MASS MOVEMENTS ON NW-SE TRENDING YARDANG-LIKE RIDGES IN NORTH-CENTRAL NEBRASKA, USA

NW-SE-trending ridges (~30 m high and 0.2-2.5 km long) are prominent on the landslide-prone Pierre Shale in Boyd County, Nebraska and adjacent areas. These ridges are interpreted as eolian erosional features (yardangs) because they lie roughly parallel to strong Late Pleistocene and Holocene winds inferred from the nearby Sand Hills and from modern wind roses. The markedly asymmetrical, transverse cross-sections of most of the ridges formed in shale (steep NE-facing slopes and gentle SE-facing slopes) suggest that secondary factors influenced their development. Oriented features developed on other geologic materials (loess, sand, and Ogallala Group bedrock) in the same area are more subdued and do not show the same kind of slope asymmetry.

Field observations and examination of aerial and satellite imagery demonstrate that slumps, slides, and terracettes are common on the NE-facing slopes of oriented ridges on the Pierre Shale, some of which have convex basal slope segments directly attributable to mass movement. Mass movements can be documented yearly on Pierre Shale slopes in the study area, both on the side slopes and the noses of the NW-SE oriented ridges, as well as on the north-facing valley wall of the Missouri River.

Various dynamics of eolian erosion may have contributed to the asymmetry of the yardangs on Pierre Shale. However, we interpret much of the causality of slope asymmetry to slope aspect and microclimate. Relative to SW-facing slopes, NE-facing slopes should: (1) undergo much less desiccation from dry southerly to southwesterly winds, (2) have more persistent winter snow cover, and (3) experience saturation in the shallow soil or subsoil more frequently (perhaps facilitated by deep cracks in weathered smectitic shale) and over longer intervals during the year. NE-facing slopes, although unforested, frequently have more woody vegetation than do SW-facing slopes. This vegetation pattern cannot, however, be verified as an original condition in the GLO survey maps of the area (ca. 1890) and may have emerged only after widespread Euramerican settlement.

The distribution of mass movements in this area illustrates that subtle differences in aspect should be considered as factors which can influence slope stability and long-term landscape evolution.

This research was supported through USGS STATEMAP.