The Loess Hills

Loess (pronounced “luss”), a German term meaning loose or crumbly, is a yellowish sedimentary deposit of wind-blown silt forming a distinctive 640,000-acre range of hills (bluffs or “miniature mountains”) up to 250 feet high and 3 to 10 miles wide, running parallel to the Missouri River 220 miles from Mound City, MO to Westfield, IA.  During the warm months of the glacial period 12,000 to 30,000 years ago, glacial melt from the north brought glacier-ground silt (or “rock flour”) down the Missouri River, where it accumulated in mud flats.  During the winter, when the water froze and glacial flow subsided, the mud dried and the silt was blown by westerly winds into dunes on the east side of the river that were eventually stabilized with grass and trees.  The loess, called “sugar clay” by the locals, is extremely hard when dry but becomes very slippery and easily eroded from the top when exposed and wet. However, when protected at the top by trees and other vegetation, the loess ravines maintain their steep, stable cliffs, up to 100 feet in height.  Loess naturally sloughs into terraces or “cat steps” along steep slopes.  While loess is found in many locations, including the Rhine River valley in Germany, only this region along the Missouri River and another near Shaanxi, China have sufficient deposits (60-200 feet) to form an extended range of hills.  It is loess that gives China’s Yellow River its color.