loess

Fine-grained, wind-deposited silt — soil carried aloft from glacial outwash plains, riverbeds, or desert surfaces and laid down in thick, uniform blankets hundreds of miles from its source. Loess is among the most fertile soils on earth. The agricultural wealth of the American Midwest, the Central European plains, and the Yellow River valley of China is built on loess deposited during the Ice Ages. It erodes vertically — loess bluffs stand in sheer faces rather than slumping — and it holds its structure when dry but collapses catastrophically when saturated.