The Loess (//, //, or //) Hills are generally located between 1 and 15 miles (2 and 24 km) east of the Missouri River channel. The Loess Hills rise 200 feet (60 m) above the flat plains forming a narrow band running north-south 200 miles (320 km) along the Missouri River. These hills are the first rise in land beyond the floodplain, forming something of a "front range" for Iowa, and parts of Missouri and Nebraska adjacent to the Missouri River.
During the last Ice Age, glaciers advanced into the middle of North America, grinding underlying rock into dust-like "glacial flour." As temperatures warmed, the glaciers retreated and vast amounts of meltwater and sediment flooded the Missouri River Valley. The sediment was deposited on the flood plain, creating huge mud flats. When meltwaters receded, these mud flats were exposed. As they dried, the fine-grained silt was picked up by strong prevailing westerly winds. Huge dust clouds were moved and redeposited over broad areas. The heavier, coarser silt was deposited close to the Missouri River flood plain, forming vast dune fields. The dune fields were eventually stabilized by grass. Due to the erosive nature of loess soil and its ability to stand in vertical columns when dry, the stabilized dunes were eroded into the corrugated, sharply dissected bluffs we see today.
The dominant features of this landscape are "peak and saddle" topography, "razor ridges" (narrow ridges, often less than 10 feet (3 m) wide, which fall off at near ninety-degree angles on either side for 60 feet (18 m) or more), and "cat-step" terraces (caused by the constant slumping and vertical shearing of the loess soil). The soil has a characteristic yellow hue and is generally broken down into several units based on the period of deposition (Loveland, Pisgah, Peoria). Loess is known locally as "sugar clay" because it can be extremely hard when dry, but when wet, loses all cohesion. The Loess Hills of Iowa are remarkable for the depth of the drift layer, often more than 90 feet (27 m) deep. The only comparable deposits of loess to such an extent are located in Shaanxi, China.
Today, the hills stretch from Westfield, Iowa in the north to Mound City, Missouri in the south. Loess topography can be found at various points in extreme eastern portions of Nebraska and Kansas along the Missouri River Valley, particularly near the Nebraska cities of Brownville, Rulo, Plattsmouth, Fort Calhoun, and Ponca. The hills are usually no more than 200 feet (61 m) above the Missouri River bottoms. However, in some areas, such as Murray Hill in Harrison County, Iowa, the Loess Hills can rise over 350 feet (110 m) above the adjacent Missouri River floodplain.
The Loess Hills have abundant oak-hickory hardwood forests and some of the last remaining stands of prairie grass in the region. The invasion of prairie and oak savanna areas by woodland species such as red cedar (not native to the Hills) is threatening the stability of the fragile soils, as well as diminishing the native ecosystems found there. The areas of native prairie comprise big bluestem and little bluestem, Indian grass, sideoats grama, and forbs such as yucca, pasque flower and lead plant (false indigo). Many of the prairie species found in the Loess Hills are outside of their normal range of distribution, with plants like spiny-leafed yucca and prickly pear cactus being more common further west, in the Sandhills of central Nebraska. In 1984, a previously undiscovered fern, the Prairie Moonwort, was discovered in the Hills and is thought to be endemic to the Hills.