Loess is a homogeneous, typically nonstratified, porous, friable, slightly coherent, often calcareous, fine-grained, silty, pale yellow or buff, windblown (aeolian) sediment. It generally occurs as a widespread blanket deposit that covers an area of hundreds of square kilometers and tens of meters thick. Loess often stands in either steep or vertical faces. The term is sometimes also refer to soils derived from such deposits. The word comes from the German Löss or Löß, and ultimately from Swiss German lösch (loose) as named by peasants and brickworkers along the Rhine Valley where this type of sediment was first recognized. It is pronounced in several different ways in English (IPA: /ˈloʊɛs/, /ˈlʌs/, /ˈlɛs/).
Loess is an aeolian sediment which forms by the accumulation of wind-blown silt and lesser and variable amounts of either sand or clay. Glacial loess is derived from either glacial or glacial outwash deposits, where glacial activity has ground rocks very fine (rock flour). After drying, these deposits are highly susceptible to wind erosion, winnowing of their silts and clays, transportation of these sediments, and deposition some distance downwind from glacial deposits. The loess deposits found along both sides of the Mississippi River Alluvial Valley are a classic example of glacial loess Nonglacial loess consists of silt-size sediments eroded by wind from either deserts, dune fields, or playa lakes. The prolonged accumulation of wind-blown volcanic ash can form loess. Some types of nonglacial loess are 1) volcanic loess in Ecuador and Argentina; 2) tropical loess in northeastern Argentina, Brazil and Uruguay: 3) gypsum loess in northern Spain; 4) trade-wind loess in Venezuela and Brazil; and 5) anticyclonic gray loess in Argentina.. The thick Chinese loess deposits are classic nonglacial (desert) loess with their sediments having been blown in from deserts in northern China. The loess covering the Great Plains of Nebraska, Kansas, and Colorado is nonglacial desert loess. Nonglacial desert loess is also found in Australia and Africa.
Loess deposits may become very deep — a hundred metres or more in areas of China and the midwestern United States. Loess deposits are geologically unstable by nature, and will erode very readily. Even well-managed loess farmland can experience dramatic erosion of well over 25 tonnes per hectare per year.