The Loess Hills are an undulating ridge of varying depths and shades of ocher soil which runs along most of the western border of Iowa. Loess is a soil type which is comprised of fine sand. It was deposited by wind approximately 18,000 years ago. Although the soil type is found in many places the only other deep deposit is in China adjacent to the Yellow River. Recently the Loess Hills region has been the subject of an evaluation by the National Park Service. The study was undertaken to determine if the region meets the criteria for inclusion as a new unit of the National Park System. Opinions of residents varied widely with preservationists in disagreement with private property advocates. The National Park Service has elected not to protect the area.