EXTENSIVE BOULDER EROSION RESULTING FROM A RANGE FIRE ON THE TYPE-PINEDALE MORAINES, FREMONT LAKE, WYOMING

In July 1988, the Flatfish Range Fire burned over the type-Pinedale mo raines at Fremont Lake, Wyoming, and caused extensive exfoliation of e xposed boulder surfaces. The mass of exfoliated material from 130 of 1 030 boulders investigated was measured and recorded with information c oncerning factors that could influence the extent of fire-induced exfo liation. The range in thickness of material removed from 98 randomly s elected boulders within the burn area (averaged over the entire expose d boulder surface area) is 6.1 to < 0.1 mm. The mean thickness loss fo r all 98 boulders is 0.9 mm/fire and the expected loss from individual boulders (median) is 0.4 mm/fire. At the 95% confidence level there i s no significant relationship between the degree of exfoliation and bo ulder size, lithology, grain size, proximity to vegetation, or vegetat ion density. The expected fire-induced boulder surface erosion rates r ange from 5.9 to 0.3 x 10(-3) mm/yr on boulders in sagebrush rangeland where fire recurrence intervals are typically every 20 to 400 yr. Fir e-induced exfoliation may account for differences in boulder size and abundance on Pinedale and Bull Lake moraines. Surface dating methods u sing varnish or cosmogenic nuclides may yield exposure ages that are t oo young if the consequences of range fires are not considered when sa mpling boulder surfaces that are within about 2 m above ground level. (C) 1994 University of Washington.