Nebraska Soils Field Trip - Virginia Tech

4th IUSS Soil Classification Conference Field Tour Guidebook
Figure WS3 presents the general loess thickness in the Midcontinent. The thickest loess occurs
in western Iowa and south central NE. Two loess sources account for this distribution. One
source is the Missouri Valley (Ruhe et al., 1967; Simonson and Hutton, 1954; Muhs and Bettis,
2000), which served as glacial outlet beginning, at least during Illinoisan time (Mason, 2006.).
Aleinikoff et al. (1998, 1999), using geochemical analysis showed that Peoria Loess across
Nebraska came from non-glacial sources of the western Great Plains (White River and Arikeree
siltstones). Mason (2001) reached similar conclusions using Peoria Loess thickness trends.
Landscape Evolution
Despite widespread occurrence of glacial and loess deposits, the field area is an erosional
landscape that displays dissection and a well-integrated drainage. The terrain resembles
erosional landscapes developed solely on relatively flat-lying sedimentary rocks. As the
landscape declines in elevation from higher primary divides to lower secondary divides, younger
stratigraphic units are truncated by erosion and older units are exhumed and exposed at the
land surface (Hallberg, 1980). Ruhe1967, following extensive field study, developed a soil
landscape and geomorphic model for southwest IA. This model is appropriate to the field area.
Fig WS4 depicts this model.
Four geomorphic surfaces descend from stream divides to major drainages. The uppermost
surface is blanketed by loess that overlies a paleosol developed on till or a clay-rich material
known as gumbotil (Kay, 1916), which is interpreted as weathering product from till. . The clayrich
material may alternatively represent an early loess deposit (Woida and Thompson, 1993).
Ruhe (1967) named the paleosol for time period it represented, the Yarmouth Sangamon (YS)
paleosol (Yarmouth is the classical North American interglacial between Kansan and Illinoisan
glaciations).
Episodic erosion cut a pediment below the YS surface and the associated materials. This
geomorphic surface Ruhe (1967) named the late-Sangamon Pediment. In Ruhe’s study area
(Greenfield quadrangle, Adair County) the Loveland Loess is absent, elsewhere this erosion
cycle removes or truncates the Loveland Loess. Hence, the paleosol formation period spans
only a portion of the Sangamon interglacial and Ruhe refers to it as the Late Sangamon. The
paleosol under this surface is formed in transported material called pedisediment (Ruhe and
Cady, 1954). A traceable stone line lies below the pedisediment, which in turn is underlain by
pre-Illinoisan till.
Episodic erosion produced a second pediment during early Wisconsin time. This erosion cycle
removed the pre-existing soil. The modern soil on this surface has reformed in the truncated,
less-weathered pre-Illinoisan till. Wisconsin age loess originally covered this surface, but
subsequent hillslope erosion removed it at the site shown in Figure WS4.
An erosional down cutting cycle accompanied by headward stream incision and hillslope
backwearing began in late Wisconsin time. All loess deposition predates this surface, hence
modern soils form into the truncated pre-Illinoisan till. This erosional surface descends to
Holocene alluvial deposits. This surface Ruhe (1967) called the late-Wisconsin dissection
slope.
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