Loess is a fine, silty, windblown (eolian) type of unconsolidated deposit, or, sometimes the term refers to the soil derived from it. It is derived from glacial deposits, where glacial activity has ground rocks very fine (rock flour). After drying, these deposits are highly susceptible to wind erosion, and downwind deposits may become very deep, even hundreds of feet, as in areas of China and midwestern USA.

Loess grains are angular, with little polishing or rounding, composed of crystals of quartz, feldspar, mica and other minerals. Because the grains are angular, loess will often stand in banks for many years without slumping. This soil has the characteristic called vertical cleavage, which makes it easily excavated to form cave dwellings, a still-popular method of making human habitations in some parts of China. But it is also highly erodible by water or wind, and soils underlain by loess tend to be excessively drained (droughty). As the grains weather, they release minerals which means that soils derived from loess are usually very rich.