Yardangs are enigmatic wind-parallel ridges found in arid environments. They often form downwind of rocks in sandy environments. They can be unusually stable structures, given their sandy / dusty composition, and the fact that the wind which forms them can also erode them.
“Yardangs come in a large range of sizes, and are divided into three different categories: mega-yardangs, meso-yardangs, and micro-yardangs. Mega-yardangs can be several kilometers long and hundreds of meters high and are found in arid regions with strong winds; meso-yardangs are generally a few meters high and 10 to 15 meters long and are commonly found carved in semiconsolidated playa sediments and other soft granular materials; and micro-yardangs are only a few centimeters high.”
Despite their prevalence, the physics behind their semi-permanence is unclear :
“No general theory exists to explain the long-term evolution of yardangs, curtailing modeling of landscape evolution and dynamics of suspended sediment release.”
“[…] major unanswered questions remain. What boundary conditions are responsible for yardangs? Can yardangs erode in perpetuity (or do they have a life span)? And are constant erosion (or sediment release) rates reasonable through yardang evolution?”
See: 'Yardang evolution from maturity to demise' in Geophysical Research Letters, 2015. [with illustrations]