Age of the Sphinx
A sketch of the Great Sphinx as it mighthave appeared in about 2,500 BC
Naturally geologists have long taken an interest in the Sphinx and the Giza Plateau as a whole, and the basic geology of the situation is well established. Dr. K.L. Gauri of the University of Louisville in Kentucky worked out the sequence of rock members in the Sphinx and its enclosure in the mid-1980s, alongside the archaeological investigations of Dr Mark Lehner and Dr Zahi Hawass.
Three rock members are represented at the monument. There is a bottom level of hard but brittle limestone, largely hidden under the lower cladding of the Sphinx body but partly seen in the enclosure walls and floor, that was formed many millions of years ago by sedimentation in shoal-water. Above that, the slightly younger middle member, which accounts for the bulk of the body of the Sphinx, is composed of seven separate beds which all have the interesting characteristic of passing from worse to better quality of rock from bottom to top, though the overall quality of the middle member is poor -- which is why the Sphinx gives a general impression of serious erosion and damage.
Joints in the limestone deposition, running criss-cross in the locality of the Sphinx, have in the past caused dislodgment of blocks of the body core and account for the large fissure at the rear of the monument. The top member of the Sphinx site turns into a rather pure and hard limestone above the neck, which makes for the finer preservation of the facial detail, give or take some wind erosion and the physical damage inflicted by the pious medieval sheikh. The stone of the head is not found elsewhere on the Giza Plateau today, probably because its outcrop was entirely quarried away by the necropolis builders with the exception of the knoll they wished to carve into the king's likeness.
The lower part of this top member is not such good rock, and the neck is badly eroded, very evidently by wind-blown sand. All the levels of rock within the Sphinx and its enclosure slope up from east to west and at the same time down from north to south.
The erosional circumstances of the Sphinx today were explored by the team of Egyptologists and geologists in the 1980s. The phenomenon of overnight condensation and absorption by capillary action was noted, with evaporation in the morning sun that leads to crystallization of salts within the rock's pores and spalling off of surface flakes as a result of the expansion of the crystals. It was also noted that condensation on the bedrock of the Sphinx and its enclosure could take place beneath a sand cover, leading to a situation in which the sand might he perfectly dry at the surface but wet through only a few centimeters beneath, while the rock itself could be soaked in water at some depth underneath the wet sand. This circumstance was judged to encourage the migration of salts from the depths of the bedrock towards the surface.
So both the geological structure and the erosional plight (at least in modern times) of the Sphinx were understood by geologists in the later 1980s. Egyptologists such as Dr Lekner concluded that erosional processes essentially similar to today's could account for the decay of the Sphinx between the end of the Old Kingdom and the early centuries of the New Kingdom, when they judged that the first major restoration of the Sphinx was undertaken.