During the ice age, glaciers moved across these plains and left silt, sand, gravel and boulders in their paths. Strong winds then drifted the fine dust particles into deep loess soils.
Loess soils are loose and silty, piled high by wind erosion.
Before European settlement, more than half this region's hills and ridges were tallgrass prairie. Today, the prairie has almost disappeared, converted into some of the most fertile agricultural land in the world. Forests wound along the slow, muddy streams and rivers. The Missouri River once was wide and shallow as it flowed through this region. Many islands, sloughs and backwaters attracted a wealth of wildlife.