Loess is nonstratified sediment composed of silt-sized particles, mainly of quartz, feldspar, and calcite, deposited by the wind. These windblown dust deposits, up to 50 meters thick, were derived from glacial materials. Loess is crumbly, extremely porous, usually yellowish and brown in color, and forms highly fertile topsoil (often chernozem), and is able to stand intact in cliffs. The largest expanse is in northern China in the valley of the River Huang He, where dust from the Gobi Desert has accumulated. The Huang is sometimes called the Yellow River because of all the loess it transports.