The action of wind or flowing water may be enough to carve majestic lions in repose out of rocks or clay 鈥� and it鈥檚 possible that this natural process gave a rocky outcrop in Egypt a lion-like shape that was further modified by the ancient Egyptians to create the Great Sphinx of Giza.
Writing in Smithsonian Magazine in 1981, geologist Farouk El-Baz conjectured that 鈥溾��. He suggested the ancient Egyptians took advantage of a naturally occurring landform to create the Great Sphinx.
Now, at New York University and his colleagues have produced evidence that may favour the idea. They saw miniature versions of lion-like landforms emerge from lumps of clay immersed in flowing water. Their tiny, water-carved 鈥渕ud lions鈥� point towards a mechanism by which wind could sculpt similar shapes out of desert rocks.
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The researchers set out to study how clay erodes when water runs over it. They fashioned oval-shaped mounds out of bentonite clay, burying a small chunk of hard, non-erodible plastic at one end of each one. They then placed each lump in a water tunnel and allowed the liquid to flow over it parallel to its long axis, with the buried plastic at the 鈥渦pstream鈥� end of the tunnel. Video recordings showed a lion-like shape appear as the water carried away more and more of the soft, erodible clay while leaving the plastic cylinder behind.
鈥淲e were struck by the resemblance to a seated lion or a lion in repose. And I was reading about this and came across the idea that perhaps even the Great Sphinx was one of these natural formations that then got, obviously, a little facelift from the ancient Egyptians,鈥� says Ristroph.
He says that across several experiments, the cylinder provided a template for the 鈥渉ead鈥� of the small mud lion. The flow of water around it carved its curved back, thin neck and broad paws 鈥渇or free鈥�.
鈥淭he fluid is eating away the solid, but the solid then forces the flow to conform to its shape. It feeds back on the flow and changes erosion rates and how it is distributed over the surface,鈥� he says. This 鈥渢wo-way conversation鈥� between the flow and the shape that it is carving also makes simulating the process on a computer or modelling it with mathematical equations challenging 鈥� and explains why practical experiments are very informative, he says.
The microscopic mechanisms by which wind erodes rocks are rather different from how water erodes clay. But at Brigham Young University in Utah says the type of geological phenomenon that the researchers saw in their lab is called a These formations exist across the world鈥檚 deserts. She believes that, in the desert, yardangs could arise from fairly even rocks if there is enough variation in the flow of wind.
While Ristroph and his colleagues are now working on developing a mathematical description of this process and understanding how it might scale from clumps of clay to Sphinx-sized rocks, Radebaugh has her sights on finding reclined lion shapes far away from the lab. She says that the lack of water on the surface of Mars seems to have allowed erosion to create many Sphinx-like yardangs there too.
Reference: Physical Review Fluids,
