A mound of clay in a flow tunnel is eroded by water.
Using fluorescent dyes in a water flow tunnel, the researchers were able to watch a Sphinx-like rock formation known as a yardang emerge from a mound of clay. Credit: NYU Applied Mathematics Lab
AGU Fall Meeting 2022: Science Leads the Future

More than 4,000 years ago, sculptors carved the visage of a massive, fantastical creature into a mound of limestone bedrock not far from the west bank of the Nile in Lower Egypt. What led the ancient Egyptians to create the half-man, half-feline Great Sphinx of Giza has long been a mystery. A recent geomorphological study indicates they may well have been inspired by the landscape around them.

On the basis of lab experiments and computer modeling, a team of researchers at New York University says that a kind of natural formation called a yardang may have served as the inspiration for the Great Sphinx. Yardangs, sometimes called mud lions, are mounds of rock eroded into distinctive shapes by wind and sand. Yardangs are fairly familiar features in the deserts of western Egypt, as well as places from Chile to China to Iran.

Yardangs are typically formed when windblown sand erodes soft sediments, leaving behind a harder layer of rock underneath. In some cases, this process leaves long embankments, like overturned boat hulls scattered across the ground, and in others it can sculpt fantastically shaped towers. These natural formations can sometimes resemble the Sphinx, with protruding heads, undercut necks, and short “paws” extending in front of the body.

“That could have put in the minds of these ancient Egyptians that there are these great, mythical animals that are sitting out there in the desert,” said Leif Ristroph, an experimental physicist and applied mathematician at New York University’s Courant Institute of Mathematical Sciences.

Some researchers have suggested the Sphinx may have started its life as a yardang, though definitive evidence has never turned up. But recent experiments in Ristroph’s lab show the theory is certainly plausible, at the very least.

Nineteenth century photo of the Great Sphinx of Giza, partly buried in the sand
Many researchers think the Great Sphinx of Giza, photographed here in the late 19th century, was carved from a yardang. Credit: National Science and Media Museum

Persistent Mysteries and Simple Structures

The team presented its yardang research at AGU’s Fall Meeting 2022, and their work addressing Sphinx theory has been accepted by Physical Review Fluids.

To assess the Sphinx theory, Ristroph and his colleagues placed a small mound of clay in a water flume in their lab. They embedded a plastic cylinder inside the clay to mimic the effects of harder rock layers. Then, they ran water over the clay while using a 3D optical scanner to monitor its shape in real time.

After a few hours in the flow chamber, the researchers ended up with a distinctly Sphinx-like shape, with a large bulge resembling a head and a smaller paw-like shape at the base, Ristroph said. The scientists scanned the formation again to create a digital reconstruction that could be fed into sophisticated models of fluid dynamics and geomorphological change.

“There is a very natural recipe for how nature can carve something that looks similar to this.”

Among the team’s discoveries is the importance of a yardang’s “head” in determining its overall structure. A yardang’s head forms at its windward end, where air currents undercut the bottom of the structure and leave a top that bulges out slightly. The head creates what Ristroph calls a flow-funneling effect that acts to accelerate and guide winds, creating what can appear to be a neck and forelimbs.

“All of that ‘anatomy’ is coming from the interesting ways that the [air] flows develop from the shape,” Ristroph said. “What you get in the end is something that looks a lot like a seated lion.”

“There is a very natural recipe for how nature can carve something that looks similar to this,” he said.

Whether the Great Sphinx started out as a yardang and the Egyptians enhanced what they saw, or whether they were inspired by other yardangs in the surrounding desert is unclear. It’s also possible that yardangs weren’t involved in the creation of the Great Sphinx at all, and the resemblance is a coincidence. We’ll likely never know for sure.

A Model for Mud Lions

Just as the origins of the Great Sphinx present a mystery to archaeologists and historians, the formation of yardangs has never been fully understood by geomorphologists.

“One of the biggest questions about yardangs is actually how they form,” said Elena Favaro, a planetary geomorphologist at the Open University in the United Kingdom who wasn’t affiliated with the new research.

“We need the mathematical backing of how these things form because our field-based observations can’t really tell us what we’re after,” Favaro said.

That mathematical model is beginning to emerge from Ristroph and his colleagues’ work, which examines what mathematicians call a moving boundary problem, in which the shape of an object alters the forces acting on it and those forces in turn alter the object. The model could be useful for a range of real-world applications, Ristroph said, including for simulating how glaciers melt underwater.

A more thorough understanding of the dynamics that create yardangs could also pay dividends beyond Earth’s surface. Fields of yardangs on Mars appeared in images from the Mariner mission in 1972, and there are indications that yardangs could also exist on Venus and Saturn’s moon Titan. A better understanding of yardangs could reveal how forces such as wind and water helped shape distant worlds over billions of years.

Satellite image of windblown rock formations called yardangs on Mars
Scientists were surprised to see yardang fields like this on Mars. Credit: NASA/JPL-Caltech/UArizona

“The form we see on the landscape is just a snapshot of all the processes that have gone into making it look the way it looks,” said Favaro.

—Nathaniel Scharping (@nathanielscharp), Science Writer

Citation: Scharping, N. (2023), Did these curious rock formations inspire the Great Sphinx?, Eos, 104, https://doi.org/10.1029/2023EO230424. Published on 3 November 2023.
Text © 2023. The authors. CC BY-NC-ND 3.0
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