EROSIONAL PROCESSES AND LANDFORMS IN BADLANDS NATIONAL MONUMENT, SOUTH DAKOTA

Abstract

Small-scale erosional features in the Big Badlands of South Dakota include miniature mountain ranges and pediments, with morphology resembling that of mature mountainous desert regions of the southwestern United States. Miniature badland pediments have steeper gradients than large-scale pediments of the arid southwest, but angles of mountain fronts average about the same for both—approximately 35°. There is an abrupt reduction of slope where the miniature mountain front meets its bordering pediment. The miniature mountain drainage is poorly integrated; it consists of closely spaced rill channels having the same gradient as the mountain slope. At the expense of the retreating miniature mountain front the pediment is extended headward by erosion at the margins of spreading sheetwash. The escarpment retreats by disaggregation through absorption and desiccation and by rill erosion. Topographic texture varies with underlying bedrock. Topography developed on the Chadron formation is fine-textured; that on the Brule formation is ultra-fine-textured. Badlands drainage is typically dendritic with regularly bifurcating finger-tip tributaries. The mean maximum angle of slope in typical Chadron topography is 26.2°, with estimated standard deviation of 4.2°. For the Brule formation mean maximum slope angles of 56.0°, 49.5°, and 51.0° were obtained in three areas. A t test revealed that the differences between means of Brule and Chadron are significant. Each slope mean represents an equilibrium condition in which the slope has been adjusted to achieve a steady state between the resistivity of the material to erosion and the intensity of erosional processes.