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Loess is a mineral silt, made by glaciers and wind, forming rare ecosystems. Loess, pronounced "luss," forms hills, mountains, bluffs, and gulleys during the slow process of its deposit and relatively rapid erosion. River valleys and plateaus of loess provide very rich soil, perfect for agriculture, and beautiful, recognizable topographical features.
Loess, meaning "crumbly" in German, is made primarily of particles of quartz, mica, feldspar, and other silicates that make it brown or yellow in color. Loess has a small amount of clay, so it's not sticky like sediment, but rather slippery; that's why it erodes so much more quickly than other types of soil. The central and northwestern areas of the United States, Ukraine, eastern China, and eastern and central Europe all have significant deposits of loess. Agriculture has thrived in these areas since prehistoric humans took advantage of the rich topsoil to grow crops. These rare geographic locations, separated by thousands of miles, all share a similar geologic history.
Migrating, melting glaciers and high winds formed loess thousands of years ago, after the last Ice Age. First, glaciers scraped along huge beds of rock, dislodging and grinding the rock into particulate minerals. Then, the melting glaciers washed this debris along channels to low-lying areas and flood plains. Eventually, as the earth's temperature continued to rise, these lakes of mud dried. Finally, swift wind carried the light, powdery loess in drifts that layered to form bluffs, hills, and mountains. This is why geologists refer to the formation of loess as eolian, or made by wind.
Loess deposits form striking, remarkable landscapes, such as the Loess Formation of Iowa. There, we see ridged hills, steep mountain drops, deep gullies, jagged crests, and ambling spurs. From the air, the wedge-shaped mountain ranges trace the history of the direction of wind. Different layers, from separate cycles of flooding and blowing, inform geologists about the earth's past. In Iowa, their deposits vary from just 12,000 years ago to 160,000 years ago. The depth of the loess reaches 300 feet (91 m) in some areas. The Shaanxi area of China has much deeper loess plateaus, up to 1,000 feet (305 m) thick, although they exhibit much more erosion.