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It is evident that most of the great cañons of the Sierra Nevada have been formed by erosion—that is, by the wearing influence of stream of water, and of the matter which they have carried down. But the Yosemite Valley is an exception. These vertical walls, half a mile deep, with numerous sharp angles, could scarcely have been formed by water, and the narrowness of the cañon below shows that there never has been any sufficient outlet for a large stream. Besides, it is evident that, since the main chasm was formed, great masses of rock have been split off, in many places, from the sides, and if the valley had been the result of erosion, these masses would have made mountains in it. The most probable explanation of its origin is the theory that it was formed by a great convulsion which tore apart the mountain to a depth very much greater than that now perceptible, that vast masses of rock fell down into the chasm, and that after a time they were covered up by the washing of the waters, leaving a level valley over them.
Professor Whitney, in the State Geological Survey Report, says:
“It appears to us probable that this mighty chasm has been roughly hewn into the present form by the same kind of forces which have raised the crest of the Sierra and moulded the surface of the mountains into something like their present shape. The Domes and such masses as that of Mount Broderick, we conceive to have been formed by the process of upheaval itself, for we can discover nothing about them that looks like the result of ordinary denudation. The Half Dome seems, beyond a doubt, to have been split asunder in the middle, the lost half having gone down in what may truly be said to have been ‘the wreck of matter and the crash of worlds.’ * * * If the bottom of the Yosemite ‘did drop out,’ to use a homely but expressive phrase, it was not all done in one piece or with one movement; there are evidences in the valley of fractures and cross-fractures at right angles to these, and the different segments of the mass must have been of quite different sizes, and may have descended to unequal depths.”
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