loess (lĕs, lōˈəs, Ger. lös) [key], unstratified soil deposit of varying thickness, usually yellowish and composed of fine-grained angular mineral particles mixed with clay. It is found in many regions of the world and is probably related to the chernozem soils of Russia; extensive deposits occur along the Mississippi River and its tributaries, on the Columbia Plateau in Oregon, Washington, and Idaho, and in China. Loess is an erosional product carried by the wind from adjacent deserts, from frost-pulverized outwash of glaciers (during the Pleistocene epoch), or from the floodplains of glacier-fed streams. Studies of particles transported by wind from plains recently denuded by tillage show that the material is sorted to about the same degree as loess. Much of the loess in the United States and Europe are of glacial origin; in China, of desert origin and may reach up to 300 feet (90 meters) thick. Loess is usually deep, fertile soil, rich in organic remains (especially the shells of snails) and characterized by slender, vertical tubes that are said to represent stems and roots of plants buried by sediment. When cut by streams or other agencies, loess remains standing in cliffs exhibiting a vertical, columnar structure; this is attributed to the vertical tubes and to the angularity of the grains and their consequent tendency to interlock. The uncompacted character of loess makes it subject to rapid erosion.