The Grain of the Plains Remains: Widespread Northwest-Southeast Ridgelines and Associated Oriented Landforms, Northern Interior Plains, USA

We have mapped >200,000 roughly northwest-southeast-oriented ridgelines, >10,000 deflation basins, the outlines of small dune fields, and associated, presumptively aeolian landforms across an area of 660,000 km² between 39° and 48° N and 93° to 108° W in the USA.  Our study area lies east of the Rocky Mountain front and, effectively, west of the Late Pleistocene limit of Laurentide glaciation. It includes parts of Iowa, Missouri, Kansas, Nebraska excluding the Sand Hills, Colorado, Wyoming, South Dakota, North Dakota, and Montana.  The oriented ridgelines lie within the western periphery of the Central Lowlands in the Dissected Till Plains (where multiple pre-Illinoian ice advances occurred) and across a large part of the Great Plains (including some of the glaciated Coteau Slope in North Dakota). Most of the ridgelines are developed on Cretaceous, Paleogene, and Neogene sedimentary rocks and Pleistocene loesses.  Northwest-southeast-oriented ridgelines are a prominent, and in some cases the dominant component (e.g., on the Pierre Shale in northernmost Nebraska and much of western South Dakota, and on loess in parts of western Iowa and east-central Nebraska) of local to regional landscapes.  Thus, the ridges impart a pronounced geomorphic “grain.”  Oriented ridgelines may be sharp- or broad-crested, straight to slightly sinuous, and they range in length from 0.1 to 15 km.  We interpret the oriented ridgelines on pre-Pleistocene strata to have resulted primarily from aeolian erosion.  Those on loess may have been produced by a combination of erosion and deposition.

Multiple lines of evidence support a hypothesis that the oriented ridgelines were produced chiefly by wind erosion during the Late Pleistocene.  The study area encompasses many Pleistocene–Holocene dune fields, including the largest dune field in the Western Hemisphere (Nebraska Sand Hills). There are also widespread dust deposits, such as the Peoria Loess.  Erosional features, ranging from local ventifacts to widely distributed deflation basins, that have already been identified as aeolian features in the study area.  Moreover, the orientations of deflation-basin axes, the long axes of small dune fields, and other presumptively aeolian features tend to accord with the general trend of the oriented ridgelines, suggesting a commonality in their geomorphic origins.  Additionally, other authors have interpreted MIS/OIS (29–14 ka), at the end of the Pleistocene, as a time of dry, windy, and dusty conditions, during which vegetation cover was reduced at least locally in the interior of North America.