Pleistocene Ventifacts East of the Big Horn Mountains, Wyoming

Abstract

Ancient ventifacts are abundant on gravel-mantled erosion surfaces along the eastern base of the Big Horn Mountains. Stones of varied lithologic characteristics and from less than 1 inch to 6 feet in diameter have wind-cut surfaces displaying the pitting, fluting, grooving, and luster that is characteristic of ventifacts. Faces making angles greater than 550 with the wind are pitted, and faces at lower angles are grooved and fluted. As many as twenty separate faces have been cut on a single stone. In this area such multiple faces cannot be explained by wind-splitting or by variable winds, so they are attributed to changes in stone position caused largely by congeliturbation (frost action) and wind scour. Ventifacts rest on erosion surfaces at four levels, 25-325 feet above stream grade. On the lower surfaces wind-cut stones are abundant and fresh, but on the higher surfaces they are sparse and deeply weathered. At least four separate periods of wind-cutting are indicated by this evidence. Cutting is attributed to sand picked up from barren flats and hurled against stones lying in the same environment. Conditions most favorable for wind-cutting prevailed during times of glaciation in the mountains, when winds were strong and floods of meltwater inundated parts of the piedmont, producing broad barren flats mantled by sand and gravel. Ventifacts on the lower erosion surfaces are probably Wisconsin, and those on the highest surfaces may be pre-Wisconsin. Pleistocene wind directions have been measured at twenty-nine separate localities by careful reference to wind-cut faces on large, presumably stable, boulders. The mean Pleistocene wind direction so determined is N. 290 W. Modern winds are also consistently from the northwest, and it appears that local orographic control was supreme in the Pleistocene as now. Neither glaciers in the Big Horn Mountains nor the continental ice sheet, 250 miles north, exerted much influence on local wind directions.