Loess is a homogeneous, typically non stratified, 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 ().
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.
Loess tends to develop into highly rich soils. Therefore under appropriate climatic conditions these areas are among the most agriculturally productive in the world.
Hungary has several areas that are covered by loess. At locations such as Dunaújváros and Balatonakarattya, loess walls are exposed as "reefs" (see illustration). Similar formations exist in Bulgaria on the south bank of the Danube.
The central part of Belgium is also covered by thick loess stacks. An interesting loess site where late Middle and Late Pleistocene Neanderthal artifacts were found within the soils between the loess layers is Veldwezelt-Hezerwater.
In several areas of the world, loess ridges have formed that are aligned with the prevailing winds during the last glacial maximum. In the midwest, these are generally called paha ridges, while they are called greda ridges in europe. The form of these loess dunes has been explained by a combination of wind and tundra conditions.
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 a characteristic called "vertical cleavage", which makes it easily excavated to form cave dwellings; this is still a 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. One theory states that the fertility of loess soils is due largely to electron exchange capacity (EEC) and pore space (the ability of plants to absorb nutrients from the soil, and the air-filled space in the soil, respectively). Unlike other soil, loess's fertility is not due to organic matter content, which actually tends to be rather low (unlike tropical soils, which depend almost wholly on organic matter for their fertility). In the Loess Hills of Iowa, the fertility of the region is owed to the prairie topsoils built by 10,000 years of post-glacial accumulation of organic-rich humus as a consequence of a persistent grassland biome. When the valuable A-horizon topsoil is eroded or degraded, the underlying loess soil is infertile, and requires the addition of fertilizers in order to support agriculture. In general, the fertility of farmland in the Loess Hills of Iowa is lower than in the adjacent alluvial floodplain of the Missouri River.
Though in geological time loess has an incredible rate of erosion, in a more human time scale loess is very durable and resistant to maltreatment. In China, for instance, loess deposits along the Yellow River have been farmed and have produced phenomenal yields for over a thousand years; though a large amount of the credit for this goes to the farmers themselves, as Chinese farmers were the first to practice active erosion control, which also started about one thousand years ago. The largest deposit of loess in the United States, the Loess Hills along the border of Iowa and Nebraska, has also survived under intensive farming and, in this case, poor farming practices. For almost 150 years this loess deposit was farmed with Mouldboard Ploughs and fall tilled (both practices are intensely erosive); at times it suffered erosion rates of over 100 tonnes per hectare per year. However, today this loess deposit is worked as low till, or no till, in all areas and is aggressively terraced.
Loess soil forms sharp topographic hills east of the Mississippi River and Yazoo River in western Mississippi north and south of Vicksburg. These deposits are in excess of 100 feet (30 m) thick (comparable to those in Iowa) immediately above the river valleys, to which they are sub-parallel, and thin to trace thickness within 25 miles (40 km) east. Streams and gulleys are incised very deeply and sharply between the linear loess ridges making topography very important in the conduct of military operations for the Vicksburg Campaign.
The loess deposits along the Mississippi River near Vicksburg, Mississippi, consist of three separate loess layers. The Peoria Loess, Sicily Island Loess, and Crowley's Ridge Loess accumulated at different periods of time during the Pleistocene. Ancient soils, called paleosols, have developed in the top of the Sicily Island Loess and Crowley's Ridge Loess. The lowermost loess, the Crowley's Ridge Loess, accumulated during the late Illinoian Stage. The middle loess, Sicily Island Loess, accumulated during early Wisconsin Stage. The uppermost loess, the Peoria Loess, in which the modern soil has developed, accumulated during the late Wisconsin Stage. Faunal remains include terrestrial gastropods and mastodons