THE SPHINX

What determines the microporosity of a particular stone? It is a function of the original constitution of the rock as formed during deposition, diagenetic changes that modify primary textures and, finally, leaching of the rock matrix. The limestones composing the core body of the Sphinx are not uniform, as Gauri and colleagues have pointed out. These authors classify the lower half of Member II (their beds I through 3) as a sparse biomicrite and the upper half of Member II (their beds 4 through 7) as a packed biomicrite. In general, packed biomicrites might be expected to have a larger volume of large-pore space and, therefore, be characterized by higher durability factors than sparse biomicrite. Even taking this into account, Gauri et al's data show a consistent trend of increasing durability factors toward the top of the section within the packed biomicrites (beds 4-7).


          How might we account for the trend noted for the packed biomicrites? I would suggest that this trend is compatible with the hypothesis that the stone was subjected to leaching of the matrix-which opened the pores and increased durability-due to precipitation. As rain fell on the back of the Sphinx (or, at least. on the stone that was to become the Sphinx), and on the Giza Plateau in general, it would soak into and leach the rock from the top down giving rise to the pore-volume distribution seen in these rocks today.


          It is interesting to note that on the wall of the Sphinx ditch the beds for which Gauri and colleagues calculated the highest durability f actors are not consistently the least weathered and receded in profile (assuming that the wall of the Sphinx ditch was originally cut vertically or nearly vertically). For instance, utilizing Gauri's own data (Gaud, 1984, 32, fig. 3C), in an east-west profile of the rear of the Sphinx and the wall of the Sphinx ditch one sees that beds 1i and 2i-which both have low durability factors of 11-are greatly receded and undercut the overlying units of higher durability (beds 1ii and 2ii). However, in the same section, bed 2ii (with a durability factor of 76) is receded further back than is bed 1 ii (durability factor of 56). Likewise, bed 3ii (durability factor of 76) is receded back further than bed 3i (durability factor of 42), and beds 4i and 4ii (durability factors of 75 and 86 respectively) are receded further back than bed 3ii. In general, the amount that a bed has receded is not so much a function of its present-day durability factor, but primarily a function of its geometric position on the exposure. It would be logical that precipitation failing down from above would preferentially weather the uppermost beds and cause them to recede back at a faster rate than the lower beds. Again, this train of thought suggests that the Sphinx and walls of the Sphinx enclosure (or ditch) were subjected to precipitation-induced weathering.