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Following the 1993 "Mystery of the Sphinx" program, scholars began to respond to the claims of a prehistoric Sphinx in a series of publications. Further research bearing directly or indirectly on the age of the Sphinx also appeared. West and Schoch responded in turn.
Rock surfaces cannot directly date an archaeological site unless the rate of erosion is known. For the Sphinx and its enclosure, erosion rates going back 4500 years or longer are not possible to reconstruct. Schoch based his dating on two sets of indirect observations: (1) the presence of surface weathering patterns that he believed to be inconsistent with the arid climate of historic times, and (2) subsurface evidence that he interpreted to be inconsistent with the traditional date.
On account of the uncertainty about rates of surface weathering, though, critics argued that the erosion visible on the Sphinx and its walls could have occurred in the last 4500 years during periods when the Sphinx was cleared of sand. Some critics also argued that the Sphinx and its enclosure could have eroded during periods of burial under sand. Critics also disputed Schoch's interpretation of his subsurface data, suggesting that he had merely detected a natural difference in the rock and not a weathered region.
The Sphinx has been buried in desert sand for about four-fifths of its known existence. Shifting of the sand cover has exposed the upper strata of the monument from time to time, but only the head seems to have been continuously exposed. The intervals in which the Sphinx is believed to have stood fully in the open were the centuries from 2500 to 2100 BCE (before the current arid regime set in) and the periods of restoration and sand clearance from about 1400 to 1200, 600 to 500, 30 BCE to 200 CE, and from the 1920s to the present.
One of the first questions in the controversy over the age was whether erosion could have occurred to the monument under the sand.
James A. Harrell, "The Sphinx Controversy: Another Look at the Geological Evidence," KMT: A Modern Journal of Ancient Egypt, Vol. 5, No. 2 (Summer 1994), pp. 70-74.
James Harrell is a professor of geology at the University of Toledo, Ohio, USA. In his summer 1994 KMT article, he questioned Schoch's geological conclusions mainly on two grounds.
First, Harrell observed, the weathering effects of atmospheric condensation, wind, and occasional rain would not have operated on the monument when it was protected by sand. But rainfall and occasional Nile river flooding could have wetted the sand that filled the Sphinx enclosure during periods of sand burial. Wetness in the sand could have dissolved minerals in the Sphinx and enclosure walls, causing the rock to weather, although less rapidly than from exfoliation during periods of exposure to the air.
Second, Harrell argued that the subsurface soundings taken by Schoch and Dobecki may have detected only a natural bedding plane. Harrell cited geological surveys of the area that identified the lower-velocity region as a more porous layer of rock overlying a less porous layer. Harrell also noted that the velocities recorded along Schoch's S3 line in back of the Sphinx were 20 percent lower than the velocities along the sides; if lower velocities indicated greater age, Harrell noted, then the back should be older rather than younger.
Robert M. Schoch, Response to James A. Harrell, KMT: A Modern Journal of Ancient Egypt, Vol. 5, No. 2 (Summer 1994), pp. 6-7.
In an initial reply to Harrell, Schoch questioned whether sand filling the enclosure ever became saturated with water. If the sandfill had been saturated in ancient times, Schoch observed, the weaker lower strata of the Member II rock should have eroded more severely than the more durable upper strata. In fact, the upper strata showed greater recession. Floodwater should have also left undercutting that is not in evidence on the rock. Schoch asked how wet sand could have widened joints into fissures.
Regarding his S3 line, Schoch acknowledged the discrepancy but noted that the line was shorter than the others because of the confined space. He observed that a shorter line or a slight difference in the natural hardness of the rock could explain the lower velocity.
James A. Harrell, Letter to the Editor, KMT: A Modern Journal of Ancient Egypt, Vol. 5, No. 3 (Fall 1994), pp. 3-4.
In a follow-up letter, Harrell noted that the greater recession of the upper strata on the enclosure walls was the result of their having originally been cut at a slope (or "batter") of about 80 degrees, as can be seen from the relatively less eroded eastern half of the southern wall. Furthermore, only if the limestone beds were all of the same durability would preferential weathering of the upper strata clearly suggest rainfall as the cause. Instead, as they slope back, what we see are harder strata overhanging weaker ones. Nile flooding could have taken the form of slackwater, wetting the enclosure sand without undercutting the rock at the base of the enclosure. Harrell attributed the fissures in the rock to earthquakes and subsurface groundwater movements going back millions of years.
Turning to Schoch's comparison of the Sphinx rock with the rock face at the Tomb of Debehen, Harrell questioned whether the two were really the same rock, given the distance and higher elevation of Debehen's Tomb. He also pointed to the quarry walls and tombs to the west and southwest, particularly the Old Kingdom Tomb of Queen Khentkawes. Although higher in elevation than the rock of the Sphinx and therefore probably composed of more durable stone, the rock in these places showed (albeit less deeply) the same horizontal rolls as the main body of the Sphinx (Fig. 4.2 below).
Fig. 4.2. The tomb of Queen Khentkawes. The base shows horizontal rolls similar to, although not as severe as, those on the Sphinx. Courtesy and copyright of John Bodsworth
Robert M. Schoch, Letter to the Editor, KMT: A Modern Journal of Ancient Egypt, Vol. 5, No. 3 (Fall 1994), pp. 4-5.
In a follow-up reply, Schoch pointed out that if wet sand caused by rain or flooding was responsible for the erosion of the Sphinx enclosure, the same strata on the southern wall should display the same degree of erosion all along their length. Instead, the erosion appeared to intensify as it moved west. Schoch also noted that fractures in the rock produced over millions of years were not the same as fissures, which are fractures widened as a result of weathering. Many of the fissures on the Sphinx and enclosure walls were wider at the top than at the bottom and suggested the action of rainwater.
Schoch acknowledged the presence of the less intense but clear horizontal banding at the base of the Khentkawes tomb. He observed that tombs closer to the Sphinx, and more likely composed of the same rock as the statue, had not eroded in this way. Schoch suggested that the Khentkawes tomb might have been built on an earlier rock exposure.
K. Lal Gauri, John J. Sinai, and Jayanta K. Bandyopadhyay, "Geologic Weathering and Its Implications on the Age of the Sphinx," Geoarchaeology, Vol. 10, No. 2 (April 1995), pp. 119-133.
In his 1995 paper, a response to Schoch, Gauri did not address weathering beneath wet sand. He and his colleagues argued that salt exfoliation induced by atmospheric condensation was sufficient to explain the eroded condition of the monument and its enclosure within the last 4500 years.
In addition to the differences in porosity that explained why some strata on the Sphinx and its enclosure walls had receded more than others, Gauri noted that strata can differ in how they grade into each other. In strata with a smoother gradient, he observed, horizontal rounding occurs, while in rock that grades sharply between hard and soft strata, the jaggedness that Schoch attributed to wind appears. Gauri argued that the protruding strata of the Sphinx and its walls were horizontally rounded because of a smoother gradient.
Gauri conceded that rainfall could have widened fractures that already existed in the rock. But he argued that sharp corners tended to wear away faster as a result of salt exfoliation, producing the spheroidal weathering pattern that Schoch attributed to rainfall. Gauri also argued that fissures could have formed and widened as a result of fluctuations in the underground water table over millions of years.
Gauri did not dispute Schoch's subsurface findings and Schoch did not respond to Gauri's 1995 article in Geoarchaeology. However, archaeologists invoked Gauri's explanation of how the Sphinx had eroded (see next page), and to these West and Schoch did respond.
David Coxill, "The Riddle of the Sphinx," Inscription: Journal of Ancient Egypt, No. 2 (Spring 1998), pp. 13-19.
Schoch's case for an earlier Sphinx prompted David Coxill, an English local government geologist, to visit the monument. He examined the surface erosion and observed fissures that did not displace strata in the rock, as might be expected if the original cracks were the result of tectonism (earthquakes). From this and other observations, Coxill concluded that the Sphinx had a prehistoric date, although he declined to evaluate Schoch's subsurface results.
A Note on Peer Review
To present their preliminary results at the October 1991 meeting of the Geological Society of America, West and Schoch submitted a proposal for a poster presentation that met the approval of a review panel on archaeological geology. The GSA did not formally endorse the argument for an earlier Sphinx. But those geologists who visited the West-Schoch poster presentation expressed an interest in their findings and many asked to be notified of further research. Schoch gave a brief update of the case for an earlier Sphinx before an audience of about 300 at the GSA meeting in 2000. At the end of his talk, two members of the audience rose to question the findings and express skepticism. The remainder of the audience listened with interest and did not approve or dissent.
Schoch's 1992 paper appeared in Geoarchaeology, a peer-reviewed journal, and Gauri and his colleagues published their response in the same journal. Colin Reader published his case for a less old Sphinx in a peer-reviewed journal, Archaeometry, in 2001 (see the page after next). Several scholars commented in the same issue. Reader and the Belgian observer Gerd Vandecruys exchanged views in the peer-reviewed online journal, PalArch, in 2006. It does not appear that any other contributions on either side of the Sphinx controversy have been published in peer-reviewed academic journals.