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The Sphinx water erosion hypothesis contends that the main type of weathering evident on the enclosure walls of the Great Sphinx could only have been caused by prolonged and extensive rainfall, and that it must therefore predate the time of King Khafra, who is held by the traditional view of modern Egyptology at large to have built the Great Sphinx in approximately 2500 BC along with the Second Pyramid at Giza.

The hypothesis is championed primarily by John Anthony West, an author and alternative Egyptologist, and Robert M. Schoch, a geologist and associate professor of natural science at the College of General Studies at Boston University.

This hypothesis is rejected by the vast majority of Egyptologists.

The water erosion hypothesis
R. A. Schwaller de Lubicz, a French mystic and amateur Egyptologist, first claimed evidence of water erosion on the walls of the Sphinx enclosure in the 1950s. John Anthony West, an author and alternative Egyptologist, investigated Schwaller de Lubicz's ideas further and, in 1989, sought the opinion of Robert M. Schoch, a geologist and associate professor of natural science at the College of General Studies at Boston University.

From his investigation of the enclosure's geology, Schoch concluded that the main type of weathering evident on the Sphinx enclosure walls could only have been caused by prolonged and extensive rainfall. According to Schoch, the area has experienced a mean annual rainfall of approximately one inch (2.5 cm) since the Old Kingdom (c. 2686–2134 BC), such that, since Egypt's last period of significant rainfall ended between the late fourth and early 3rd millennium BC, the Sphinx's construction must date to the 6th millennium BC or 5th millennium BC.

Colin Reader, a British geologist, agrees that the suggested evidence of weathering indicates prolonged water erosion. Reader found, inter alia, that the flow of rainwater causing the weathering had been stemmed by the construction of 'Khufu's quarries', which lie directly "upstream" of the Sphinx enclosure, and therefore concludes that the Sphinx must predate the reign of Khufu (2589–2566 BC), and certainly Khafra, by several hundred years. Reader however disagrees with Schoch's palaeometeorological estimates, and instead concludes that the Sphinx dates to the Early Dynastic Period (c. 3150–2686 BC).

Similarly, David Coxill, a geologist working independently of both Schoch and Reader, has concluded from the evidence of weathering in the enclosure that "[t]he Sphinx is at least 5,000 years old and pre-dates dynastic times [before 3100 BC]."

Response of Egyptologists
This hypothesis is rejected by the vast majority of Egyptologists.

Dr Zahi Hawass, former Egyptian minister of state for antiquties affairs and secretary-general of the Supreme Council of Antiquities, was asked in an interview with NOVA if it was possible that a more ancient civilization might have sculpted the Sphinx. Hawass replied: “Of course it is not possible for one reason …. No single artefact, no single inscription, or pottery, or anything has been found until now, in any place to predate the Egyptian civilization more than 5,000 years ago.” This reasoning and conclusion was supported in a similar NOVA interview by Dr Mark Lehner, another senior Egyptologist.

Kenneth Feder, professor of archaeology at Central Connecticut State University, has criticised Schoch on the grounds that he is unable to produce archaeological evidence to support his geological theories, particularly on the basis that "only a culture with a pattern of social stratification and the capability to enlist the labor of a large pool of workers would have been capable of building the Great Sphinx, and for the period predating 2500 BCE, there is no evidence at all of such a culture."

Another argument used by Egyptologists to ascribe the Sphinx to Khafra is the “context” theory, which notes that the Sphinx is located in the context of the funerary complex surrounding the Second Pyramid, which is traditionally connected with Khafra. Apart from the Causeway, the Pyramid and the Sphinx, the complex also includes the Sphinx Temple and the Valley Temple, both of which display the same architectural style, with 200-tonne stone blocks quarried out of the Sphinx enclosure. A diorite statue of Khafre, which was discovered buried upside down along with other debris in the Valley Temple, is claimed as support for the Khafra theory. Reader agrees that the Sphinx Temple and Valley Temple are closely associated with the Sphinx, as is the Causeway and even part of the Khafra Mortuary Temple. However he states that this evidence merely indicates that these structures also predate Khafra, and that it does not link the Sphinx in any way to Khafra.

Zahi Hawass points to the poor quality of much of the Giza limestone as the basis for the significant erosion levels. He has concluded, from the present-day rapid rate of erosion on the Member II surface of the Sphinx, that "[t]he eleven hundred years between Khafre and the first major restoration in the Eighteenth Dynasty, or even half this time, would have been more than enough to erode the Member II into the deep recesses behind Phase I restoration masonry". However Schoch states that other structures and surfaces on the Giza Plateau are made from the same band of limestone as the Sphinx enclosure, but they do not evidence the same erosion as the walls of the Sphinx enclosure.

Peter Lacovara, an Egyptologist and curator at the M. C. Carlos Museum in Emory University, assigns "some of the erosional features" on the enclosure walls to quarrying activities rather than weathering, and states that other wear and tear on the Sphinx itself is due to groundwater percolation and wind erosion.

Response of other geologists
Various geologists have proposed alternative explanations for the evidence of weathering in the Sphinx enclosure.

One of the alternative erosion mechanisms proposed is called haloclasty. Moisture on limestone will dissolve salts, which are then carried by percolating moisture into the spaces inside the porous limestone. When the moisture dries the salt crystallises, and the expanding crystals cause a fine layer of surface limestone to flake off. It is accepted by Schoch et al that this mechanism is evident in many places on the Giza Plateau. One proponent of the haloclasty process is Dr James A. Harrell of the University of Toledo, who advocates that the deep erosion crevices were caused by the haloclasty process being driven by moisture in the sand that covered the carved rock for much of the time since it was exposed by quarrying. Lal Gauri et al, also favour the haloclasty process to explain the erosion features, but have theorised that the weathering was driven by moisture deriving from atmospheric precipitation such as dew.

Haloclasty is rejected as an explanation by Schoch et al because it doesn’t explain all the visible evidence, namely that the water erosion features are not evenly distributed, being concentrated in those areas that would have been particularly exposed to running water, whereas the haloclasty process should have operated evenly on all exposed limestone surfaces. Similarly, Schoch points out that the alternative explanations do not account for the absence of similar weathering patterns on other rock surfaces in the Giza pyramid complex which were cut from the same limestone beds.

Reader, who agrees that the Sphinx predates Khafra but prefers a construction date within the Early Dynastic Period, points to the tombs dug into the enclosure walls during Dynasty XXVI (c. 600 BC), and notes that the entrances of the tombs have weathered so lightly that original chisel marks are still clearly visible. He points out that if the weathering on the enclosure walls (up to a metre deep in places) had been created by any of the proposed alternative causes of erosion, the tomb entrances would have been weathered much more severely.

It is also agreed that wind erosion has played a significant role in eroding the Sphinx. Schoch however points out that wind erosion forms distinctive horizontal bands, whereas the water erosion features are clearly vertical.

Response of climatologists
Recent studies by German climatologists Rudolph Kuper and Stefan Kröpelin, of the University of Cologne, and geologist Judith Bunbury, of St Edmund's College, Cambridge, dispute Schoch's paleoclimatological conclusions. Kuper et al. suggest that the change from a wet to a much drier climate may have occurred later than is currently thought, and that Dynasty IV (the traditional era of the construction of the Sphinx) may still have been a period of significant rainfall; a conclusion also accepted by Mark Lehner. However, Schoch points out that fragile mudbrick structures nearby, indisputably dated to Dynasties I and II, have survived relatively undamaged, indicating that no heavy rainfall has occurred in the region since the Early Dynastic Period, and nor was any heavy rain anticipated by those Early Dynastic Period communities who built those structures.