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Transition from shear moraines to rock glaciers

Published online by Cambridge University Press:  30 January 2017

P. E. Carrara
P. E. Carrara*
Affiliation:
Institute of Arctic and Alpine Research, University of Colorado, Boulder, Colorado 80302, U.S.A.
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Abstract

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Type
Correspondence
Information
Journal of Glaciology , Volume 12 , Issue 64 , 1973 , pp. 149
Copyright
Copyright © International Glaciological Society 1973

SIR,

Several authors have discussed the origin of rock glaciers and stated that they are of glacial origin, in other words, a stagnant glacier which had become debris-covered (Reference BrownBrown, 1925; Reference Outcalt and BenedictOutcalt and Benedict, 1965). The fact that rock glaciers may formerly have been active ice glaciers would seem rather welt established by Reference BrownBrown (1925). Miners tunneling into a rock glacier in the San Juan Mountains of Colorado exposed a central core of ice, which Brown concluded to be of glacial origin.

If an ice glacier can in time evolve into a rock glacier, the question then arises of how it becomes so ladened with superglacial debris. In many cases the inactive or stagnant glacier may become covered by rock falls from the surrounding slopes.

However, the possibility exists that the glacier may be responsible for its own surficial cover. If the glacier’s terminus is stationary (a condition which would favor the formation of shear moraines), shear moraines may develop. As the glacier slowly retreats a succession of shear moraines may form up-valley, producing what is referred to as a rock glacier.

It is known that many rock glaciers possess arcuate ridges in their terminal areas. Reference Wahrhaftig and CoxWahrhaftig and Cox (1959) stated that these ridges form when the basal shear of the rock debris exceeds the internal friction of the material. However, Wahrhaftig and Cox believed rock glaciers to consist of debris cemented with interstitial ice, and not old glacial ice covered by debris. For those rock glaciers which are indeed old glaciers covered by debris these arcuate ridges may well be a series of shear moraines.

A photograph in the paper by Reference Outcalt and BenedictOutcalt and Benedict (1965) shows debris bands in the ice of a rock glacier. These debris bands may well be shear planes which would be one mechanism for obtaining a surficial mantle on the ice body.

This idea is put forward only as a theory and certainly not as a rule of formation for rock glaciers.

20 July 1972

References

Brown, W. H. 1925. A probable fossil glacier. Journal of Geology, Vol. 33, No. 4, p. 464–66.CrossRefGoogle Scholar
Outcalt, S. I., and Benedict, J. B. 1965. Photo-interpretation of two types of rock glacier in the Colorado Front Range, U.S.A. Journal of Glaciology, Vol. 5, No, 42, p. 849–56.Google Scholar
Wahrhaftig, C., and Cox, A. 1959. Rock glaciers in the Alaska Range. Bulletin of the Geological Society of America. Vol. 70, No. 4, p. 383–436.Google Scholar