Loess hills are composed of dust formed when glaciers advanced into the middle of North America and ground the rocks into very fine "glacier flour". As the glaciers melted, large amounts of water and sediment flooded the Missouri River Valley forming mud flats. When the waters receded, these mud flats were exposed and, as they dried, the fine-grained flour was picked up by strong prevailing westerly winds. Huge dust clouds were moved and deposited over broad areas. The heavier, coarser particles formed large dune fields that were eventually stabilized by grass. The stabilized dunes were eroded into the steep bluffs seen today. Memphis is called the "Bluff City", because it is located on top of the Loess Bluffs.