what is loess in geology

Full Answer

What is loess and why is it important?

Loess soils are among the most fertile in the world, principally because the abundance of silt particles ensures a good supply of plant-available water, good soil aeration, extensive penetration by plant roots, and easy cultivation and seedbed production.

How do you identify loess?

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 can be described as a rich, dust-like soil.

What are loess deposits?

Loess is a deposit of wind-blown silt that blankets large areas of the continents. It is often light brown in color, consisting predominantly of quartz, feldspars, micas, and calcium carbonate.

What is the difference between loess and silt?

Loess is a geologic term that refers to deposits of silt (sediment with particles 2-64 microns in diameter) that have been laid down by wind action (aeolian activity to geologists). Extensive, thick loess deposits generally formed in areas bordering large, continental glaciers.

What type of soil is loess?

The hills grew during the last ice age as loess—a dusty, wind-blown silt—piled into high dunes, which later hardened and were sculpted by erosion. In some parts of the world, windblown dust and silt blanket the land. This layer of fine, mineral-rich material is called loess.

What color is loess soil?

loess, an unstratified, geologically recent deposit of silty or loamy material that is usually buff or yellowish brown in colour and is chiefly deposited by the wind. Loess is a sedimentary deposit composed largely of silt-size grains that are loosely cemented by calcium carbonate.

Is loess a rock?

Depending on the researcher's background, loess is classified as sediment, soil or rock.

Why are loess sediments important?

Loess sediments cover extensive areas on the Earth's surface (Muhs et al., 2014). They can provide insights into dynamical sedimentation processes of the past and serve as terrestrial archives for studying dust deposition and atmospheric circulation (Muhs & Bettis, 2000; Muhs et al., 2014; Schaetzl et al., 2018).

Why is loess a problem?

However, loess soils often contain little clay, which leads to loss of organic matter from soil types other than chernozems under arable cultivation; the resulting structural instability of the surface soil causes problems of crusting, poor germination of crops and erosion.

What is the other name of loess?

clay, gault. [chiefly British], gley, guck.

What minerals are in loess?

Loess (pronounced "luss"), is German for loose or crumbly. It is a gritty, lightweight, porous material composed of tightly packed grains of quartz, feldspar, mica, and other minerals.

Is loess sandy soil?

The loess layers were characterized by a sand content of about 35 %. In the soils, however, the sand content rose up to 65 %. With intensive weathering and soil formation, one might assume that the fine sandy loess will weather into even finer material.

Which of the following best describes loess?

This is Expert Verified Answer Loess is aeolian deposit determined by the multiplication of wind-blown deposit, approximately in the 20– 50-micrometer size limit, 20 % or less slip and the surplus even separates dirt and sediment that are loosely connected by calcium carbonate.

What is the difference between loess and Delta?

Loess is a sediment formed by the accumulation of wind - blown dust. While a river delta is a landform formed from the deposition of sediment carried by a river.

How did the Loess Hills form?

These large dust clouds were moved eastward by prevailing westerly winds and were redeposited over broad areas. Heavier, coarser silt, deposited closest to its Missouri River flood plain source, formed sharp, high bluffs on the western margin of the Loess Hills. Finer, lighter silt, deposited farther east, created gently sloping hills on the eastern margin. This process repeated for thousands of years, building layer upon layer until the loess reached thicknesses of 60 feet or more and became the dominant feature of the terrain.

What is the origin of the Loess Hills?

Origin of the Loess Hills. Although early geologists assumed loess was either fluvial (deposited by a river) or lacustrine (formed in a lake), today we know that loess was eolian (deposited by the wind). During the Ice Age, glaciers advanced down into the mid-continent of North America, grinding underlying rock into a fine powderlike sediment ...

Why are the Loess hills rough?

Today, the Loess Hills are rough and jagged, the result of erosion by the very elements that created themãwind and water. The pie-crust shapes of these hills are the result of extreme erodibility of loess by wind, water, gravity-induced slipping, and human activity. When dry, loess particles form stable surfaces.

Why is the loess so susceptible to erosion?

Wet loess, however, is very susceptible to collapse and erosion because of a lack of clay particles which normally bond wet soils together. Color-infrared image taken 40,000 feet above the Loess Hills. Courtesy of the Geological Survey Bureau, Iowa Department of Natural Resources.

What is the name of the hill on the western side of the Missouri River?

The steep angles and sharp bluffs on the western side of the Loess Hills are in sharp contrast to the flat rectangular cropfields of the Missouri River flood plain. From the east, gently rolling hills blend into steep ridges. Loess (pronounced "luss"), is German for loose or crumbly.

How far above the Missouri River is the Loess Hills?

However, in westernmost Iowa the Loess Hills rise 200 feet above the flat plains forming a narrow band running north-south 200 miles along the Missouri River. The steep angles and sharp bluffs on the western side of the Loess Hills are in sharp ...

Why are the Loess hills called peaks and saddles?

Erosion has sculpted the Loess Hills into unusual shapes. The irregular ridge crests are called peaks and saddles because they resemble an animal's back. Distinctive staircases-like features on the slopes of some hills are known as catsteps and are actually the result of slipping soil.

What are the forms of carbonates in loess?

Carbonates are present in loess in a variety of forms, primarily as incrustations on quartz grains and clay-particle aggregates and as small granules and shell fragments. Secondary concentrations include concretions of nodules (Loess-doll) and layers of lime accumulation (caliche).

What percentage of loess is quartz?

The porosity of sandy loess is about 60 percent. Loess contains 60 to 70 percent quartz with extremes of 40 and 80 percent. Feldspars and micas make up 10 to 20 percent and carbonates 5 to 35 percent. About 2 to 5 percent of the silt is composed of such heavy minerals as amphiboles, apatite, biotite, chlorite, disthene (cyanite), epidote, garnet, ...

How thick are loess blankets?

Thick loess blankets are composed of loess packets 1 to 5 m (about 3 to 16.5 feet) thick, each of which contains intercalated strata of loessial and loesslike sediments, paleosols (ancient soils), sand layers, and similar material. The totality of these constitutes the loess complex. There are several regional variants of loess that, together with true loess, constitute a loess series, including loessial sand, sandy loess, loess loam, and clayey loess. The individual elements of the loess series are hard to distinguish, and the several sediment types composing it are interpreted differently by workers in different regions or countries.

What is the porosity of loess?

Its porosity is 50 to 55 percent, decreasing slightly downward to a depth of about 10 m (33 feet). Below this depth, porosity varies as a function of the grain-size distribution. If the loess is enriched with clay, then the porosity may decrease to 34 to 45 percent. The porosity of sandy loess is about 60 percent.

What is the carbonate content of loess?

The characteristic carbonate content of loess depends on the nature of the dust source, on geochemical and biological processes that occur during and after deposition, and on precipitation and leaching by groundwater. Carbonates are present in loess in a variety of forms, primarily as incrustations on quartz grains and clay-particle aggregates and as small granules and shell fragments. Secondary concentrations include concretions of nodules (Loess-doll) and layers of lime accumulation (caliche). Lime forms frequent tubular incrustations along decayed plant roots, fissure fillings, and similar avenues in loess.

What does "loess" mean?

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. Thick loess blankets are composed of loess packets 1 to 5 m (about 3 to 16.5 feet) thick, each of which contains intercalated strata of loessial and loesslike sediments, ...

What is the grain size of loess?

The dominant grain-size fraction of loess, called the loess fraction, ranges from 0.02 to 0.05 mm (0.0008 to 0.002 inch) and includes grains of coarse and medium-grained dust. Grain-size analyses by various methods indicate that the abundance of this fraction is about 50 weight percent. Clay-sized particles (less than 0.005 mm [0.0002 inch]) make up another 5 to 10 percent. In some loess regions, the grain-size distribution shifts toward finer grains with increasing distance from the source of dust ( e.g., eastward from Sand Hills, Neb.).

What is the use of radiocarbon dating in Loess?

Besides luminescence dating methods, the use of radiocarbon dating in loess has increased during the past decades. Advances in methods of analyses, instrumentation and refinements to the radiocarbon calibration curve have enabled to obtain reliable ages from loess deposits for the last 40-45 ka. However, the use of this method relies on finding suitable in situ organic material in deposits such as charcoal, seeds, earthworm granules or snail shells.

What are loess ridges called?

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.

Why are the Loess Hills fertile?

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 .

Why is loess rich?

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 rich. One theory states that the fertility of loess soils is due largely to cation 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.

What is a loess?

Loess ( US: / lɛs, ˈloʊ.əs, lʌs /, UK: / lɜːs /; from German: Löss [lœs]) is a clastic, predominantly silt -sized sediment that is formed by the accumulation of wind-blown dust. Ten percent of Earth's land area is covered by loess or similar deposits.

Where is the largest loess deposit in the United States?

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.

What is loose sediment?

Loess is an aeolian (windborne) sediment being an accumulation of 20% or less clay and the balance of mainly equal parts sand and silt ( grain size typically from 20 to 50 micrometers ), often 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 .

What are geodes in rock?

Geodes are commonly found in these gravel deposits. These come from a variety of sources and in a variety of forms. They can be lined with quartz, hematite or chalcedony among many other minerals. Chert, agate, jasper, petrified wood and many rocks containing fossils are among the material that make up the pre-loess terrace deposits. The abundance of chert, includes banded chert, oolitic chert and chert containing fossils. For the most part, these cherts all started as limestone and the calcareous minerals was replaced with silica over time. Several types of agates are found in the pre-loess gravel deposits. Agates are distinguished from banded chert by a very distinct banding known as iris banding. Agates are considered a semi-precious stone.

What is the loess bluff?

The loess bluffs trend north-south throughout the state, bordering the river to the south and the Delta to the north. The loess soil has several defining characteristics. It is well drained and rich in nutrients, making it ideal for agriculture, such as growing cotton.

Where did the loess deposits of western Mississippi originate?

The loess deposits of western Mississippi originated to the north during the Pleistocene Ice Age. As the glaciers in what is now Canada and the northern Untied States moved, they would grind up the bedrock and other rock debris into a fine flour-like soil.

What type of rock is found in the Mississippi River?

These rocks create a diverse deposit with sources across the Mississippi River drainage basin.

Where is Sioux quartzite found?

These gravels contain several interesting rocks. Pieces of Sioux quartzite are found in these deposits, which originates in southwestern Minnesota. This rock is one of the oldest rocks found in the gravel deposits (and one of the oldest known rocks on Earth) dating at about 1.7 billion years old.

When was petrified wood found?

It was most likely petrified around the time the gravel was deposited, about 1 million years ago.

Where did the sporadic boulders come from?

Among the mostly hand-sized and smaller gravel are the sporadic boulders. Many of these can be sourced back to Minnesota and other northern states. These rocks mainly arrived here in Mississippi through a process known as ice-rafting. Basically, the boulder had been picked up by a glacier (this occurred during the Pleistocene Ice Age when glaciers covered much of Minnesota) and eventually that chunk of ice and debris containing the boulder broke away from the glacier and floated down the Mississippi River before being deposited in Mississippi.

Overview

Further reading

• Smalley, I. J. (editor) 1975. Loess Lithology & Genesis. Benchmark Geology 26. Dowden, Hutchinson & Ross 454pp.
• Smalley, I. J. 1980. Loess: A Partial Bibliography. Geobooks/Elsevier. ISBN 0 86094 036 5. 103pp.
• Rozycki, S. Z. 1991. Loess and Loess-like Deposits. Ossolineum Wroclaw ISBN 83-04-03745-9. 187pp.

Properties

Etymology

History of research

Formation

Fertility

Large areas of loess deposits and soils