These hills are located just east of the Missouri River mainly in Iowa. They rise nearly 200 feet (61 m) above the flat plains and form a narrow band of hills running north-south 200 miles (320 km) along the Missouri River. They are the first rise in land beyond the flood plain, forming something of a "front range" for Iowa, and parts of Missouri and Nebraska adjacent to the Missouri River.
The word loess comes from German meaning loose or crumbly and in this case it refers to a windblown silt that once covered this area. It is a gritty, lightweight, porous material composed of tightly packed grains of quartz, feldspar, mica, and other minerals. Loess is eolian (deposited by the wind) rather than fluvial (deposited by a river) or lacustrine (formed in a lake)
The Loess Hills are comprised of three major layers. From oldest to youngest, the layers are known as the Loveland Loess, (120,000 to 159,000 years old), the Pisgah Loess (25,000 to 31,000 years old), and the Peoria Loess (12,500-25,000 years old)
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. Can you imagine dune fields here in Iowa? The dune fields were eventually stabilized by grass and then bushes and trees. Due to the erosive nature of loess soil and its ability to stand in vertical columns when dry, the stabilized dunes were eroded into corrugated, sharply dissected bluffs that now make up these hills.
The dominant features of this landscape are "peak and saddle" topography, "razor ridges" (narrow ridges, often less than ten feet wide, which fall off at near ninety-degree angles on either side for 60 feet or more), and "cat-step" terraces (caused by the constant slumping and vertical sheering 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 ninety feet deep. The Loess Hills of Iowa are extremely fragile having one of the highest erosion rates in the U.S., almost 40 tons/acre/ year. Because the hills were deposits from Ice-Age winds they hold a record of past climates. Scientists are gathering geologic information critical to testing computer models of past climates. Come explore these marvelous hills. Iowa has designated a Scenic byway through the area that is well worth visiting.
Questions based on signage at this location:
1. Where is the only other place in the world with loess hills similar to these?
2. Based on the pictures on the signs how would you describe the change in the hills over the last 15,000 years?
3. What were the names of the two glacial periods when huge quantities of loess accumulated to create these hills?
4. How long ago was this?