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
References in periodicals archive ?
The precipitation events on the Loess Plateau can be divided simply into two categories: small rainfall events occurring at a higher frequency and large rainfall events at a lower frequency.
In this paper, PF[C.sup.2D] is used by selecting the appropriate particle contact constitutive model and determining the microscopic parameters; the ideal numerical calculation model is established to simulate the loess splitting grouting and further analyze the mechanism of loess splitting grouting.
In general, annual precipitation exhibited no obvious trend in the Loess Plateau for the last decades.
On the condition that the water permittivity is 81 and the loess permittivity is 5, so (4) can be simplified as
The most direct method is to classify the gush strata of spring; it can be divided into outcropping spring of Salawusu formation ([Q.sub.3]s), outcropping spring of loess and hipparion laterite, outcropping spring of burnt rocks ([J.sub.2]y), outcropping spring of Jurassic ([J.sub.2]z or [J.sub.2]y), and outcropping spring of other layers.
The soil profile is dominated by silt fraction, which is characteristic for soils developed on loess. The proportion of sand fraction is small.
From Table 1 and Figure 8, we can see that the window size can influence greatly the result of main trend extraction by PMU, Loess, and MA methods.
First, the Mualem model could not be directly applied to the remolded loess used in the test, as (2) had set [beta] and [gamma] in correspondence with the Mualem model and the L value obtained through curve fitting ranged between 1.39 and 3.53, which is significantly different from the L value of 0.5 in the Mualem model.
Shear Wave Velocities of Loess. Dynamic soil stiffness, as indicated by either shear modulus or shear wave velocity, is a prerequisite parameter for dynamic analysis of site seismic response.
Ryegrass, which is one of the most important kinds of grass for grassland animal husbandry and green-recovery, was chosen as the research object to investigate the influences of biochar amendment and water stress on the plant growth process in two kinds of soils from the Loess Plateau.
The depth of the loess soil in the AVA allows grapevine roots to grow without being blocked by other dense soils or bedrock and also permits water to drain quickly from the vines.
rhamnoides under different soil moisture conditions in loess plateau region Datong, Xining, Qinghai Province.