loess

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loess

(lĕs, lō`əs, Ger. lös), unstratified soil deposit of varying thickness, usually yellowish and composed of fine-grained angular mineral particles mixed with clay. It is found in many regions of the world and is probably related to the chernozemchernozem
or black earth,
variety of soil rich in organic matter in the form of humus. It is generally a modified type of loess. True chernozem is black in color, but there are various grades, shading off into gray and chestnut-brown soils.
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 soils of Russia; extensive deposits occur along the Mississippi River and its tributaries, on the Columbia Plateau in Oregon, Washington, and Idaho, and in China. Loess is an erosional product carried by the wind from adjacent deserts, from frost-pulverized outwash of glaciers (during the Pleistocene epoch), or from the floodplains of glacier-fed streams. Studies of particles transported by wind from plains recently denuded by tillage show that the material is sorted to about the same degree as loess. Much of the loess in the United States and Europe are of glacial origin; in China, of desert origin and may reach up to 300 feet (90 meters) thick. Loess is usually deep, fertile soil, rich in organic remains (especially the shells of snails) and characterized by slender, vertical tubes that are said to represent stems and roots of plants buried by sediment. When cut by streams or other agencies, loess remains standing in cliffs exhibiting a vertical, columnar structure; this is attributed to the vertical tubes and to the angularity of the grains and their consequent tendency to interlock. The uncompacted character of loess makes it subject to rapid erosion.

Loess

 

unstratified, homogeneous, calcareous sedimentary deposit, light yellow or buff in color. Particles 0.01–0.05 mm in size predominate; clay particles smaller than 0.005 mm are present in an amount of 5–30 percent. A certain quantity of particles 0.01–0.05 mm in size is represented by aggregates formed during the coagulation of the colloidal part of the sediment. The porosity of loess is 40–55 percent; it is traversed by narrow tubes (macro-pores, traces of plant remains).

In terms of its composition loess is usually classified as a loam or, more rarely, as a sandy loam. The large particles in the loess consist mostly of quartz and feldspar, and to a lesser degree of mica, hornblende, and so forth. In some interlayers there are many grains of volcanic ash that has been carried by the wind for hundreds of kilometers from the site of eruption. The fine particles in the loess consist of various clay minerals (hydromica, kaolinite, and montmorillonite). Sometimes calcareous concretions (loess dolls, lime nodules), the shells of terrestrial mollusks, and the bones of mammals, particularly rodents and mammoths, are encountered in the loess.

Loess is found in Europe, Asia, North America, and South America, predominantly in the steppe and semisteppe regions of the temperate zone. It occurs in the form of a blanket from several meters to 50–100 m thick on water divides, slopes, and ancient valley terraces.

The question of the origin of loess has still not been finally settled. Its formation has been associated with different geological processes (on land with the activity of wind, rainwater and snow meltwater, soil formation and weathering, volcanism, the settling of cosmic dust, and sedimentation in lakes and seas) as well as stages of rock formation. In 1877 the German scientist F. Richthofen proved the subaerial origin of Chinese loess (it was formed on land, and water played a limited role). Also popular are theories of the eolian (V. A. Obruchev), pedogenic (L. S. Berg), and polygenetic (eolian, deluvial, and pedogeniceluvial processes occurring in an arid climate) origin of loess. The loess has intercalations with a clearly expressed soil profile, that is, buried soils that testify to the existence of warmer and wetter periods (interglacials) than the time during which the loess formed (ice ages).

Loess is the parent rock of chernozem and sierozem soil. It is used for making brick (adobe) and cement and as a filler for levees and dams. After wetting, the loess is often compacted under the pressure of its own weight or the weight of structures, and the ground subsides, which can cause severe damage to structures.

REFERENCES

Obruchev, V. A. Izbrannye raboty po geografii Azii, vol. 3. Moscow, 1951.
Larionov, A. K., V. A. Priklonskii, and V. P. Anan’ev. Lessovye porody SSSR i ikh stroitel’nye svoistva, part 1. Leningrad, 1959.
Berg, L. S. Izbr. trudy, vol. 3. Moscow, 1960.
Kriger, N. I. Less, ego svoistva i sviaz’s geografi-cheskoi sredoi. Moscow, 1965.
Cailleux, A. Les Loess et limons éoliens de France. Paris-Liège, 1954.
Guenther, E. W. Sedimentpetrographische Untersuchung von Lössen, part 1. Cologne-Graz, 1961.
Richthofen, F. von. China, vol. 1. Berlin, 1877.

N. I. KRIGER

loess

[les]
(geology)
An essentially unconsolidated, unstratified calcareous silt; commonly it is homogeneous, permeable, and buff to gray in color, and contains calcareous concretions and fossils.

loess

A uniform wind-deposited accumulation of silty material having an open structure and relatively high cohesion due to cementation of clay or calcium-like material at grain contacts.

loess

a light-coloured fine-grained accumulation of clay and silt particles that have been deposited by the wind
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Loessial soils in this area are dominated by the xerohalophytes Anabasis syriaca and Peganum harmala.
For the purpose of this discussion, we have separated the soils by loessial and glacial till parent material (Table 7).
On the loessial soils, Grantsburg, Hoyleton, Rozetta, Sharpsburg, and Wymore, depth to CaC[O.
Few-to-common silt-size, rounded, compound particles (aggregates) are randomly distributed in the loessial soil fabrics, whereas aggregates with high degrees of subplasticity and dispersion-stability dominate in many parna areas (Butler 1976).
Wei XR, Shao MA (2007a) Distribution of soil properties as affected by landforms in watershed of loessial gully region.
The high silt contents of all but some of the deepest horizons are indicative of the loessial origin of parent materials.
The region's soils are largely loessial, having developed from primitive or secondary loess parent materials (Cheng et al.
They applied this procedure to calcareous soils from the loessial plain of North China, with Ca[CO.
The loessial landforms include yuans (large, flat surface with little erosion), ridges, hills, and gullies at elevations ranging from 100 to 3000m (Yang and Shao 2000).
The predominant soil is loessial soil, with a dry bulk density averaging 11.
The experimental field was flat according to the FAO/UNESCO Soil Classification (FAO/UNESCO 1993), and the soil was a dark loessial soil with 26.
There are two main soil types in this catchment, a loessial and an aeolian sandy soil.