Under Cairo’s asphalt, under the reverberations of its street traffic, rocks are flowing, compacting, shearing and warping.

The city is built on millions-of-years-old fossil limestone. You have to travel to the outskirts, though, to get a sense of the Cairo’s terra firma — at Moqattam, where the limestone outcrops from the necropolis; on Ain Sokhna Road, where the cuttings hold petrified tree trunks; or at Wadi Degla, a natural valley on the outskirts of Maadi.

One week ago, Egypt Independent joined American oil geologist Bill Bosworth and Egyptologist Ahmed Seddik on a tour of what Seddik called “Egypt’s Grand Canyon,” Wadi Degla.

The story of Cairo’s geology is less well-known than that of most capital cities. The first geologic map was completed in 1983. Since then, the science has improved, but the city has spread outwards, covering the secrets of its underlying rock from prying hammers and eyes.