A yardang is a wind-abraded ridge found in a desert environment. Yardangs are elongate features typically three or more times longer than they are wide, and when viewed from above, resemble the hull of a boat. Facing the wind is a steep, blunt face that gradually gets lower and narrower toward the lee end.
The word itself is of Turkic
origin, meaning ‘steep bank’, and was first introduced to the English-speaking world by the Swedish explorer Sven Anders Hedin
A yardang is formed in cohesive material. Hedin
first found the wind-sculptured "clay terraces" or yardangs in the dried up riverbed
of the Kurruk-daria in Central Asia
. However, yardangs can be found in most deserts
across the globe. Depending upon the winds and the composition of the weakly indurated deposits of silt and sand from which they are carved, yardangs may form very unusual shapes — some resemble various objects or even people.
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, meso-yardangs are generally a few meters high and 10 to 15 meters long, and micro-yardangs are only a few centimeters high.
A large concentration of mega-yardangs are found near the Tibesti Mountains in the central Sahara. There is a famous yardang at "Hole in the Rock" in Papago Park in Phoenix, Arizona, a rock formation with a roughly circular hole in it. Another yardang in Arizona is Window Rock, near the town of Window Rock. It is a 60-meter sandstone hill with a very large circular hole in the middle of it. Some geologists believe that Great Sphinx of Egypt is an augmented yardang. Pictures from Mars show that the yardang ridges occur on a massive scale there, giving visual support to the theory that Mars has once had groundwater.
Yardangs form in environments where water is scarce and the prevailing winds are strong, unidirectional and carry an abrasive sediment load. The wind cuts down low lying areas into parallel ridges which gradually erode into separate hills that take on the unique shape of a yardang. This process yields a field of yardangs of roughly the same size, commonly referred to as a fleet due to their resemblance to the bottoms of ships. Alternatively, one can be formed by the migration of a dune
that leaves behind a cemented core. As the process of formation continues, typically a trough will form around the base of the yardang.
They are more commonly created from softer rock types like siltstone, sandstone, shale and limestone, but have also been observed in crystalline rocks such as schist and gneiss.