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, ...

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