Loess

loess, an unstratified, geologically recent deposit of silty or loamy material that is usually buff or yellowish brown in colour and is chiefly deposited by the wind. Loess is a sedimentary deposit composed largely of silt-size grains that are loosely cemented by calcium carbonate. It is usually homogeneous and highly porous and is traversed by vertical capillaries that permit the sediment to fracture and form vertical bluffs. The word loess, with connotations of origin by wind-deposited accumulation, is of German origin and means “loose.” It was first applied to Rhine River valley loess about 1821.

Thick loess blankets are composed of loess packets 1 to 5 m (about 3 to 16.5 feet) thick, each of which contains intercalated strata of loessial and loesslike sediments, paleosols (ancient soils), sand layers, and similar material. The totality of these constitutes the loess complex. There are several regional variants of loess that, together with true loess, constitute a loess series, including loessial sand, sandy loess, loess loam, and clayey loess. The individual elements of the loess series are hard to distinguish, and the several sediment types composing it are interpreted differently by workers in different regions or countries.

Loess covers extensive areas in Asia, Europe, and North America. Most widespread in today’s temperate zones and in the marginal semiarid zones of the deserts, loess covers about 10 percent of the land surface of the Earth. Loess usually exhibits a surface cover of fertile soil that is conducive to intensive agriculture. The capacity of loess to retain vertical or even overhanging walls is especially evident on the Loess Plateau in China, where some loess bluffs stand 150 m high and contain innumerable cellarlike dwellings excavated by the local inhabitants. In semiarid regions people such as the Pueblo Indians made houses and fortresslike closed edifices from loess-based adobe.