Loess Hills, National Natural Landmark in western Iowa, United States

The Loess Hills form a narrow band up to 15 miles east of the Missouri River, rising about 200 feet above the floodplain and running roughly 200 miles north to south from the Blood Run Site in South Dakota to Mound City, Missouri. Wind-deposited loess, called "sugar clay" locally, stands in vertical columns when dry and slumps when wet, producing razor ridges under 10 feet wide, peak-and-saddle topography and cat-step terraces. The Iowa drift layer often exceeds 90 feet deep.

The hills formed during the last Ice Age, when glaciers ground bedrock into fine silt that meltwater carried down the Missouri River and spread as mud flats. Westerly winds lifted the dried silt and redeposited it as dunes, which grass stabilized; erosion then cut the bluffs into razor ridges and cat-step terraces. The range runs from the Blood Run Site in South Dakota to Mound City, Missouri.

The name "Loess Hills" comes from the German word Löss, for the wind-blown silt that forms them; locally the soil is called "sugar clay" because it is hard when dry and loses cohesion when wet. The Loess Hills Scenic Byway runs from Westfield to Hamburg, and Broken Kettle Grasslands Preserve, owned by The Nature Conservancy, is the largest contiguous tract of native prairie left in Iowa.

Multiple trailheads with parking areas give easy access to explore the hills on foot, with routes ranging from short walks to full-day hikes. The terrain can be muddy and steep after rain, so proper footwear and water are important for any outing.

The Loess Hills of Iowa are notable for the depth of the drift layer, often more than 90 feet (27 m) deep; the only comparable loess deposits of such extent are in Shaanxi, China. The soil is known locally as "sugar clay" because it is extremely hard when dry but loses all cohesion when wet.