SURFICIAL GEOLOGIC MAPPING IN THE MISSOURI NATIONAL RECREATIONAL RIVER IN NEBRASKA AND LANDSCAPE EVOLUTION AT THE EASTERN MARGIN OF THE GREAT PLAINS

Mapping in the Missouri National Recreational River corridor in northeastern Nebraska, funded by the National Park Service, clarifies geologic and geomorphic relationships. We identify ~900 landslides, ranging from <1 to 33 ha, and almost entirely in the Pierre Shale, on: (1) the upland rim north of the Niobrara River (NR), (2) two segments of the south valley wall of the Missouri River (MR), and (3) within the lower Verdigre (VC) and Bazile (BC) creek basins.

The tops of late Pleistocene fill-terraces on both sides of the NR are at ~497 m MSL, ~90 m above present river level; sands east of NR and VC lie at similar elevations. The terrace on the north side of the NR is dissected. The terrace on the south side is nearly continuous and at least 8 km in width. Broad, south-bank terraces are the rule upstream on the NR for hundreds of kilometers. North of the NR, Ponca Creek (PC) also flows into the MR, but there is only a single remnant of high-elevation fluvial sands on the PC-MR divide, which is intensely dissected. The PC-MR divide is marked by NNW-SSE-oriented ridges (and subsidiary, shorter NW-SE crests), which appear to coincide with the axes of strong late Pleistocene winds. There is an isolated remnant of a fill terrace, consisting of glacially derived, gravelly to bouldery sand, ~85 m above the present MR on the north side of the PC-MR divide; there is also a discernible NNW-SSE grain in topography east of the NR. More than one hundred small (<1-46 ha) alluvial fans are mapped along MR, on the south side of NR, and along VC and BC. There are very few remnants of Ogallala Group strata (Miocene) in the uplands.

We conclude that late Pleistocene landscape evolution at the easternmost margin of the Great Plains involved: (1) deep incision of the MR and NR, (2) differential dissection of the PC-MR and NR-PC divides, (3) eolian erosion of the Pierre Shale on the PC-MR divide, and (4) widespread mass movements on valley-side slopes after they were steepened.