Loess is windborne dust found in vast deposits in North America, north-central Europe, and Asia. Blown from glacial outwash plains at the edge of Pleistocene ice sheets, from deserts, and from river ﬂoodplains, it has accumulated in vast deposits. Thirty percent of the United States is blanketed in this ice-age eolian silt, which, along with other elements, has developed into the fertile agricultural soils of the Mississippi Valley, parts of the Great Plains, and the Palouse Hills of the Columbia River basin. Iowa’s Loess Hills feature an extraordinarily deep deposit, two hundred feet thick in places. Loess is generally yellowish brown. Its silty particles make it porous, crumbly, and prone to erosion. Still, loess can stand in vertical, albeit vulnerable, cliffs. Scientists believe this is because loess is well sorted, allowing particles of similar size and geometries to pack together.