Loess
Wind-deposited silt (quartz-rich silt with clay and carbonate)
A loose, wind-blown silt deposit, typically buff-colored and very fertile, that forms thick blankets and stands in steep cliffs.
- Mohs hardness
- Soft, unconsolidated (~1-2)
- Color
- Yellowish-buff, tan to pale brown
- Type
- sedimentary
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Overview
Loess is a wind-deposited sediment made predominantly of silt-sized particles, mostly quartz with some feldspar, clay and calcium carbonate. It is loose, porous and uniform, with a characteristic yellowish-buff to tan color, and it forms extensive blankets over the landscape in many parts of the world.
Despite being unconsolidated, loess has a remarkable ability to stand in near-vertical cliffs and faces because of its angular grains and weak carbonate and clay cement. It is highly fertile, supporting some of the most productive farmland on Earth.
Loess deposits also serve as valuable archives of past climate and wind patterns.
Formation & geology
Loess forms when fine silt produced by glacial grinding, desert weathering or river floodplains is picked up by wind and carried, sometimes great distances, before settling out as a blanket of dust. Vegetation traps and stabilizes the accumulating silt, allowing thick deposits to build up over thousands of years.
Many loess deposits are linked to ice-age conditions, where glaciers ground rock into rock flour that was then deflated by strong winds. Extensive loess occurs in the Loess Plateau of China (the thickest in the world), the central United States (Mississippi Valley and Great Plains), central Europe, and Argentina.
How to identify it
Loess is recognized by its uniform, fine silty texture (gritty but not sandy, smooth but not plastic like clay), its pale yellowish-buff to tan color, and its loose, porous, easily crumbled nature.
Its most distinctive field characteristic is the way it stands in steep, vertical bluffs and erodes into sharp-walled gullies despite being unconsolidated. It often effervesces gently in acid due to carbonate content. Compared to clay it is non-plastic and silty; compared to sand it is much finer and more uniform. The combination of fine silt, buff color, and vertical cliff-forming habit is diagnostic.
Uses & significance
Loess is best known for forming exceptionally fertile soils; many of the world's great agricultural regions, such as the American Midwest, the Ukrainian steppe and the Chinese Loess Plateau, owe their productivity to deep loess. Its silty, well-aerated, mineral-rich nature makes it ideal for crops.
Loess has been used as a building material, notably in the cave dwellings carved into the Loess Plateau of China. Scientifically, loess sequences are prized records of past climate, wind direction and glacial cycles. Its tendency to collapse and erode, however, makes it a geotechnical hazard requiring careful engineering.
Frequently asked questions
What is loess?
It is a loose, wind-blown silt deposit, mostly quartz, that forms thick fertile blankets over the land in many regions.
Why is loess so fertile?
Its fine silty texture, good aeration and mineral content make it excellent for plant growth, supporting some of the world's best farmland.
How can loess form vertical cliffs if it is loose?
Its angular silt grains interlock and are weakly bound by clay and carbonate, giving it cohesion that lets it stand in steep faces.
Where are the biggest loess deposits?
The Loess Plateau of China has the thickest loess in the world, with other major deposits in the central United States, Europe and Argentina.
Loess guides
In-depth guides for identifying, valuing, and understanding Loess.
