Loess (pronounced /ˈloʊ.əs/, /lʌs/, or /lɛs/) is an aeolian sediment formed by the accumulation of wind-blown silt and lesser and variable amounts of sand and clay.[1] that are loosely cemented by calcium carbonate. It is usually homogeneous and highly porous and is traversed by vertical capillaries that permit the sediment to fracture and form vertical bluffs.
The word loess, with connotations of origin by wind-deposited accumulation, is of German origin and means “loose.” It was first applied to Rhine River valley loess about 1821.[2]
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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.[3] 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.
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.
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.[4][5]
Non-glacial loess can originate from deserts, dune fields, playa lakes, and volcanic ash.
Some types of nonglacial loess are:[6]
The thick Chinese loess deposits are non-glacial loess having been blown in from deserts in northern China.[7] 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.[8] and Africa[9]
Loess tends to develop into highly rich soils. Under appropriate climatic conditions it is some of the most agriculturally productive terrain in the world.[10]
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 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.
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.[11]
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.
The Loess Plateau (simplified Chinese: 黄土高原; traditional Chinese: 黃土高原; pinyin: huángtǔ gāoyuán), also known as the Huangtu Plateau, is a plateau that covers an area of some 640,000 km² in the upper and middle of China's Yellow River and China proper. The soil of this region has been called the "most highly erodible soil on earth".[12] The Loess Plateau and its dusty soil cover almost all of Shanxi, Shaanxi, and Gansu provinces, the Ningxia Hui Autonomous Region, and parts of others.
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 Romania (Walachian plain) and 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.
LOESS (Ger. Loss), in geology, a variety of loam. Typical loess is a soft, porous rock, pale yellowish or buff in colour; one characteristic property is its capacity to retain vertical, or even over-hanging, walls in the banks of streams. These vertical walls have been well described by von Richthofen (Fiihrer fiir Forschungsreisende, Berlin, 1886) in China, where they stand in some places Soo ft. high and contain innumerable cave dwellings; ancient roads too have worn their way vertically downwards deep into the deposit, forming trench-like ways. This character in the loess of the Mississippi region gave rise to the name "Bluff formation." A coarse columnar structure is often exhibited on the vertical weathered faces of the rock. Another characteristic is the presence throughout the rock of small capillary tubules, which appear to have been occupied by rootlets; these are often lined with calcite. Typical loess is usually calcareous; some geologists regard this as an essential property, and when the rock has become decalcified, as it frequently is on the surface by weathering, they call it "loessloam" (lOsslehm). In the lower portions of a loess deposit the calcium carbonate tends to form concretions, which on account of their mimetic forms have received such names as losskindchen, losspuppen, poupees du loess, " loess dolls." In deposits of this nature in South America these concretionary masses form distinct beds. Bedding is absent from typical loess. The mineral composition of loess varies somewhat in different regions, but the particles are always small; they consist of angular grains of quartz, fine particles of hydrated silicates of alumina, mica scales and undecomposed fragments of felspar, hornblende and other rock-forming silicates.
In Europe and America loess deposits are associated with the margins of the great ice sheets of the glacial period; thus in Europe they stretch irregularly through the centre eastwards from the northwest of France, and are not found north of the 57th parallel. In both regions loess deposits are found within and upon glacial deposits. For this reason the loess is very commonly assigned to the Pleistocene period; but some of the loess deposits of northern Europe have been in process of formation intermittently from the Miocene period onward, and in South America the great loess formations known as the Pampean or Patagonian belong to the Eocene, Oligocene and Pleistocene periods. Most geologists are agreed that the loess is an aeolian or wind-borne rock, formed most probably during periods of tundra or steppe conditions. The capillary tubules are supposed to have been caused by the roots of grass and herbage which kept growing upon the surface even while the deposit was slowly increasing. Others contend that loess is of the nature of alluvial loam; this may be true of certain deposits classed as loess, but it cannot be true of most of the typical loess formations, for they lie upon older rocks quite independently of altitude, from near sea level up to 5000 ft. in Europe and to 11,500 ft. in China; they are often developed on one side of a mountain range and not upon the other, and in a series of approximately p arallel valleys the loess is frequently found lying upon one side and that the same in each case, facts pointing to the agency of prevalent winds.
The thickness of loess deposits is usually not more than 33 ft., but in China it reaches 1000 ft. or more; it also attains a great thickness in South America. Numerous proboscidian and other mammalian fossils have been found in the loess of Europe; the tapir, mastodon and giant sloths occur in South America, but the most common fossils are small land shells and such amphibious pond forms as Succinea. Certain loess deposits in Turkestan have been attributed to rain-wash, this is the so-called "` lake-loess" (see-loss); according to Tukowski the difference between sub-aerial and lake loess is that the former is porous, dry and pervious, while the latter is laminated, plastic and impervious. Two types of loess have been recognized in Russia, the Hillor Terrace-loess and the Low-level-loess, a product of the weathering of underlying rocks. In South Germany the following order has been recognized: (I) an upper unbedded, noncalcareous loess, (2) the gehangloss, mixed with subsoil rocks, and (3) the sand or thal-loss, with some gravel. The effect of vegetation on the upper layers of loess is to produce soils of great fertility, such as the black earth (Tschernozom) of southern Russia, the dark Bordeloss of the Magdeburg district, and the black "cotton soil" (regur) of the Deccan.
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