LANDFORMS GENERATED BY WIND EROSION OF NAVAJO SANDSTONE OUTCROPS AT THE WAVE (COLORADO PLATEAU, UTAH/ARIZONA BORDER)

Along the eroded crest of the East Kaibab Monocline, sand driven by strong southwesterly winds has abraded Navajo Sandstone outcrops, generating distinctive features on both a small and a large scale. The exceptional exposure of Jurassic sedimentary structures in this area has resulted from sand-blasting of friable outcrops. Sloping rock surfaces commonly display a “stepped” morphology: steep, laterally continuous, 3-4 mm-high risers face the dominant wind direction and are spaced 10-15 cm apart; they cut Navajo Sandstone sedimentary structures at a high angle. The steps may be the erosional equivalent of wind ripples: high-energy impacts cut the steps, and each step shields a downwind flat from the low-angle approach of impacting grains.

Where echo dunes approach steep, southwest-facing sandstone escarpments, eddying airflow has cut deep recesses and alcoves that are tens of meters wide and up to 10 m high. Fluted scours line the inner walls of alcoves and a dune or an abraded, conical sandstone mass occupies the central floor. In one alcove, rockfalls have generated large arches in abrasion-thinned walls. The Wave, a site within Vermillion Cliffs National Monument that is well-known to hikers and photographers, lies within a kilometer (below and downwind) of three adjacent alcoves. At The Wave, abrasion by saltating and suspended sand has removed a sandstone wall that once stood between two adjacent, wind-abraded “channels”. Removal of the wall generated an undrained depression with four points of entry. Iron-oxide-cemented sandstone clasts form a lag on the floor of the depression. Sand that exits The Wave accumulates in a falling dune that lies directly northeast of the site.

Although sand-blasting may be an ineffective agent of erosion over broad areas of the Colorado Plateau, it is important to landscape development at our sites because Navajo Sandstone outcrops there are dominated by thick, very weakly cemented eolian grainflows. These outcrops are much more friable than those dominated by finer-grained, less-well-sorted, and better-cemented wind-ripple strata. Removal of grains from the friable sandstone provides a positive feedback for the erosion process because it supplies additional tools for abrasion.