DEBRIS-COVERED GLACIERS IN THE SIERRA-NEVADA, CALIFORNIA, AND THEIR IMPLICATIONS FOR SNOWLINE RECONSTRUCTIONS

Ice-walled melt ponds on the surfaces of active valley-floor rock glac iers and Matthes (Little Ice Age) moraines in the southern Sierra Neva da indicate that most of these landforms consist of glacier ice under thin (ca. 1-10 m) but continuous covers of rock-fall-generated debris. These debris blankets effectively insulate the underlying ice and gre atly reduce rates of ablation relative to that of uncovered ice. Such insulation explains the observations that ice-cored rock glaciers in t he Sierra, actually debris-covered glaciers, are apparently less sensi tive to climatic warming and commonly advance to lower altitudes than do adjacent bare-ice glaciers. Accumulation-area ratios and toe-to-hea dwall-altitude ratios used to estimate equilibrium-line altitudes (ELA s) of former glaciers may therefore yield incorrect results for cirque glaciers subject to abundant rockfall. Inadvertent lumping of deposit s from former debris-covered and bare-ice glaciers partially explains an apparently anomalous regional ELA gradient reported for the pre-Mat thes Recess Peak Neoglacial advance. Distinguishing such deposits may be important to studies that rely on paleo-ELA estimates. Moreover, Ma tthes and Recess Peak ELA gradients along the crest evidently depend s trongly on local orographic effects rather than latitudinal climatic t rends, indicating that simple linear projections and regional climatic interpretations of ELA gradients of small glaciers may be unreliable. (C) 1994 University of Washington.