Millions of years have passed since the sediments on top of Member III were deposited and later uplifted (probably starting in Oligocene/Miocene) and exposed to weathering and erosion by wind, sand and water.
This history is largely unknown. The landscape we see today is a result of natural processes and extensive human activity from at least around 3,000 BC. One notable observation is that the sediments once deposited flat in a marine environment are now tilted gently to the south-east. Human activity has not only been limited to sculpturing the Sphinx and its enclosure.
From the plateau of Giza was also moved a considerable amount of rocks, in the course of landscape modeling and extraction of building materials and blocks for building another monument near the Sphinx. In addition, this area experienced significant influx of sand, as it gradually changed from fertile land to desert after the last ice age. Occasionally the whole Sphinx enclosure was filled with sand in periods when people were not taking care of these magnificent monuments.
Greath Sphinx - Analysis of Member II
Several factors contributed to the current poor rock quality of Member II, but they all have their origin in the composition of the fine grained muddy limestone matrix that hampered the initial lithification process.
Greath Sphinx - fractures
The load of overburden on the water saturated mudstone and the resulting pressure and stress caused the Member II limestone to fail, forming a network of fractures. Visible chemical weathering of the rock around the fractures tells us that water has penetrated into the formation, as can be seen on the walls of the enclosure, and on the Causeway, where a network of fractures are visible on the surface, uninfluenced by the carving of the enclosure wall.
Greath Sphinx - limestone pavement
The Causeway immediately to the south of the Sphinx has a hard nodular crust developed on the top layer of the otherwise very fragile Member II. This nodular crust is a result of chemical weathering, when softer parts of the limestone were dissolved by acid rain through a mantle of topsoil. This surface shows remarkably few signs of wear from the millions of tourists who have walked on the Causeway to admire the Sphinx.
Greath Sphinx - water weathering
The fractures through Member II allowed acid rain water to penetrate the formation. Limestone is particularly susceptible to deterioration, principally through the effects of chemical dissolution. Even unpolluted rain contains carbon dioxide, creating a weak carbonic acid which is able to dissolve calcite, the main mineral component of the limestone. Jørn Christiansen found two pieces of evidence that shows that this chemical weathering of Member II started long before the carving of the Sphinx and its enclosure.
On the southern enclosure wall there are numerous exposures of colored, gently curved stripes, often several parallel similar to tree rings, crossing the stratigraphy. Some of these are slightly elevated compared to the surrounding, indicating a harder composition. The stripes are following the fractures and they are the visible evidence of a carbonic acid invasion zone resulting from rain water finding its way into the fractures from above. By the shape of the striping pattern Jørn Christiansen determined that they existed prior to the carving of the wall and is then the evidence that the chemical alteration took place before the excavation of the Sphinx and its enclosure.
Source: GEO Expro