Solving a Periglacial Puzzle: Pleistocene Polygonal Ground in North-Central Nebraska

ABSTRACT:This article is the first documentation of widespread, discontinuous relict polygonal ground (RPG) in north-central Nebraska and southern South Dakota. RPG formed under periglacial (persistently cold, but not glacial) conditions during the Late Pleistocene. It is now discernible chiefly in high-resolution digital aerial imagery taken within the past 15 years, although some fields of RPG were identified ex post facto during this study in wet-film aerial photographs taken in 1967 and viewed under magnification. Fields (as large as 65 ha) of both well-defined and indistinct RPG exist on comparatively stable, 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. These surfaces are on exposed or very shallow bedrock, chiefly of the Ash Hollow Formation of the Ogallala Group (upper Miocene). The present-day visibility of RPG depends on seasonal and yearly environmental conditions and land use. Individual polygons are rectangles and slightly irregular pentagons, hexagons, and heptagons 4 to 50 m in maximum width. These characteristics are shared with extant periglacial polygonal ground. Our results verify that Late Pleistocene periglacial conditions existed in a zone extending some 150 km southwest of the Laurentide Ice Sheet.