The limestone on the Causeway represents a more resistive layer within Member II. However, the fractures remained open and allowed the rain water to find its way through this layer long before the carving of the enclosure walls.
In several places under the harder Causeway layer we find both large and small cavities inside Member II where total dissolution of the limestone has taken place below vertical fractures. This is the evidence of the important role the vertical fractures played in the dissolution process of the cement.
The Great Sphinx and carbonic acid erosion
We know that there was a lot more rain prior to carving the Sphinx than after, hence most of the damage to the formation along the fractures must have taken place before the Sphinx and its enclosure were created. The rock matrix has experienced variable degrees of dissolution of the cementation between the limestone grains, which resulted in later erosion along the fractures when they were exposed after the excavation of the enclosure. In addition the alteration of softer and harder layers in Member II has resulted in variable degree of horizontal erosion, all of which are nicely exposed in many photos of the Sphinx.
From a geological point of view Jørn Christiansen did not find any evidence that could date the carving of the Sphinx to a time earlier than any of the other monuments on the Giza plateau. It has been demonstrated how the Member II has gone through a chemical weathering process that generally follows the network of the fractures of the formation. Without being specific this alteration process has taken a long time, geologically speaking – long before human activity on the Giza Plateau began.
Then, at some point in time the Sphinx and its enclosure was excavated and before long the wind, sand and occasional rain made the weaknesses on the edifice visible. Erosion took place horizontally along poorly cemented sub-units of Member II, and vertically where carbonic acid had been allowed to work along the fractures over geologic time. The latter is not unlike what would be expected from water erosion, but the alteration pattern and cavities in the rock proves that the weakness along the fractures of the formation existed prior to the carving of the Sphinx and its enclosure.
Likewise, the Valley and the Sphinx Temples, which were constructed with stones quarried from the Sphinx enclosure, were soon in ruins as a result of erosion and weathering. The Valley Temple was then ‘dressed’ in carefully custom-cut granite repair blocks from Aswan; incomprehensible precision work and a masterpiece of masonry, like so many other monuments in Egypt.
Considering the results of this analysis and interpretation of the rocks of the Sphinx monument and its surroundings, it is concluded that the amount of erosion observed and its expression cannot be used in estimating the age of the Sphinx. Superficial geological methods and probably also detailed petrophysical analysis (physical and chemical analysis of rock properties and their interactions with fluids) are unsuited for dating the Sphinx with any precision since acid water had penetrated the formation and caused weathering long before its creation. The challenges to isolate and quantify the effect of the various parameters at work makes it simply too difficult to estimate the age.
However, what the rocks do tell us is that the southern enclosure wall, when carved, was perfectly aligned along the straight west-north-west to east-south-east running Causeway leading from the Valley Temple to the Khafra Pyramid, pointing 14° south of the eastwards looking Sphinx itself. That angle is no coincidence; it is the direction of the rising Sun on the 22 October, the day of the Hep Set Festival. So the Sphinx was carved as an integral part of a larger construction master plan on the Giza Plateau.