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Comments on the geological evidence for the Sphinx's age by Professor James Harrell
(Written for Ian Lawton's website and reproduced here with Professor Harrell's permission)


When geologist Robert Schoch and pyramid-power proponent John Anthony West first proposed their radical reinterpretation of the age of Giza's Great Sphinx in 1991-92 (1-3), I was the first geologist to challenge their geological arguments (4). My position was then, and still is, that the degradation pattern seen on the limestone of the Sphinx's body and enclosure walls are consistent with a Fourth Dynasty date for this monument. I, thus, see no reason to doubt that the Sphinx was carved as part of king Khafre's funerary complex. This interpretation was subsequently supported by geologist Lal Gauri and co-workers (5) and again more recently, with slight modification, by geological engineer Colin Reader (6). My purpose in writing the present paper is to comment on some of the geological evidence currently under discussion in the ongoing debate about the Sphinx's age.

Although the current geological and archaeological evidence is not as complete or definitive as any of us would like, it is still more than sufficient to rule out the 7000+ B.C. date of Schoch and West (and the latter's notion that the Sphinx was carved by refugees from Atlantis which was originally settled by immigrants from the planet Mars!) It is indeed surprising that the ideas of Schoch and West are even still being debated. Colin Reader, following a much more reasonable line of inquiry, has suggested that the Sphinx is indeed older than Khafre's reign but only by a few hundred years (i.e., it is an Early Dynastic artifact). His dating is based primarily on geologic evidence and it is this that I want to focus on first in this paper.

An Early Dynastic Age for the Sphinx?

The following two facts are undisputed. First, it is on the western wall and the western portions of the northern and southern walls of the Sphinx enclosure that ones sees the greatest degree of weathering and erosion: that is, deeper recesses along the less durable limestone layers and wider sub-vertical fissures cutting across all the layers. The same degradation pattern, but less pronounced, occurs on the Sphinx's body and on the eastern portions of the northern and southern walls. Second, the surface of the Giza Plateau around the Fourth Dynasty Khufu pyramid slopes generally to the southeast and toward the Sphinx, and so would tend to direct rainfall runoff to the Sphinx enclosure. With these points in mind, Reader attributes the greater weathering and erosion of the western walls to the overland (surface) flow of rain runoff, but concludes that this could not have happened after construction of the Khufu pyramid; hence, his Early Dynastic age for the Sphinx. What precludes a later date, according to him, is the presence of a sand- and debris-filled limestone quarry, dating to the Fourth Dynasty, just off the southeast corner of the Khufu pyramid and directly west of the Sphinx. This quarry supposedly collects the runoff and prevents it from reaching the Sphinx.

I believe Reader is mistaken in his conclusions for two reasons. First, rainfall runoff does reach the Sphinx enclosure today and so probably did in the past as well. The evidence for this is anecdotal and comes from Ian Lawton and Chris Ogilive-Herald, the co-authors of the book "Giza the Truth". In a note to me, they reported that during a sound-and-light show at the Sphinx in 1990 or 1991, Egyptologist David Rohl told one of them he saw a "torrent of water cascading into the enclosure during a rain storm". This is precisely what one would expect to happen given the slope of the ground northwest of the Sphinx. For further discussion of rainfall runoff at Giza see the later section on subsurface weathering. A second and greater problem for Reader's argument is his assumption that all or most of the rainfall would be converted to overland flow. I suspect that much of it will quickly sink into the limestone through its myriad fractures (joints) and then travel through these as well as along the bedding planes between the limestone layers. The latter have a southeasterly inclination and so would direct the groundwater toward the Sphinx. The Khufu quarry would be no barrier to the subsurface flow of water and might even serve to collect the surface runoff and then channel it through the limestone on the west side of the Sphinx enclosure. I would expect it to emerge on the western walls as spring-like seepages along the bedding planes. The role played by the Khufu quarry is only an educated guess on my part and so more work is needed to confirm or refute it. In any event, it is clear that the geology does not necessarily support Reader's Early Dynastic age for the Sphinx.

Another argument Reader makes for his earlier date has to do with the location of the causeway connecting Khafre's pyramid and valley temple. He says because there are Fourth Dynasty limestone quarries on both the causeway's northern and southern sides it must predate them, and as part of an important religious monument of the Early Dynastic period later quarrying would not have cut through it. This sounds reasonable but Reader overlooks two other, and I would say even more likely, explanations. Khafre was Khufu's son and so it is entirely possible that the future location for Khafre's causeway was known and so excluded from quarrying while Khufu's pyramid was being built. Alternatively, if Khafre's causeway was not already planned in Khufu's time then the quarry on the north side may not cut the causeway simply because there was no need to quarry that far to the south. There was ample limestone available for quarrying to the west and closer to Khufu's pyramid.

Subaerial Weathering of the Sphinx Lal Gauri and co-workers have nicely elucidated the weathering processes currently affecting the Sphinx (7-10). According to them, dew forming on exterior limestone surfaces on cool nights dissolves and draws out salts from the rock interior which then crystallize as halite and gypsum when the dew evaporates in the morning sun. The repeated episodes of expansive salt crystallization cause small-scale spalling or exfoliation of the limestone. The degradation is greater for the lower part of each of the several limestone beds or layers and so these are the recessed intervals visible on the Sphinx's body and enclosure walls. In contrast, the upper parts of the same beds are protruding. The geologic differences between the two parts of each bed are that the lower has a higher non-carbonate content (more quartz sand and silt, and clay minerals), a higher soluble salt content (sodium chlorite and calcium sulfate), a finer-grained texture (more micritic with fewer fossils), smaller pores, and higher overall porosity. The greater degradation of the lower parts of beds is due to their smaller pore sizes, and the rounded outcrop profiles are caused by the gradational changes in composition and texture between beds. The rapid weathering rate observed today may be due in part to the air pollution and increased humidity levels associated with modern development in the Cairo area. There can be no doubt, however, that the same weathering processes operated in the distant past, at least during the rainy season between October and March when cool nights and humid air are the norm.

For the above subaerial weathering processes to operate, the limestone surfaces cannot be covered by sand. I think it is now universally agreed that the Sphinx spent most of its 4,500 year history buried in sand, certainly up to at least the level of the 5 meter-high northern and southern enclosure walls and probably at times up to the top of the western wall and the Sphinx's back, another 6-7 meters higher. It may be that the limestone surfaces were free of their sand covering for no more than a few centuries total, but even this should be long enough for significant amounts of weathering to occur. During the far longer period when the monument was buried under sand, it seems to me that weathering of a different kind would also have taken place.

Subsurface Weathering of the Sphinx

In 1994 (4) I invoked what has become known as the "wet sand" hypothesis to explain how subsurface weathering might have affected the Sphinx since the time of Khafre. The sand filling the enclosure and lying in contact with the walls and Sphinx body must have been wet much of the time. This was certainly the situation when the sand was cleared from around the Sphinx in the early 1980's. At that time it was observed that the sand was "completely soaked with water a few inches below the surface [and also that] the bedrock in contact with the sand was soaked" (11; see p. 4-5).

I originally identified two sources of water that could wet the enclosure sand: Nile River floods and rainfall. I no longer consider the floods an important source even though Nile floodwaters in 1874 and 1938 rose up to about one meter above the floor of the Sphinx enclosure, and capillary action would have carried the water at least another meter upwards into the very-fine to fine-grained sand (with grains smaller than 0.25 mm and not "coarse-grained" as claimed by Reader) and even further into the limestone bedrock. Such extraordinary floods are quite rare and probably would seldom wet more than just the lower part of the sand fill, and so cannot be a significant factor.

A more frequent and more thoroughly wetting source of water for the enclosure sand is the rain that falls on the Giza plateau. Climatic records show that the mean annual rainfall for Cairo was 2.97 cm for 1835-1841 and 3.40 cm for 1887-1922 (12; see Table 2), and, according to another source, 2.14 cm for 1931-1981 (13; see p. 157-159). For the 1902-1947 period the mean annual rainfall at Giza was 2.60 cm (13; see Table 6). Much of the little rain that Cairo and Giza receive comes in brief but intense storms. Shata (13; see p. 169) reports that "Cairo is affected by torrential winter and [summer] monsoon rains (roughly every seven or ten years) and the desert drainage lines (wadis) become suddenly flooded". He mentions one particular storm that dropped 5.0 cm of rain in the Cairo area on December 6, 1951. Sutton (12; see p. 4, 69-70; Table 13) gives the following rainfall amounts for torrential storms at the Giza station: 3.6 cm fell on January 17, 1919; 4.5 cm fell over 36 hours in December 1921; 5.0 cm fell in 80 minutes on October 27, 1937; and 5.0 cm fell over 1.5 hours on May 10, 1943. Lesser amounts of rain that cannot be considered torrential downpours, such as 1-2 cm over a 24 hour period, must occur more frequently and would produce significant subsurface flows. There is no reason to think that the rain falling on the Giza plateau over the last 4,500 years was significantly different from what has been observed during the last century. Thus, it seems that every several years, if not more frequently, there were rainfalls that wetted the sand in the Sphinx enclosure. Depending on the intensity of the rainfall, the water would have arrived either mainly as overland (surface) runoff or subsurface flow. Most of the water will enter the Sphinx enclosure around the western walls and so, understandably, it is here that most of the weathering and erosion has occurred. Once soaked, the sand would remain wet for many weeks or months due to capillary retention.

It is easy to see how water running over the surface can erode the western walls if they are not covered by sand, but what effect would the water have on limestone buried under the sand? One could reasonably assume that the naturally mildly acidic rainwater would cause dissolution of calcite in the limestone. Relative to the upper part of each limestone bed, the lower part has a greater porosity and smaller pore size and, hence, a greater internal surface area. This fact alone would cause greater amounts of dissolution in the lower part. Of far greater importance, I suspect, is the subsurface water that flows into the enclosure. This would move not only through the sub-vertical fractures but also along the bedding planes that separate the upper part of each bed from the lower part of the overlying one. The water would thus be flowing through the weaker lower parts of beds causing further dissolution. During those periods when the sand was removed from the enclosure, either by wind erosion or human excavation, the more deeply weathered material in the lower parts of beds would be removed as well.

It is worth noting that the same type of degradation pattern (alternating protruding and recessed limestone layers) seen in the Sphinx enclosure is also observed, although less pronounced, on the walls of the Fourth Dynasty quarry below the early Fifth Dynasty Khentkawes tomb, a mastaba-like structure to the southwest of the Sphinx. The underlying limestone bedrock occurs as an isolated and elevated cube-like mass and so, like the Sphinx itself, could not have been weathered by subsurface flows along bedding planes or eroded by overland runoff. The quarry walls have probably been buried under sand for most of their history and so have been weathered in the same way as the Sphinx's body; that is, by a combination of subsurface (wet sand) and subaerial weathering.

A Weathering Zone in the Floor of the Sphinx Enclosure?

Schoch, with the assistance of geophysicist Thomas Dobecki, conducted a seismic survey of the limestone bedrock below the floor of the Sphinx enclosure (3). The resulting seismic profiles revealed a low-velocity layer that extends from the floor to a depth of 1.8-2.5 meters on the east (front), north and south sides of the Sphinx, and 1.2 meters on the west side (back). The lower seismic velocities are probably due to greater porosity in the near-surface limestone. Schoch interprets the low-velocity layer as a zone of weathered limestone, where the weathering began after the Sphinx enclosure was first cut and the depth of weathering is directly related to the length of time since the enclosure floor has been exposed. The roughly two-fold difference in the depth of this layer between the front and back of the Sphinx is, he says, an indication that the Sphinx's rump, which he accepts as Old Kingdom in age, is about 4,500 years younger than the front half of the Sphinx's body.

The low-velocity layer is the most critical piece of evidence in Schoch's older Sphinx argument. What he does not tell readers and hopes that they will never ask is "how does he know the low-velocity layer represents weathered limestone?" Nowhere has he ever given any evidence to support this claim. He has not dug or drilled into this layer and so has no idea of what is really down there. Schoch (14; see p. 7), in answer to my criticism of his seismic interpretation (4; see p. 73-74), says he "has good reason to believe that the low-velocity layer is the result of weathering" but he will not tell us what these reasons are! Weathered limestone is what he wants (and needs) the layer to be and so his interpretation is not to be trusted, especially when there is a more likely explanation.

As I stated in my 1994 paper (4), the low-velocity layer is probably just a reflection of the original bedding in the limestone. The lower portions of the Sphinx's body and enclosure walls and the bedrock underneath to a depth of a few meters consists of an irregular bed of limestone that geologist Thomas Aigner (15; see p. 355) called the "shoal reef" facies of the limestone underlying the Giza Plateau. He described the layer as consisting of "isolated patches ([up] to a few meters in diameter) of coral colonies that float in a variable matrix of ... reef debris" with the coral colonies especially abundant in the upper part of the layer. The top of this layer, which is well exposed at the base of the enclosure walls, is a hummocky surface with up to 1 m of relief (16; see p. 27). The base of Schoch's low-velocity zone, as depicted in his seismic profiles, shows a hummocky relief that is identical in scale to that seen at the top of the shoal reef facies. I therefore think it quite likely that the bottom of the low-velocity zone corresponds to the base of the shoal-reef facies. Schoch interprets the higher velocity zone in the lower part of his seismic profiles as "sound limestone", but most likely what he is actually seeing is the less porous "nummulite bank" limestone facies that Aigner says immediately underlies the shoal reef facies (15; see p. 349-350). The direction and angle of dip for the limestone beds in the Sphinx enclosure are about south 45 degrees east and 2.5 degrees, respectively (17). The enclosure is oriented due east-west and so the limestone has an apparent bedding dip of about 1.2 degrees to the east and this easily accounts for the increasing depth of Schoch's low-velocity zone from west to the east across the enclosure.

Where Do We Go From Here? I think the explanation for the degradation pattern seen in the Sphinx enclosure is connected to the behavior of water, both the surface runoff and subsurface flow. A better understanding of where the water is going and how it promotes weathering will allow us to decide whether a Fourth Dynasty date for the Sphinx is early enough to account for the pattern now observed. I believe it is. This would be easy enough to verify if some resident of Cairo would volunteer to visit the Sphinx after each rainstorm and observe what the water is doing. It would also be useful to create computer-generated topographical and hydrogeologic models of the northern part of the Giza plateau for different time periods and then see, using rainfall simulations, where the water goes.

References cited

(1) Schoch, R. M. and J. A. West, 1991, Redating the Great Sphinx of Giza, Egypt: 103rd annual national meeting of the Geological Society of America, San Diego, California; abstract in `Abstracts with Programs', v. 23, no. 5, p. A253

(2) Schoch, R. M., 1992, A modern riddle of the Sphinx: Omni, v. 14, n. 11, p. 68-69

(3) Schoch, R. M., 1992, Redating the Great Sphinx of Giza: KMT, v. 3, n. 2, p. 52-59.

(4) Harrell, J. A., 1994, The Sphinx controversy another look at the geological evidence: KMT, v. 5, n. 2, p. 70-74 (see also p. 3-4 in v. 5, n. 3 of KMT for Harrell's reply to a rebuttal of this paper on p. 5-7 in v. 5, n. 2).

(5) Gauri, K. L., J. J. Sinai and J. K. Bandyopadhyay, 1995, Geologic weathering and its implications on the age of the Sphinx: Geoarchaeology, v. 10, n. 2, p. 119-133.

(6) Reader, C., 1999, Khufu Knew the Sphinx: a paper published only on this web site.

(7) Gauri, K. L., A. N. Chowdhury, N. P. Kulshreshtha and A. R. Punuru, 1988, Geologic features and the durability of limestones at the Sphinx; in ? Marinos and ? Koukis (eds.), "Engineering Geology of Ancient Works, Monuments and Historical Sites": A. A. Balkema, Rotterdam, p. 723-729.

(8) Chowdhury, A. N., A. R. Punuru and K. L. Gauri, 1990, Weathering of limestone beds at the Great Sphinx: Environmental Geology and Water Science, v. 15, n. 3, p. 217-223.

(9) Punuru, A. R., A. N. Chowdhury, N. P. Kulshreshtha and K. L. Gauri, 1990, Control of porosity in the durability of limestone at the Great Sphinx: Environmental Geology and Water Science, v. 15, n. 3, p. 225-232.

(10) Gauri, K. L., A. N. Chowdhury, N. P. Kulshreshtah and A. R. Punuru, 1990, Geologic features and durability of limestones at the Sphinx: Environmental Geology and Water Science, v. 16, n. 1, p. 57-62.

(11) Gauri, K. L., G. C. Holdren and W. C. Vaughry, 1986, Cleaning efflorescences from masonry; in J. R. Clifton (ed.), 'Cleaning Stone and Masonry': ASTM Special Technical Publication 935, Philadelphia, p. 3-13.

(12) Abdu A. Shata, 1988, "Geology of Cairo", Bulletin of the Association of Engineering Geologists, vol. 25, no. 2, pp. 149-183.

(13) L. J. Sutton, 1949, "Rainfall in Egypt statistics, storms, runoff", Ministry of Public Works, Physical Department Paper No. 53, Government Press, Cairo.

(14) Schoch, R. M., 1994, letter in the Reader's Forum: KMT, v. 5, n. 2, p. 6-7

(15) T. Aigner, 1983, Facies and origin of nummulitic buildups an example from the Giza pyramids plateau: Neus Jahrbuch Geologie und Palaeontologie Abhandlung, v. 166, p. 347-368.

(16) K. L. Gauri, 1984, Geologic study of the Sphinx: Newsletter of the American Research Center in Egypt, n. 127, p. 24-43.

(17) K. L. Gauri, personal communication.

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