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Nebraska Soils Field Trip - Virginia Tech

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4th IUSS Soil Classification Conference <strong>Field</strong> Tour Guidebook<br />

Overview of the Loess Hills<br />

By Richard Lensch<br />

Soil Scientist (Retired)<br />

Iowa Loess Hills<br />

The loess hills of western Iowa are divided into two Major Land Resource areas-MLRA<br />

107A and MLRA 107B. MLRA107A is located on the Northern Iowa drift plane and MLRA107B is<br />

located on the Southern Iowa drift plane.<br />

The Northern Iowa drift plane, which occupies Northwestern Iowa, was covered by the<br />

Tazewell ice sheet approximately 40,000 BP. Little modification of the landscape occurred<br />

between the recession of the early Wisconsin ice sheet and the beginning of Wisconsin loess<br />

deposition about 31,000 years BP. It is characterized by broad loess covered flats with glacial till<br />

at depths of 1 and 8 meters.<br />

In stark contrast to the Northern Iowa drift plane, the Southern Iowa drift plane is a much<br />

more mature landscape that is heavily dissected by geologic erosion. Erosion began on the<br />

Southern Iowa drift plane in Pre-Illinoian time, and is characterized by hills with narrower<br />

interfluves and steeper side slopes than the Northern Iowa drift plane. Loess thickness ranges<br />

from more than 22 meters to 2 meters on the Southern Iowa drift plane.<br />

Loess deposition started approximately 159,000 yeas BP on the Southern Iowa drift<br />

plane. The Loess Hills are comprised of three major layers. From oldest to youngest, the layers<br />

are known as the Loveland Loess, (120,000 to 159,000 years old), the Pisgah Loess (25,000 to<br />

31,000 years old), and the Peoria Loess (12,500-25,000 years old). (1)<br />

Thickness of the loess, in MLRA 107B, ranges in from more than 22 meters at the bluffs<br />

to about 5 meters at the eastern boundary. As the loess thickness thins with the distance from<br />

the source area, sand content and coarse silt to fine silt ratios decrease, and clay content<br />

increases, within the solum. Soil solums show increased weathering and maturity as the distance<br />

increases from the source area.<br />

Erosion has exposed other geologic formations beneath the loess, such as glacial<br />

deposits of sand and gravel, glacial till, and paleosols. Limestone and shale bedrock can be<br />

observed in deep quarries along the western edge of the loess hills. A thin layer of volcanic ash<br />

can be seen in some road cuts. This ash layer is the result of eruption of a mega volcano near<br />

Yellowstone National Park in Wyoming more than 640,000 years ago.<br />

In MLRA 107B, gullies are an integral part of the landscape. Loess is eroded from the<br />

side hills and deposits this sediment in drainage ways and on floodplains. Many gullies can be<br />

many miles long and pose a serious problem. Some gullies in the Monona-Ida association, near<br />

the loess bluffs can be more than 100 feet wide, and as deep as 80 feet. Bridges and roads can<br />

collapse as the gullies widen restricting travel.<br />

59<br />

(1) U.S. Department of the Interior; U.S.<br />

Geological Survey Information Handout,<br />

July 1999. This page is<br />

http://pubs.usgs.gov/info-handout/loess/.<br />

Maintained by Eastern Publications Web<br />

Team. Last revised 8-18-99

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