Transport direction of Peoria Loess in Nebraska and implications for loesssources on the central Great Plains

In the midwestern United States, large rivers draining the Laurentide Ice S heet (LIS) were the most important sources of Peoria Loess, deposited durin g the last glaciation. Loess deposition near those rivers may have responde d primarily to ice-sheet dynamics rather than direct effects of climatic ch ange. In contrast, it has been proposed that thick Peoria Loess on the cent ral Great Plains was derived mainly from unglaciated landscapes northwest o f the main loess deposits. In this study, transport directions inferred fro m more than 600 measurements of Peoria Loess thickness in Nebraska are used to test the hypothesis that much of the Peoria Loess on the Great Plains i s nonglaciogenic. A strong northwest to southeast thickness trend indicates that most Peoria Loess in Nebraska was transported from one or more unglac iated northwestern source areas rather than from glacially influenced river floodplains. The Missouri River (draining the LIS), the Platte River (drai ning alpine glaciers), and the Elkhorn River (unglaciated basin) were secon dary sources. Their contribution is not detectable beyond a distance of 40- 60 km. Peoria Loess deposition on the central Great Plains was largely a di rect response to climatic change in the unglaciated source region. (C) 2001 University of Washington.