The steep hillsides south of Lanzhou are composed of loess, which where not terraced and planted, is a vivid yellow-brown in the morning sunlight. Loess is a wind-blown sediment. In 1877, the geologist Franz von Richthofen concluded that the yellow soils of central China were very similar to the Rhine Loesch. Loess covers about 440,000 square kilometers in China.
All loess is eolian, transported by the wind.
Dust storms in northern China are still common and impressive, reminiscent of the American Dust Bowl of the 1930s. However, the origin of the Chinese loess is different from other parts of the world. The loess of North America and Europe is derived from glacial material. As the Pleistocene glaciers retreated, the sparsely vegetated plains left behind were a ready source of wind-blown dust. Most of the Chinese loess originated in the deserts and gobis of Central Asia northwest of the loess plateau, as indicated by decreasing average grain sizes toward the southeast.