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The Loess Hills: A Geologic View
 

Red ball iconThe Loess Hills: A Geologic View

Compiled by Jean Cutler Prior and Deborah J. Quade

 

Landform regions map

Landform Regions of Iowa

The Loess Hills of western Iowa are one of the state’s seven major topographic regions. The steep, sharply ridged hills extend in a narrow band along the length of the Missouri River valley. The level lowland of the Missouri Alluvial Plain is a separate landform region, yet it is intricately tied to the geological origins of the Loess Hills. (Geological Survey Bureau illustration)

 

Loess Hills and Alluvial Plain satellite photo

Loess Hills and Alluvial Plain topography

This color-infrared photo shows the meandering Big Sioux River, its cultivated floodplain, and a sharp boundary with the adjacent Loess Hills region. The hills display alternating ridge crests, a dense network of drainageways, and irregular land-use patterns. The photo was taken in April 1980 from an altitude of 40,000 feet, approximately 17 miles northwest of Sioux City. (Iowa Geological Survey photo)

 

3-dimensional diagram of loess hills

Inside view of the Loess Hills

This 3-dimensional diagram illustrates the geologic materials found beneath the Loess Hills. The landscape is composed of unusually thick deposits (60-150 ft) of wind-blown silt known as loess. The silt was carried by the wind from the floor of the Missouri Valley following periods of glacial meltwater flooding. The accumulated loess was later carved by erosion into narrow ridges and steep sideslopes. The loess mantles older glacial deposits consisting of pebbly clay and beds of sand and gravel. (Illustration by Pat Lohmann)

 

Underlying Bedrock

Layers of sedimentary rock, reflecting ancient marine and coastal environments, are occasionally seen in the Loess Hills, usually in quarries and roadcuts along the bluffs of the Missouri and Big Sioux valleys. The northern Loess Hills region is underlain by shales and chalky limestones of Cretaceous age (90 million years old). From Harrison County on south, the region is underlain by limestones of Pennsylvanian age (310 million years old).

 

Plymouth County aerial view

Plymouth County aerial view

The slopes of this loess ridge are crossed with a series of stair-like features known as "catsteps." Native prairie occupies the drier, exposed summits and sideslopes, while trees and shrubs grow along the more moist, protected ravines and backslopes. (Gary Hightshoe, Iowa State University photo)

 

Monona County aerial view

Monona County aerial view

The crinkled topography of the Loess Hills exhibits narrow ridge crests, branching sidespurs, steep slopes, and a dense drainage network. In the background, the floodplain of the Little Sioux River joins the Missouri River Valley. Interesting patterns of vegetation and land use reflect the changes in topography. (Gary Hightshoe, Iowa State University photo)

 

gully

Gully erosion

Loess is highly erodible and unstable when wet, producing serious hazards to land use in the region. Deep, narrow gullies, which can lengthen and widen quickly after rainstorms, are characteristic erosional features of drainageways. (Stan Mitchem photo)

 

Woodbury County landscape

Woodbury County landscape

Steeply pitched, prairie-covered slopes are seen west of Smithland in Woodbury County. These picturesque peaks and saddles are sculpted from thick deposits of windblown silt that were swept from the nearby Missouri River valley following episodes of glacial meltwater flooding. (Don Poggensee photo)

 

Loess exposure

Loess exposure

Loess is uniformly gritty in texture, dominated by silt-sized particles that are composed mostly of quartz. The lightweight loess also tends to stand in nearly vertical faces when exposed, often forming slabs and columns as it erodes. Three major episodes of deposition include: 1) the Peoria Loess (12,500 to 21,000 years old), which is the thickest and most common loess unit in Iowa; 2) the Pisgah Formation (24,000 to 42,000 years old); and 3) the Loveland Loess, which accumulated 140,000 to 160,000 years ago. (Brian Witzke photo)

 

Loess kindchen

Loess kindchen

Loess deposits sometimes contain "pebbles" called "loess kindchen" (or "loess dolls"). These nodules of lime (calcium carbonate) range in size from peas to baseballs or grapefruit. They were formed by infiltrating precipitation that dissolved and leached carbonate grains in the loess. As water moved downward, the lime was redeposited around some nucleus to form the unusually shaped concretions. (John McNeilly photo)

 

County Line ash site

County Line ash site

A roadcut along the base of the loess bluffs at the Harrison-Monona County line reveals a grayish layer of volcanic ash, another interesting wind-deposited sediment within the Loess Hills region. The ash deposit is approximately 15 inches thick and originated 620,000 years ago from a now-extinct volcano complex in the Yellowstone National Park area of Wyoming. (Jean C. Prior photo)

 

Fossil Clam

Fossil Clam

This fossil clam (Inoceramus sp.) is found in the thin-bedded chalky limestones of Cretaceous age that occur close to the land surface in the northern Loess Hills. Exposures of these sedimentary rocks can be seen in the Sioux City area. (Paul VanDorpe photo)

 

Cretaceous strata exposure

Cretaceous strata near Stone State Park

Limestone beds belonging to the Greenhorn Formation of Cretaceous age are seen at the top of this bluff near the entrance to Stone State Park. The remaining strata include shale and thin limestones of the Graneros Formation, and sandstones of the underlying Dakota Formation. These marine deposits formed as the last of the great inland seas advanced eastward over Iowa about 90 million years ago. (Greg Ludvigson photo)