Abstract

Alluviation along the North and Middle Loup rivers in Valley County, Nebraska, produced a series of alluvial silt beds on which are developed five interstadial soils of Brady (?), Cary and Mankato (?), Mankato (?), early Recent and late Recent age. No deposits of Illinoian age were found and in at least one place the Brady (?) soil is superposed on a gleyed horizon developed on the Sappa formation, which overlies the Grand Island formation. Post-Yarmouth erosion removed the Sappa formation and part of the Grand Island formation at most places. The characteristic Sangamon soil was not developed on uneroded remnants of the soil of Yarmouth age because of a wet, poorly drained environment. A large collection of molluscs obtained from this gley soil supports stratigraphic evidence indicating a Yarmouth age.

Alluviation during the glacial substages of Wisconsin time and soil formation during the interglacial substages were uninterrupted by erosion until after development of the soil of Mankato age. The Wisconsin deposits were dissected to below the Brady soil in places, and the resulting gullies were filled with later alluvium. The late Recent fill terrace lies 15 feet below the top of the late Wisconsin terrace. Subsequent cutting and filling in late Recent time produced another terrace 11 feet below the older terrace. The modern gully was cut during the last century.

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