November 22, 2004
Yardangs, in spite of their evocative name, are geologic landforms that are of special interest to geologists working on the surface features of Mars. These long ridges are only found in places that have very little rain, a source of sand, and strong prevailing winds. They were first named for their occurrence in the Middle East. However, on occasion, they're observed in North America.
The yardang above is at the down-wind end of Edwards Dry Lake in the Mojave Desert of California, where the space shuttle occasionally lands. Granite outcrops along the far end of the lake are swept by winds into this area of sandstone. The erosional process has shaped these hard rocks into parallel ridges, some of which are shaped like an inverted boat hull, sometimes with a keel. They're as large as 8 m tall and 50 m long. Much larger yardangs occur in dry areas of Turkey, Iran and Peru in rocks including limestone and granite. Many have flat tops. The Sphinx is probably a limestone yardang that was long ago carved into its reclining animal shape by humans.
While Mars shows clear evidence of ancient river erosion and deposition, the Martian surface today is dry, and long, wind-formed yardangs cover considerable areas of the Red Planet. When the Martian water dried up, where it went, and if it can be found under the planet's surface are questions of special interest to geologists studying Mars. [Revised November 2017]