Iowa’s Loess Hills
What is Loess?
The Loess Hills of Iowa is a geologic formation spanning most of the western edge of the state. Loess (pronounced "luss") is a German word meaning "loose" and it is the name of a type of soil. Loess is a deposit of fine, yellowish-gray, clay-like sediment which can be found from north central Europe to eastern China and in the American Midwest. Loess deposits are especially common at the edges of large river basins and are generally thought to be made up of material carried by winds that went through the area during and after glacial periods.
What Are the Loess Hills of Iowa?
The story of the Loess Hills of Iowa started more than 25,000 years ago when a large glacier began to retreat from the area. As the glacier melted, water filled the Missouri River valley. When the water level dropped, large amounts of silt were left behind. Much of that silt was swept up by winds and dropped to the east of the Missouri River Valley. Most of the loess piled up within two to ten miles of the river in a corridor running about 200 miles north to south along the river valley, creating the Loess Hills of Iowa.
How the Loess Hills Were Formed
Twelve to thirty thousand years ago glaciers were moving and melting over parts of the Iowa landscape and the states to the north. Due to changes in temperature, the front of the glaciers would melt in the summer and huge amounts of meltwater would flow down the Missouri River valley.
In the wintertime, the glaciers stopped melting. The flow of water into the rivers slowed down significantly. This exposed sandbars and silt material on the floor of the valley.
Wind Creates Hills
Winds from the west were very strong during these times and they would whip through the Missouri River valley, pick up the exposed silt material and deposit it on the east side of the valley. This cycle was repeated over thousands of winters until about 12,000 years ago.
The glaciers disappeared and the wind diminished. The Loess Hills of Iowa remained. As time marched on, water carved the hills by eroding the silt. Creeks and rivers, fed by rainwater and snowmelt, rushed down the slopes, creating most of the distinctive shapes of the Loess Hills that we see today.
What makes the Loess Hills of Iowa a unique landscape is the depth of the loess. Only one other location in the world, near the Yellow River in China, has loess deposits greater than the 100 to 200 foot depths in the Loess Hills of Iowa. This makes the Loess hills unique and globally significant. Endangered animals and rare prairie grasses can be found on these lands. These hills are very fragile and are vulnerable to water erosion. Human activity and tree invasion are also a concern. They cause the Loess Hill's prairie ecosystem to shrink—several species are in danger of extinction.
The Prairie Ecosystem in the Loess Hills
The Loess Hills are home to some of the largest tracts of prairie in the state. Prairies are landscapes where the soil, weather, and other conditions favor grasses over trees. Although prairie areas can be found anywhere along the Loess Hills, they dominate the southern and western slopes. These areas are exposed to hot summer afternoon sun and drying winds that make most plants wither and die. Grasses dominate the area because they can survive dry climates, fire and grazing better than trees. Prairie plant adaptations include a deep root system and narrow leaves that reduce water loss, allowing them to survive where other plants can't.
Threats to the Hills
Small towns, big cities, farmers, business owners, parks and preserves combine to make the Loess Hills a huge working landscape. Even though humans have worked this land for hundreds of years, its unique qualities have only come into the spotlight in the last 30 years. Current threats include erosion, mining, urban sprawl, poor conservation practices, and bad land-use decisions.