Loess

Loess is a sedimentary body, primarily found in semiarid or semihumid regions covering almost 10% of the earth’s surface. Geologists found out the thickest deposit of up to 300 m, spreading over 5,00,000 sq. km. in China. While these landmasses can be thicker than 60 m, thinner and less large scale ones are distributed throughout the earth.

This article includes every detail on this topic essential to preparing for UPSC Exam.

What Is Loess?

Loess is an aeolian landform due to sedimentary deposition of silty and loamy substances. Some significant examples of this landmass are as follows-

• China’s Huang Ho river banks
• Edge of Continental deserts of Inner Asia
• Great Plains in North America
• Snake River Plain and Palouse of the Pacific Northwest of United States
• Eastern part of Caspian Sea
• Vast areas of central Asia that include the peninsula of Tien Shan, Kazakhstan and Uzbekistan
• Mid region of Lena River and Baikal Lake
• Pampas of Argentina
• Southern catchment of Yenisey and Ob rivers
• Parts of Ukraine in Europe
• Danube Basin of Rhine river
• Paris basin
• Canterbury Plains

Characteristics of Loess

• It is a homogeneous, pale or light yellowish landmass with high porosity and friability. Generally, it is non-stratified and consists of clay, silt and sand, but is often calcareous.
• According to geological timescale, it is much recently formed than other sedimentary bodies.
• Most loess dunes are formed by aligned with antediluvian wind flows during the last ice age.
• Morphologists named these “greda ridges” and “paha ridges”, respectively in Europe and America. Also, they identified these formations as a combined process of wind and tundra climate.

Physical Properties of Loess

• Grain-size Fraction: Medium-sized, coarse grain with a fraction ranging from 0.02 mm to 0.05 mm and clay particle of a fraction below 0.005 mm.
• Composition: 50% of grain particles and 5% to 10% of clay.
• Porosity: 50% to 55% (34% to 45% in case of clay-rich loess).
• Moisture Retention: low porosity of 10% to 15% but may increase with decreasing porosity.

Chemical Properties of Loess

• Silica: 50% to 60%
• Alumina: 8% to 12%
• Iron oxide (Fe₂O₃): 2% to 4%
• Iron oxide (FeO): 0.8% to 1.1%
• Manganese and Titanium dioxide: 0.5%
• Lime: 4% to 16%
• Magnesium oxide: 2% to 6%

Mineral Composition of Loess

• Quartz: 60% to 70% but sometimes may vary between 40% and 80%
• Micas and Feldspars: 10% to 20%
• Carbonates: 5% to 35%
• Silt: 2% to 5%

Formation of Loess

Loess requires 4 fundamental factors to form according to Pye, which are as follows-

1. Source of dust particles
2. Sufficient arid power for weathering and transportation of mass
3. Suitable area of deposition
4. Time.

These results in 2 types of formations such as periglacial and non-glacial.

Periglacial Loess

Glacial loess is formed due to the accumulation of debris driven from the glacial braided rivers’ flood plain. These rivers usually deposit an enormous volume of sediments carried out with glacier meltwater from mountain ice caps and continental glaciers.

The water flow in these rivers largely decreases during autumn and winter due to a pause in glacier melting. Consequently, large areas of these braided rivers’ floodplains are exposed to wind activities. The wind picks up the silt and clay from the plain and deposits towards its flowing direction.

Morphologists found a great example of such a landmass on the Mississippi River Alluvial Valley banks.

Non-glacial Loess

These typically originate from sedimentary accumulation of sand from deserts, volcanic ash, playa lakes and dune fields. Strong wind force transport weathered particles from vast arid and semiarid regions exposed to aeolian attrition.

Some subtypes of this landform are- desert, tropical, volcanic, trade wind, gypsum and anticyclonic. Morphologists found volcanic variation in Argentina and Ecuador, gypsum in Spain and tropical in Brazil, Argentina and Uruguay.

Moreover, the formation of this landscape is quite common on the earth’s surface due to a very natural denudation process. The wind is one of the strongest weathering agents that erode and transport dust particles from arid to moisture ridden areas. Following that, those areas’ moisture and natural vegetation accumulate the particles and forms loess.

These are all essential details about loess for UPSC that can be helpful for aspirants. They must also follow up on some more UPSC Notes on this topic to substantiate their knowledge.