In addition to Muir's theory that Yosemite Valley was created by glacial carving he also believed that there were still "living glaciers" in the high Sierra. In his August-October 1872, Yosemite Creek, etc. journal, he notes that the stakes he placed in a straight line across the McClure Glacier on August 21 were now pushed down the mountain indicating a moving glacier and not simply a patch of snow.
For more information...
Read Geology Underfoot in Yosemite National Park, 2010 by Allen F. Glazner and Greg M. Stock.
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