WEATHERING AND THE AGE OF THE SPHINX

The 4,500 year old Great Sphinx in Egypt exhibits a degree of weathering not seen elsewhere on the Giza Plateau. Schoch and West¹ suggested that weathering is principally the product of precipitation. Yet annual precipitation has been estimated to be about 2.5 cm over the past 5,000 years, far too low to explain the degree of weathering. Based on higher rates of precipitation prior to 5,000 years ago Schoch and West concluded that the Sphinx was constructed by an unknown civilization perhaps as long as 11,000 years ago.

The assumption that weathering is principally the result of precipitation may not be correct. Additional sources of weathering have been identified including: dew, groundwater, and even the Nile River. Unlike the pyramids the Sphinx is a statue that was excavated out of the limestone bedrock. The 13 meter excavation significantly decreased the depth to the water table putting the Sphinx within the upper limits of the annual Nile flood waters. Even if the floods did not reach the excavation they still would have replenished groundwater. In addition the Sphinx spent most of the past 4,500 years buried up to its neck in sand.

SESOIL vadose zone computer modeling was performed to evaluate groundwater recharge for various scenarios. Model results indicate that in carving the Sphinx the floor of the excavation encountered the capillary rise zone. This turned on weathering as shallow groundwater wicked up and evaporated at the surface. The wicking process was turned off whenever the excavation filled with sand. Only to be turned on again once the sand was removed. Rather than the sole product of precipitation weathering appears to be the result of a sporadic interaction of various processes; thus it cannot to be used to estimate the age of the Sphinx.

Today the sand is gone, so weathering is turned on. Dewatering is being used to lower the water table and turn off the weathering process. Modeling was used to determine how far dewatering needs to lower the water table to turn off weathering.

1. R. M. Schoch and J. A. West, 1991, Redating the Great Sphinx of Giza, Egypt. Geological Society of America Annual Meeting, San Diego, October 1991, v. 23, no. 5, p. A253