Wind Abrasion and Ventifact Formation in California

Abstract

Ventifacts are found in several physical settings in California: in formerly glaciated areas, in periglacial areas above or beyond glacier limits, in presently semiarid areas, along the coast, and in true deserts. In several localities, both active and fossil forms are found. Ventifacts and abraded surfaces develop wherever strong winds, laden with abundant sediment, erode resistant boulders or bedrock. In California, as elsewhere, the abrasive agent is most commonly a fine- to medium-grained aeolian sand. In arid areas, ventifacts occur near to Pleistocene lake shorelines, downwind of alluvial rivers, near dune fields, or in corridors of regional sand transit. They formed principally during a drier middle Holocene period from 8 to 5 ka. Staining and discoloration of the abraded face, patchy granular disintegration and spalling of abraded surfaces, and stabilized aeolian sand provide evidence for current inactivity of wind erosion. Ventifacts found on moraines beyond receding ice fronts are much older, dating from Pleistocene cold stages.

A long-term study in the Little Cowhole Mountains of the Mojave Desert seeks to further understand ventifact formation by studying active processes. Contemporary winds are monitored by a weather station and 11 anemometers. Strong winds blow from the north and south, and to date maximum gusts of 27 m s⁻¹ have been recorded. Grooves are aligned with the strongest winds and ventifacts are abraded on the windward face: along the topographic ridge a sharp keel separates the facets. Polished ventifact surfaces that are very smooth to the eye or touch are quite rough at the microscale. Scanning electron microscopy shows that sand grains abrade by chipping, breaking along cleavage planes, rubbing, and microgouging. High-energy impacts near boulder tops causes cleavage to be the main process of abrasion, whereas near the boulder base rubbing and microgouging dominate. Future work will examine the formation of flutes and helices, abrasion maxima heights in hilly terrain, rates of abrasion, and the evolution of ventifact form.