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Debris-Covered Glaciers in the Sierra Nevada, California, and Their Implications for Snowline Reconstructions

Published online by Cambridge University Press:  20 January 2017

Douglas H. Clark ,
Malcolm M. Clark  and
Alan R. Gillespie
Douglas H. Clark
Affiliation:
Department of Geological Sciences, AJ-20, University of Washington, Seattle, Washington 98195 E-mail:doug@oz.geology.washington.edu
Malcolm M. Clark
Affiliation:
U.S. Geological Survey, M/S 977, 345 Middlefield Road, Menlo Park, California 94025
Alan R. Gillespie
Affiliation:
Department of Geological Sciences, A J-20, University of Washington, Seattle, Washington 98195
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Abstract

Ice-walled melt ponds on the surfaces of active valley-floor rock glaciers and Matthes (Little Ice Age) moraines in the southern Sierra Nevada 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 greatly reduce rates of ablation relative to that of uncovered ice. Such insulation explains the observations that ice-cored rock glaciers in the Sierra, actually debris-covered glaciers, are apparently less sensitive to climatic warming and commonly advance to lower altitudes than do adjacent bare-ice glaciers. Accumulation-area ratios and toe-to-headwall-altitude ratios used to estimate equilibrium-line altitudes (ELAs) of former glaciers may therefore yield incorrect results for cirque glaciers subject to abundant rockfall. Inadvertent lumping of deposits from former debris-covered and bare-ice glaciers partially explains an apparently anomalous regional ELA gradient reported for the pre-Matthes Recess Peak Neoglacial advance. Distinguishing such deposits may be important to studies that rely on paleo-ELA estimates. Moreover, Matthes and Recess Peak ELA gradients along the crest evidently depend strongly on local orographic effects rather than latitudinal climatic trends, indicating that simple linear projections and regional climatic interpretations of ELA gradients of small glaciers may be unreliable.

Type
Articles
Information
Quaternary Research , Volume 41 , Issue 2 , March 1994 , pp. 139 - 153
Copyright
University of Washington

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