Loess

Loess is a pale, fine-grained sediment made mostly of silt-sized particles, with no more than about a fifth clay and the rest split roughly evenly between sand and silt, built up over long periods purely by the settling of wind-blown dust. Forming it takes four things in the same place: a source of loose dust, wind strong enough to carry it, a spot where it can settle undisturbed, and enough time for the dust to accumulate into a thick deposit; periglacial loess comes from fine sediment exposed on glacial meltwater floodplains and later picked up by wind, while non-glacial loess is blown in from desert surfaces or volcanic ash falls. Loess deposits cover roughly a tenth of the world's land surface. China's Loess Plateau is the largest and best known example, an area of about 640,000 square kilometers built from wind-blown material off the surrounding deserts that in turn feeds the sediment load of the Yellow River; other major deposits form the Loess Hills of Iowa and Nebraska in North America and a belt running from England across northern Europe into Ukraine, with further significant loess country in Argentina, Central Asia, New Zealand and Australia.