Unwrapping an enigma to find the riddle inside, geologists studying the Great Sphinx of Egypt have found patterns of weathering and erosion that they say show that the imposing monument was created thousands of years earlier than is generally thought.
Dr. Robert M. Schoch, a Boston University geologist who directed the research, reported Wednesday that an ancient civilization carved the Sphinx between 5000 B.C. and 7000 B.C., long before the dynasties of pharaohs. Archaeologists have long contended that it was built by Pharaoh Khafre about 2500 B.C.
Dr. Schoch suggested that Khafre had merely restored the Sphinx, a mythological creature with the body of a lion and a human head, and perhaps made some alterations. The monument, 66 feet high and 240 feet long, stands at Giza with the Great Pyramids.
The research findings were announced at the annual meeting of the Geological Society of America in San Diego. They immediately drew fire from Dr. Mark Lehner, an Egyptologist at the University of Chicago who is a leading expert on the Sphinx.
The Associated Press quoted Dr. Lehner as saying that there was "overwhelming evidence," including samples of rock from the same quarry used for the Sphinx and other monuments at the site, to support Khafre's role in the construction.
In a telephone interview, Dr. Schoch defended his research methods, which involved the use of sound waves to probe subsurface rock and identify the depth and the distinctive pattern of weathering attributed to water.
He worked with Dr. Thomas L. Dobecki, a geophysicist at McBride-Ratcliff & Associates, a seismic surveying concern in Houston, and John Anthony West, an independent Egyptologist who had developed the theory that the Sphinx was much older than archaeologists had said.
If their findings are substantiated by other research, archaeologists may have to revise their interpretations of the Middle East before the rise of Egyptian civilization in about 3000 B.C.
Little is known of these Neolithic cultures, but if some were capable of engineering projects on the scale of the Sphinx, then they could no longer be viewed as simple hunters and gatherers.
The massive stone wall and tower of Jericho, dating to the ninth millennium B.C., are among the few artifacts in the region that tend to support the notion that some of these early cultures might have been capable of conceiving and executing a construction project on the scale of the Sphinx.
Dr. Schoch, an associate professor of science, said that subsurface limestone at the front and sides of the Sphinx showed structural signs of water weathering as deep as 6 to 8 feet. Limestone at the back of the Sphinx, carved from the same bedrock, shows weather only 4 feet deep.
"The dramatic weathering we found on the body of the Sphinx is not seen on other structures in the immediate vicinity," Dr. Schoch said, "even though many of them appear to have been cut or built from very similar or identical limestones and are supposed to have been built during the same period."
Dr. Schoch said this suggested that at first the front of the body and head of the Sphinx were carved free from the surrounding bedrock. The rear of the creature remained merged with the surrounding rock.
It is a "reasonable hypothesis," Dr. Schoch said, that Khafre repaired and refurbished the Sphinx and the two nearby temples, the Sphinx Temple and the Valley Temple, and also had the back, or western end, of the Sphinx carved out and freed from the cliff.
Since the monument was built by excavating surrounding limestone, exposing the core out of which the Sphinx was carved, the rock floor has presumably been exposed directly to weathering since the construction began.
The patterns, Dr. Schoch said, had all the marks of being "precipitation induced" weathering.
And since the effects were detectable at such great depths, he said, it indicated that work on the Sphinx had begun in the period between 10,000 B.C. and 5,000 B.C., when the Egyptian climate was wetter.