has some serious flaws - even without getting into the cultural context debate.

Gauri et al. (whose response was to Schoch, written before Reader's paper) nicely described the process of salt crystal stress induced exfoliation (SCrySIE) as the principal weathering agent. In this process, various salts naturally present in the limestone and dissolved in groundwater, precipitate at the surface. As these crystals grow during precipitation, they force other grains in the limestone matrix apart, destroying the structural integrity of the surface. The broken up fragments of limestone are then free to simply fall off or be stripped away by the wind.

If you look closely at the partings between the limestone blocks anywhere in the enclosure, you will note they are subvertical, not vertical (Reader acknowledges this in his paper). If flowing water was the principal agent which cut these partings, the fissures should be vertical or very nearly so. Water flows fastest, generating the most energy, when it is accelerated most rapidly by gravity - directly towards the center of the Earth. Simply - water (like everything else) tends to fall straight down. If it's what's cutting the rock, the fissures should be very close to vertical.

But the partings or fissures are often up to 45 degrees from the vertical (and in at least one case, actually subhorizontal). In many cases, they actually cross one another. It has been recognized for a long time that the limestones at Giza are criss-crossed by joints. It is also accepted these joints were formed as the limestone indurated (turned to stone). Groundwater solution over millions of years since the limestone formed turned these joints into open cracks. During our debate, Colin partially modified his earlier suggestions and now thinks these cracks are very wide (and the limestone on either side, very rounded) on the west wall of the enclosure because sheetwash cleaned them over the first couple hundred years of their exposure - allowing other weathering processes to occur more rapidly here than elsewhere on the Plateau.

My contribution to this debate, in truly Egyptian fashion, has focused on the sun. Or, more specifically, on which rock spends the most time in the sun. I generally agree with Dr. Gauri's team that SCrySIE is the principal culprit at work. But then, why are the blocks in the west wall of the enclosure so much more "rounded" than on the south wall? (And it's time to drop the term rounded - what were looking at is spherical weathering - the shape rocks assume as their corners are dissolved by primarily chemical processes.)

I have posited this difference can be explained through the position of the sun (Uh, oh - yet another astronomical hypothesis!). The west wall of the Sphinx enclosure differs most significantly from the south wall in when and for how long it spends, "soaking up rays. "

Here's what I think happened, and continues today. Khafre or his artisans decided at some point that the hard, resistant Member III limestone chunk o' rock from which the Sphinx's head was eventually carved was not monumental enough to make a truly imposing figure. Certainly not, when contrasted with Khufu's already built pyramid and Khafre's rising one. In the inimitable style of the IV Dynasty, they came up with a clever solution - make the rock bigger. How do you do that? Cut away the surface around it. In the process of making it so, the AE's formed two nearly vertical walls, one oriented north/south (the west wall) and one oriented ESE/WNW (the south wall). Why the south wall is not perfectly perpendicular to the west wall is irrelevant to the current discussion. After the rock is made bigger, the AE's get to work, carve the Sphinx's head out of the Member III rocks and the body out of the Member I and II rocks they had exposed.

Then nature went to work - dismantling their creation. As the sun rises in the morning, the west wall gets the full brunt of its spectral assault - most importantly, the rays in the infrared range. And what happens? First, the rock (which has been cooling since noon the day before) warms up and any moisture that has either condensed on the rock during the night or seeped through from the groundwater present in the Plateau west of the Sphinx, evaporates rapidly - SCrySIE ensues - breaking up the surface of the limestone. As the sun climbs, the rocks get even hotter, grains in the matrix expand differentially - pushing against each other - breaking up the surface of the limestone. Remember, this is a vertical face, you don't need wind or water to move these grains once they are no longer attached to each other - just gravity - they fall off. Sometimes, whole sheets of limestone detach and fall off. And this happened over and over again over the last 4500 years (roughly 1,642,500 times minus how ever many rare cloudy mornings Giza experiences and minus the periods when the enclosure was full of sand). Why hasn't it happened as much to the south wall? Because the sun's rays never hit it directly, only obliquely during the summer and not at all during the winter. In fact, the south wall is essentially in its own shadow for most of the day.

This is clearly why the west wall is more weathered than the south wall. It also explains why the top of the west wall is the most severely eroded place anywhere in the enclosure. Even when the enclosure is partially filled with sand, this rock is exposed to the sun. As it spherically weathers, its exposed to longer and longer periods of sunlight.

So, bottom line, you don't need either Schoch's frequent mid-Holocene rainstorms or Reader's focused runoff to explain conditions observed at the Sphinx. That leaves the traditional interpretation of the Sphinx's age (and it's pretty obvious associations with Khafre's funerary complex) intact.

[Note that I tackled Alex on the objection that Colin Reader originally put to me which convinced me to set aside my previous reliance on Guari and SCrySIE, that being the fact that the natural fissures that cross the enclosure can be observed both in the west wall and in the Sphinx's rump, and that the weathering of these same fissures is completely different in the wall to how it is on the rump - implying that the wall has been subject to different processes, namely running water. His response was that the rump itself would be in shade for much of the day, therefore SCrySIE would operate on it less - IL]

23 Feb 2001

Response to Alex Bourdeau by Colin Reader, originally posted to the Amun discussion group

Alex confirms a very important point that the 'west wall' of the Sphinx enclosure is unique at Giza, in his own words "unable to find any quarry face as weathered as the west wall of the enclosure." Bear this in mind. Alex appears also to hold two other fundamental misunderstandings:

1. When referring to Gauri's chemical weathering hypothesis, he cites the source of water as groundwater ("In this process, as salt-rich groundwater evaporates at the surface of the limestone, salt crystals precipitate between the particles"). Although in his earlier papers (1981) Gauri refers to 'subsurface' water, by the time of the conclusion of his studies (1995), because of the fact that similar chemical weathering was observed on freshly cut masonry (not in contact with groundwater) he firmly sees the source of moisture as dew condensing from humid air at nightfall ("The indicated mechanism of this exfoliation is that in the cool of night, with the arrival of the dew point, water vapour condenses on the stone surface"). Groundwater is not therefore a factor in Gauri's hypothesis.

2. Contrary to Alex's understanding, it is not only the western wall, however, that is heavily degraded, so too is the western end of the southern enclosure wall (up to the main fissure - of which more later). ("The problem with ScrySIE being the only process weathering the limestone is the difference between the west wall and the south wall of the enclosure. The west wall is clearly more weathered than the south wall").

So if you consider aspect as the cause of the unique degradation in the west of the Sphinx enclosure - you are faced with two major difficulties.

First, according to the TmW theory, the southern enclosure wall, which faces just east of north, should not be heavily degraded ("The south wall, through much of the year, gets blasted by this same radiation - but at a very oblique angle - the rock warms up more gradually, allowing relief of the stresses without fracturing"). And yet approximately 40% of the southern enclosure wall (the bit in the west) is heavily degraded. This is not consistent with TmW.

Secondly, the effects of all this serious infra red radiation as the sun breaks would be felt by ALL east facing exposures at Giza - of which there are scores. Some, in the central field cemetery, just sw of the Sphinx, are cut in the same limestone beds. It's not possible to be so certain about the stratigraphic correlation of other more distant exposures, such as the facade of the tomb of Debehen, but they are likely to be cut into similar, if not the same, units. But as Alex confirms, the degradation of the western Sphinx enclosure is not to be found elsewhere at Giza. Again the TmW theory is not consistent with the observable facts. Chris Ogilvie-Herald, kindly reminded me also that the chest of the Sphinx (no doubt that this is cut from the same limestone beds) faces east too - it does not show the same intensity of degradation as the western wall of the enclosure - TmW can't explain this.

Alex does not engage on two other important bits of evidence, which we discussed and which also tell against the TmW theory.

1. This theory does not explain how the Fourth Dynasty cutting (dated by Lehner and Hawass) in the Member I strata north of the Sphinx temple shows so little signs of degradation, yet the face into which this cutting is made is heavily degraded. Under any sensible interpretation this unweathered 4th dyn face has been cut into an older face.

2. There is evidence of water running in the Sphinx enclosure - again courtesy of Dr Lehner. There's a shallow drainage channel crossing the rocky floor of the Sphinx enclosure - issuing from the main fissure. This explains why the degradation of the southern enclosure reduces dramatically at this point. Water from up slope, encountered the main fissure and was directed into the enclosure and across the floor. So, as water-erosion has been present at some time in the area of the Sphinx, why should it not have affected the western enclosure walls?

[Alex Bourdeau replied to this posting on the Amun list, his main point being that the interaction of weathering agents renders the analysis so complex that far more detailed measurement of the erosion surfaces at the Plateau is needed if proper conclusions are to be derived. I support this point of view entirely, but for what it is worth I feel that as the debate currently stands Colin somewhat has the edge - IL]