Loess, a deposit of earthy material consisting mainly of silt (particles intermediate in size between sand and clay). Loess is light, porous, and uniform in character from top to bottom. Although it is easily worked, the particles cling together so tightly that loess forms steep cliffs when eroded. When sufficiently watered, loess is extremely fertile.

Loess covers a vast area in the central United States, chiefly in Iowa, Illinois, Kansas, Nebraska, Missouri, Tennessee, and Mississippi, and is found also in Washington and Idaho. There are great deposits also in central Europe. Loess darkened by organic matter forms the black earth lands of Russia. The largest of all loess deposits occur in northern China.

In the United States loess deposits range up to more than 100 feet (30 m) in thickness. In China they are more than 300 feet (90 m) thick in spots. The Chinese loess colors the Yellow River and the Yellow Sea. The loess of China was carried by winds from the deserts of Mongolia. In the United States and Europe, loess deposits were transported by the wind from material piled up at the edges of the ice sheets in the last great Ice Age and from materials deposited on the flood plains of large streams.