THE RIDGES OF BOYD COUNTY: ORIENTED LANDFORMS AND LATE QUATERNARY EOLIAN PROCESSES IN NORTH-CENTRAL NEBRASKA, USA

Oriented topography trending NW-SE is prominent in Boyd County, Nebraska, west of the Laurentide glacial limit. Oriented streamlined ridges and intervening narrow valleys are prominent on the Ponca Creek-Niobrara River divide (PND) between the towns of Spencer and Monowi, where they have been eroded from the Cretaceous Pierre Shale. Several of these ridges have thin caps of late Pleistocene loess, which locally overlies alluvial sands, some of which have been reworked by wind. Single oriented ridgelines can be traced for ~0.2-2.5 km, and the intervening valleys range from ~0.12-0.45 km in width. The upper slope on the NE-facing side of a ridge is usually significantly steeper than the gentler, continuous slope of the SW-facing side. Terracettes, slumps, and slides are prominent on NE-facing slopes. Oriented valleys on the PND show a parallel drainage pattern of dry or ephemeral streams.

Nearby areas with slightly thicker or more widespread deposits of loess and sand exhibit subdued oriented topography as well as NW-SE streaks in aerial photographs. Some first-order drainages trend parallel to these oriented features. Further to the west on the PND, between the towns of Spencer and Butte, Nebraska, subtle NW-SE-oriented ridges, as well as shallow valleys, appear on loess, sand, and even bedrock of the Neogene Ogallala Group (the Harvey Buttes). A subtler NW-SE “grain” in the landscape is also visible to the east on the PND into Knox County, where loess-mantled interfluves are broad and flat. Oriented topography and streaks in aerial photographs persist northward on the Ponca Creek-Missouri River divide.

Considering the similarity of these features to others described from Nebraska, and the predominant Late Pleistocene-Holocene wind directions interpreted from dunes in the Sand Hills, we attribute the oriented topography of Boyd County to late Quaternary eolian erosion. Mass wasting and other processes have modified the ridges, in part according to slope aspect. Similarly oriented eolian erosion landforms appear to be widespread in northeastern to north-central Nebraska, and the effects of eolian erosion and deposition should be considered simultaneously in interpretations of landscape development. This research was supported through the U.S. Geological Survey’s STATEMAP cooperative mapping program.