Loess Hills Geology Page
The Landscape Today
The Loess Hills are a rare and unusual landform, but they are not permanent; loess terrain is dynamic and rapidly evolving. When originally deposited, the loess was smooth like a sand dune or a snow drift. Today, the Loess Hills are rough and jagged, the result of erosion by the very elements that created themãwind and water.
The pie-crust shapes of these hills are the result of extreme erodibility of loess by wind, water, gravity-induced slipping, and human activity. When dry, loess particles form stable surfaces. Wet loess, however, is very susceptible to collapse and erosion because of a lack of clay particles which normally bond wet soils together.
Humans inhabited the Loess Hills as early as 12,000 years ago, but the earliest written descriptions of the Loess Hills appear in journals from the Lewis and Clark Expedition, which travelled through western Iowa in 1804. Later immigrants who settled in and around these hills farmed the fertile rockless soil and grazed cattle on hillsides too steep for planting. The Loess Hills also provide natural resources such as building materials and fill dirt for highways and railroad construction.
Issues Facing the Loess Hills
The Loess Hills of Iowa are extremely fragile. They have one of the highest erosion rates in the U.S., almost 40 tons/acre/ year. Erosion from rainfall and flooding removes loess from the hills and redeposits it on the flood plain from which it came. Increased sediment in the streams can be harmful to fish and plants, and drainage ditches must be continually dredged as they become choked with sediment.
Gullies, although a natural part of this landscape, are a serious problem. They can be many miles long, more than 100 feet wide, and as deep as 80 feet. Bridges and roads collapse as the gullies widen, restricting farmers' access to their fields.
Human activities such as farming and mining can exacerbate erosion. Today, farmers practice terracing and contour farming in an effort to limit erosion.
The Loess Hills are home to several endangered plant and animal species. The uniqueness of the landscape and biota led to recognition of portions of the Loess Hills as a National Natural Landmark in 1986 by the U.S. Department of the Interior.
USGS Research in the Loess Hills
As the Nation's largest water, earth, and biological science, and civilian mapping agency, the USGS provides reliable, impartial, scientific information to minimize the loss of life and property from natural disasters, to contribute to the sound conservation, economic, and physical development of the nation's natural resources, and to enhance the quality of life. Current USGS geologic research projects in the Loess Hills include the following:
Loess Hills Erosion
The clearing of land for agricultural use has accelerated the rate of gully formation. Evidence of soil loss and the susceptibility of loess to gully erosion has prompted new research into landscape processes and preservation strategies. The USGS, in conjunction with the Iowa Department of Natural Resources, the Agricultural Research Service, and university researchers, is conducting detailed investigations on erosion rates and the effects of erosion on soils.
Testing Ice-Age Climate Models
Because loess was deposited by Ice-Age winds, it contains a record of past climates in the form of wind direction. Studies show that some of the loess in the Loess Hills came from sources much farther west than the Missouri River valley, indicating the presence of a westerly wind during that time. However, computer simulations show that winds would have come from the northeast. The USGS, in cooperation with the Iowa Department of Natural Resources, is gathering geologic information critical to testing computer models of past climates.