FIRST REPORT OF WIDESPREAD POLYGONAL GROUND IN NEBRASKA AND ITS ASSOCIATION WITH EOLIAN LANDFORMS

This study identifies numerous fields (as large as 65 ha) of both well-defined and indistinct relict patterned ground (RPG) on flattish upland surfaces in the middle Niobrara River and Keya Paha valleys in Boyd and Cherry counties in Nebraska, and northwestward toward St. Francis, South Dakota. Individual polygons are rectangles and slightly irregular pentagons, hexagons, and heptagons 4 to 50 m in maximum width. Indistinct RPG is defined as incomplete polygons, rounded polygon outlines, or “blotchy” land-surface patterns. RPG-bearing surfaces are on exposed or very shallow bedrock, chiefly of the Ash Hollow Formation (upper Miocene). The visibility of RPG depends on seasonal and yearly environmental conditions and land use (as is typically the case elsewhere). RPG is discernible in high-resolution digital aerial imagery taken within the past 15 years, but chiefly in 2013 and 2015. Some RPG can even be resolved in 1967 wet-film aerial photographs under magnification. The RPG described herein formed under periglacial conditions during the Late Pleistocene, when a periglacial zone extended approximately150 km southwest from the margin of the Laurentide Ice Sheet.

Observations also suggest an extensive imprint of eolian processes on landscapes north of the Niobrara River in Nebraska. Rolling uplands consist of shallow canyons and low divides with prominent NW–SE orientations, which are oriented roughly parallel to the strong Pleistocene winds that shaped the Nebraska Sand Hills south of the river. Broad, elongate, bedrock ridges on divides and as high as 24 m are interpreted as yardangs. Fields of small dunes alternate with RPG-bearing areas of very shallow or exposed soft bedrock in the uplands, and even the geometries of these two landform associations locally NW–SE trends. Tributary canyons leading from the uplands to the river also trend NW–SE. Mappable exposures of the slightly resistant Cap Rock Member (Ash Hollow Formation) are mostly on S- to SW-facing segments of canyon rims, suggesting a subordinate slope-aspect geomorphic control.

The association of Pleistocene periglacial and eolian features, first realized by W.J. Wayne in the 1990s, is widespread in northern Nebraska. Nebraska’s landscapes were shaped by eolian erosion and periglaciation far more than conventional wisdom holds.