The lengths of alpine glaciers quickly adjust to changes in temperature and precipitation, making them sensitive indicators of climate.
Recording changes in length of current glaciers is a matter of visual observation. However, for glaciers that existed in the past, documenting the changes in length often requires rare geological and glaciological circumstances.
The Animas River drainage of Colorado's San Juan Mountains allows documentation of the demise of a large alpine valley glacier from its Last Glacial Maximum extent. Approximately 20,000 years before present (B.P.), the Animas Valley glacier lobe of the San Juan Ice cap extended 91 kilometers from the ice divide, and pervasively polished crystalline bedrock.
To document the retreat history of this glacier, Guido et al. measured the concentration of cosmogenically produced 10Be in polished bedrock to deduce the duration of exposure to cosmic rays since the glacier receded past each of eight locations. This yielded a record of retreat that began at approximately 19,500 years B.P and ended at roughly 12,500 years B.P., when the San Juan Mountains became largely devoid of ice.
This history implies that the demise of the Animas Valley glacier was protracted, coincided with a gradual rise in solar radiation, and was perhaps fastest during a time of regional drying recorded in shoreline elevations of lakes in western North America.
Reference: "Pacing the post--Last Glacial Maximum demise of the Animas Valley glacier and the San Juan Mountain ice cap," Zackry S. Guido et al., University of Colorado, Geology, Pages 739-742.
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