Loess (, , or ) is an aeolian sediment formed by the accumulation of wind-blown silt and lesser and variable amounts of sand and clay. Loess sometimes refers to these deposits and the soil derived from them.

Properties

Loess is homogeneous, porous, friable, pale yellow or buff, slightly coherent, typically non-stratified and often calcareous. Loess grains are angular with little polishing or rounding and composed of crystals of quartz, feldspar, mica and other minerals.

Loess deposits may become very thick; more than a hundred meters in areas of China and the Midwestern United States. It generally occurs as a blanket deposit that covers areas of hundreds of square kilometers and tens of meters thick.

Loess often stands in either steep or vertical faces. 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, a popular method of making human habitations in some parts of China. Loess will erode very readily.

In several areas of the world, loess ridges have formed that are aligned with the prevailing winds during the last glacial maximum. These are called paha ridges in America and greda ridges in Europe. The form of these loess dunes has been explained by a combination of wind and tundra conditions.

Etymology

Loess comes from the German Löss or Löß, and ultimately from Alemannic lösch meaning loose as named by peasants and masons along the Rhine Valley.

Formation

Glacial

Glacial loess is derived from the floodplains of glacial braided rivers that carried large volumes of glacial meltwater and sediments from the annual melting of continental icesheets and mountain icecaps during the summer. During the fall and winter, when melting of the icesheets and icecaps ceased, the flow of meltwater down these rivers either ceased or was greatly reduced. As a consequence, large parts of the formerly submerged and unvegetated floodplains of these braided rivers dried out and were exposed to the wind. Because these floodplains consist of sediment containing a high content of glacially ground flour-like silt and clay, they were highly susceptible to winnowing of their silts and clays by the wind. Once entrained by the wind, particles were then deposited downwind. The loess deposits found along both sides of the Mississippi River Alluvial Valley are a classic example of glacial loess.

Non-glacial

Non-glacial loess can originate from deserts, dune fields, playa lakes, and volcanic ash.

Some types of nonglacial loess are:

• Volcanic loess in Ecuador and Argentina.

• Tropical loess in Argentina, Brazil and Uruguay.

• Gypsum loess in Spain.

• Tradewind loess in Venezuela and Brazil

• Anticyclonic loess in Argentina.

The thick Chinese loess deposits are non-glacial loess having been blown in from deserts in northern China. The loess covering the Great Plains of Nebraska, Kansas, and Colorado is non-glacial desert loess. Non-glacial desert loess is also found in Australia. and Africa

Fertility

Loess tends to develop into highly rich soils. Under appropriate climatic conditions it is some of the most agriculturally productive terrain in the world.

Soils underlain by loess tend to be excessively drained. The fine grains weather rapidly due to their large surface area making soils derived from loess very rich. One theory states that the fertility of loess soils is due largely to electron exchange capacity (the ability of plants to absorb nutrients from the soil) and porosity (the air-filled space in the soil). The fertility of Loess is not due to organic matter content, which tends to be rather low unlike tropical soils, which derive their fertility almost wholly from on organic matter.

Even well managed loess farmland can experience dramatic erosion of well over 2.5 kg per square meter per year. Although in geological time loess has an incredible rate of erosion, in a more human time scale loess is durable and resistant to maltreatment. In China loess deposits along the Yellow River have been farmed and have produced phenomenal yields for over one thousand years. A large amount of the credit for this goes to the farmers; Chinese farmers were the first to practice active erosion control. The largest deposit of loess in the United States, the Loess Hills along the border of Iowa and Nebraska, has survived intensive farming and poor farming practices. For almost 150 years this loess deposit was farmed with mouldboard ploughs and fall tilled, both intensely erosive. At times it suffered erosion rates of over 10 kilograms per square meter per year. Today this loess deposit is worked as low till or no till in all areas and is aggressively terraced.

Sites

The Loess Hills of Iowa owe their fertility 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 fertilizer in order to support agriculture.

The loess along the Mississippi River near Vicksburg, Mississippi consist of three 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. Animal remains include terrestrial gastropods and mastodons.Miller, B.J., G.C. Lewis, J.J. Alford, and W.J. Day, 1985, Loesses in Louisiana and at Vicksburg, Mississippi. Guidebook, Friends of the Pleistocene Field Trip, 12-14 April, 1985. LA Agricultural Experimental Station, Louisiana State University, Baton Rouge, Louisiana. 126 pp.
Loess soil forms sharp hills east of the Mississippi River and Yazoo River in western Mississippi north and south of Vicksburg. These deposits are more than 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 40 km to the 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 Palouse Hills of eastern Washington and northern Idaho is a fertile agricultural region based on loess deposits.
Hungary has several areas that are covered by loess. At locations such as Dunaújváros and Balatonakarattya, loess walls are exposed as loess reefs. Similar formations exist in Bulgaria on the south bank of the Danube.

The central part of Belgium is covered by thick loess stacks. Neanderthal artifacts were found within the soils between the loess layers of the Veldwezelt-Hezerwater.