Nebraska Soils Field Trip - Virginia Tech

4th IUSS Soil Classification Conference Field Tour Guidebook
Overview of the Loess Hills
By Richard Lensch
Soil Scientist (Retired)
Iowa Loess Hills
The loess hills of western Iowa are divided into two Major Land Resource areas-MLRA
107A and MLRA 107B. MLRA107A is located on the Northern Iowa drift plane and MLRA107B is
located on the Southern Iowa drift plane.
The Northern Iowa drift plane, which occupies Northwestern Iowa, was covered by the
Tazewell ice sheet approximately 40,000 BP. Little modification of the landscape occurred
between the recession of the early Wisconsin ice sheet and the beginning of Wisconsin loess
deposition about 31,000 years BP. It is characterized by broad loess covered flats with glacial till
at depths of 1 and 8 meters.
In stark contrast to the Northern Iowa drift plane, the Southern Iowa drift plane is a much
more mature landscape that is heavily dissected by geologic erosion. Erosion began on the
Southern Iowa drift plane in Pre-Illinoian time, and is characterized by hills with narrower
interfluves and steeper side slopes than the Northern Iowa drift plane. Loess thickness ranges
from more than 22 meters to 2 meters on the Southern Iowa drift plane.
Loess deposition started approximately 159,000 yeas BP on the Southern Iowa drift
plane. 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). (1)
Thickness of the loess, in MLRA 107B, ranges in from more than 22 meters at the bluffs
to about 5 meters at the eastern boundary. As the loess thickness thins with the distance from
the source area, sand content and coarse silt to fine silt ratios decrease, and clay content
increases, within the solum. Soil solums show increased weathering and maturity as the distance
increases from the source area.
Erosion has exposed other geologic formations beneath the loess, such as glacial
deposits of sand and gravel, glacial till, and paleosols. Limestone and shale bedrock can be
observed in deep quarries along the western edge of the loess hills. A thin layer of volcanic ash
can be seen in some road cuts. This ash layer is the result of eruption of a mega volcano near
Yellowstone National Park in Wyoming more than 640,000 years ago.
In MLRA 107B, gullies are an integral part of the landscape. Loess is eroded from the
side hills and deposits this sediment in drainage ways and on floodplains. Many gullies can be
many miles long and pose a serious problem. Some gullies in the Monona-Ida association, near
the loess bluffs can be more than 100 feet wide, and as deep as 80 feet. Bridges and roads can
collapse as the gullies widen restricting travel.
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(1) U.S. Department of the Interior; U.S.
Geological Survey Information Handout,
July 1999. This page is
http://pubs.usgs.gov/info-handout/loess/.
Maintained by Eastern Publications Web
Team. Last revised 8-18-99