Loess is a geologically material which is usually yellowish or brown in color and consisting of tiny mineral particles brought by wind to the places where they now lie. It is a sedimentary deposit of mineral particles which are finer than sand but coarser than dust or clay, deposited by the wind. Loess is a type of silt which forms fertile topsoil in some parts of the world. The soil has few clay particles to hold it together. It is composed mainly of quartz crystals which slide easily against each other, and is therefore very subject to erosion.