Yardangs

Yardangs are perhaps, the most neglected and misunderstood of the Earth’s landforms. The existence of large fields of yardangs ranging from meters to kilometers in length is not well known to American geologists. In the United States many early workers overstated the role of the wind in landscape development, leaving a legacy of uncertainty about the effectiveness of the wind as an agent of surface sculpture.

Yardangs were first described along the eastern edge of the Takla Makin Desert by the explorer Sven Hedin in 1903. The term is considered applicable to all positive, streamlined, aerodynamically shaped hills regardless of size and type of bedrock in which they form (Yardang is the ablative form of the Turkestani word yar which means ridge or steep bank – the ablative in this case expresses the sense of removal – an appropriate connotation for wind erosion features). Wind sculptured hills have been described, often under different names, in the desert regions of every continent except Australia. They are restricted to those parts of deserts that are extremely arid where plant cover and soil development are minimal and strong unidirectional winds occur throughout much of the year. Major localities are known in Iran, Egypt, Arabia, Chad, Namibia and Peru. Scanty data suggests that they also occur in Afghanistan, Libya, and Mauritania. Minor localities have been described at Rogers and Coyote Lakes, California.

The somewhat imperfect yardangs of the Talara region of northern Peru have been known for more than fifty years.234 This region is subject to torrential heavy rains about every 20 years so that these windforms bear the imprint of episodic running water as do many of the African and Iranian examples. The coastal desert of central and southern Peru is essentially rainless (less than 10 mm of precipitation reported at Pisco per year and this is in the form of condensate from the coastal fog). In the Ica Valley region of south central Peru there are thousands of small to large totally ungullied hills oriented parallel to one another. They occur in clusters with individual hills showing a high degree of streamlining in both plan and profile. These wind erosion forms occur in the upper Tertiary Pisco Formation which consists of white to yellow sandstones, siltstones, bentonites and thin layers of conglomerate. In other deserts yardangs are known to occur in Paleozoic and Mesozoic sediments and also even in crystalline rocks such as gneisses and granites.

There is much confusion in the existing literature on the origin of yardangs. Although abrasion and deflation have long been considered integral parts of the wind erosion regime, many authors have stated that abrasion alone is responsible for yardangs. New field and experimental modeling data suggests that deflation is far more important in yardang development than previously realized.