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Ruck glaciers—some comments about age

Published online by Cambridge University Press:  30 January 2017

Paul Carrara
Paul Carrara*
Affiliation:
U.S. Geological Survey, Denver Federal Center, Lakewood, Colorado 80225, U.S.A.
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Type
Correspondence
Information
Journal of Glaciology , Volume 22 , Issue 87 , 1979 , pp. 411
Copyright
Copyright © International Glaciological Society 1979

The Editor,

Journal of Glaciology

Sir,

I wish to make several comments concerning the age of rock glaciers in the San Juan Mountains of Colorado. Work in this region suggests that many rock glaciers may have originated in late Pinedale time, before 10000 b.p.

Glacial ice apparently disappeared from valleys in the San Juan Mountains before 9000 b.p. Reference MaherMaher (1972) obtained ages of 15450 220 b.p. (Y-1147) and 13 360±120 b.p. (Y-1437) for organic material in Molas Lake, at 3200 m, indicating that the San Juan highland icefield near Molas Divide was dissipated before 15500 b.p. Radiocarbon dates from bogs within 1 km of the cirque headwalls (9 620± 400, St-3909; 8785 ± 95, SI-1241; 8430 ± 85, SI-1558) suggest that ice was confined to northeastward-facing cirques along the Continental Divide in the San Juan Mountains before 9000 b.p. (Reference Andrews, Andrews, Carrara, King and StuckenrathAndrews and others, 1975; Reference Carrara and AndrewsCarrara and Andrews, [1976]), The same authors found no evidence of post-altithermal glacial activity in this mountain range.

Many rock glaciers may have originated in late Pinedale time prior to 9000 b.p. On the basis of pollen analysis and bog stratigraphy (Reference Andrews, Andrews, Carrara, King and StuckenrathAndrews and others, 1975; Maher, unpublished), it appears that the most severe periglacial climate in Holocene time occurred immediately after Pinedale deglaciation, between 15000 and 9000 b.p. Reference Wahrhaftig and CoxWahrhaftig and Cox (1959) noted that rock glaciers in the Alaska Range approximately 500 m below present snowline are currently active. Hence, during Pinedale deglaciation the perennial snowline may have been above the land surface and yet conditions conducive to formation and/or preservation of rock glaciers may have persisted. In the San Juan Mountains, it is argued that the time immediately after Pinedale deglaciation was the most likely time of origination of rock glaciers. Relative weathering of clasts on several rock glaciers in the San Juan Mountains indicates that the rock glaciers may have originated prior to the altithermal (8000–5000 b.p.) (Reference Carrara and AndrewsCarrara and Andrews, [1976]). Reference BirkelandBirkeland (1973) also cited evidence for the formation of Pinedale rock glaciers on Mount Sopris in the Elk Range of Colorado.

The majority of rock glaciers in the San Juan Mountains are presently active. If they were formed in late Pinedale time, as I believe, it is not necessary to assume that they were inactive during the altithermal. The altithermal may not have been significantly warmer than the present in the San Juan Mountains (Reference Andrews, Andrews, Carrara, King and StuckenrathAndrews and others, 1975; Maher, unpublished). Because rock glaciers are relatively insensitive to climatic change (Reference PotterPotter, 1972). it does not seem likely that they were deactivated during the altithermal. Hence it is suggested that many rock glaciers in the San Juan Mountains are time- transgressive periglacial features that originated during the late Pinedale and have been active to the present.

Paul Carrara

U.S. Geological Survey,

Denver Federal Center

Lakewood. Colorado 80225, U.S.A.

30 June 1978

References

Andrews, J. T., and others. 1975. Holocene environmental changes in the alpine zone, northern San Juan Mountains, Colorado: evidence from bog stratigraphy and palynology, by Andrews, J. T., Carrara, P. E., King, F. B., and Stuckenrath, R. Quaternary Research. Vol. 5, No. 2, p. 173–97.Google Scholar
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