Thus a
small rapid stream with abundance of loose transportable material within
its reach may fill up an extensive basin in a few centuries, while a
large perennial trunk stream, flowing over clean, enduring pavements,
though ordinarily a hundred times larger, may not fill a smaller basin
in thousands of years.
The comparative influence of great and small streams as lake-fillers is
strikingly illustrated in Yosemite Valley, through which the Merced
flows. The bottom of the valley is now composed of level meadow-lands
and dry, sloping soil-beds planted with oak and pine, but it was once a
lake stretching from wall to wall and nearly from one end of the valley
to the other, forming one of the most beautiful cliff-bound sheets of
water that ever existed in the Sierra. And though never perhaps seen by
human eye, it was but yesterday, geologically speaking, since it
disappeared, and the traces of its existence are still so fresh, it may
easily be restored to the eye of imagination and viewed in all its
grandeur, about as truly and vividly as if actually before us. Now we
find that the detritus which fills this magnificent basin was not
brought down from the distant mountains by the main streams that
converge here to form the river, however powerful and available for the
purpose at first sight they appear; but almost wholly by the small local
tributaries, such as those of Indian Canon, the Sentinel, and the Three
Brothers, and by a few small residual glaciers which lingered in the
shadows of the walls long after the main trunk glacier had receded
beyond the head of the valley.
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