Fine-grained sedimentary clay. It is earthy, porous and crumbly, and usually yellowish or brown. Loess consists of mainly quartz and calcite from glaciated areas, blown by the wind and often built up into thick layers. The largest expanse is in the Huang He valley, N China.

LOESS (PRONOUNCED “LUSS”) is soil comprised almost entirely of silt (soil particles 2–50 µm diameter), with some very fine sand and coarse clay, that has been deposited by wind, often during glacial periods. Most loess soils on the present global landscape are geologically recent, having been deposited during and after the most recent Ice Age (1.6 million to 10,000 years ago). As the climate gradually warmed and glaciers retreated, massive deposits of glacial till and outwash were left on the landscape and in river valleys. Windstorms plucked fine soil particles from these enormous, barren expanses of glacial debris and deposited them equator-ward, where they were layered as bluffs.

Usually several meters thick, extensive loess deposits are found in the central, midwestern, and northwestern United States, eastern and central Europe, Argentina, northwestern Africa, and central Asia. The extensive loess deposits in the central plains of the United States account in part for the high agricultural productivity in that region of the world. Some of the largest loess deposits are located in China, where they range from 30–100 meters thick and extend approximately 800,000 kilometers2. (Unlike loess deposits that originate from glacial till and outwash, loess in China originated from erosion of silt from central Asian deserts to its west.)

Over time, loess deposits may take on a stepped appearance in the landscape, because mineralogically, silt particles are typically microscopic quartz grains that slide easily past one another. This mineral structure of silt also accounts for the soft, lubricious feel of loess when rubbed between the fingers. Loess soils are fertile and productive for agriculture, particularly when combined with sufficient clay and soil organic matter.

Loess soils are also highly vulnerable to erosion, however, because silt particles are much smaller than sand grains, and they are not as tightly bound to one another (or to organic matter) as clay particles. Because silt is not sticky and plastic like clay, loess cracks when it settles and dries. Thus, water entering cracks in settled loess quickly erodes silt particles and carries them away. As erosion widens the cracks, more surface area is exposed and erosion proceeds more rapidly in a positive feedback process termed piping.

Soil texture plays a central role in determining agricultural productivity, and texture is also critical to consider when making decisions about human infrastructure development. This was highlighted by a recent case study in the United States: In 1977, a large earth-fill dam on the Teton River in Idaho failed, killing 11 people and leaving 25,000 people homeless. The dam failed unexpectedly because it had been built on loess, which, unlike clay, cannot be compacted into a water-impermeable mass. Cracks formed in the silt underlying the dam, and piping of silt from these cracks compromised the integrity of the soil base. In a similar vein, much attention has been paid recently to ensuring that enough clay is incorporated into the replacement levees surrounding the U.S. city of New Orleans, where storm surges and flooding from Hurricane Katrina destroyed protective levees in August 2005, killing 1,000–1,500 people and leaving tens of thousands of people homeless.