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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 Chinamarker 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 slump. 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 mason along the Rhine Valleymarker.

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:

The thick Chinese loess deposits are non-glacial loess having been blown in from deserts in northern Chinamarker. The loess covering the Great Plainsmarker of Nebraskamarker, Kansasmarker, and Coloradomarker 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 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 depositsmarker along the Yellow Rivermarker 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 Iowamarker and Nebraskamarker, 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 Iowamarker 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, Mississippimarker 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.

Loess soil forms sharp hills east of the Mississippi River and Yazoo Rivermarker in western Mississippimarker north and south of Vicksburgmarker. 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 Campaignmarker.

The Palouse Hills of eastern Washingtonmarker and northern Idahomarker is a fertile agricultural region based on loess deposits.

Hungarymarker has several areas that are covered by loess. At locations such as Dunaújvárosmarker and Balatonakarattya, loess walls are exposed as loess reefs. Similar formations exist in Bulgariamarker on the south bank of the Danube.

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

References

  1. Richthofen, F. von. 1882. On the mode of origin of the loess. The Geological Magazine, Decade II, 9(7):293-305.
  2. Neuendorf, K.E.K., J.P. Mehl, Jr., and J.A. Jackson, 2005, Glossary of Geology. Springer-Verlag, New York, 779 pp., ISBN 3-540-27951-2
  3. Bettis, E.A., III, D.R. Muhs, H.M. Roberts, and A.G. Wintle, 2003, Last glacial loess in the conterminous U.S.A. Quaternary Science Reviews. vol. 22, pp. 1907-1946.
  4. Muhs, D.R., and E.A. Bettis, III, 2003, Quaternary loess-paleosol sequences as examples of climate-driven sedimentary extremes. Geological Society of America Special Paper no. 370, pp. 53-74.
  5. Iriondo, M.H., and D.M. Krohling, 2007, Non-classical types of loess. Sedimentary Geology. vol. 202, no. 3, pp. 352-368.
  6. Ding, Z., and J. Sun, 1999, Changes in Sand Content of Loess Deposits along a North–South Transect of the Chinese Loess Plateau and the Implications for Desert Variations. Quaternary Research. vol. 52, pp. 56–62.
  7. Muhs, D.R., and E.A. Bettis, III, 2003, Quaternary loess-paleosol sequences as examples of climate-driven sedimentary extremes. Geological Society of America Special Paper no. 370, pp. 53-74.
  8. 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.


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