Post 00054
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On some points concerning the origin of these taluses I was long in doubt. Plainly enough they were derived from the cliffs above them, the size of each talus being approximately measured by a scar on the wall, the rough angular surface of which contrasts with the rounded, glaciated, unfractured parts. I saw also that, instead of being slowly accumulated material, weathered off, boulder by boulder, in the ordinary way, almost every talus had been formed suddenly, in a single avalanche, and had not been increased in size during the last three or four centuries; for trees three or four hundred years old were growing on them, some standing at the top close to the wall, without a bruise or broken branch, showing that scarcely a single boulder had fallen among them since they were planted. Furthermore, all the taluses throughout the range seemed, by the trees and lichens growing on them, to be of the same age. All the phenomena pointed straight to a grand ancient earthquake. But I left the question open for years, and went on from cañon to cañon, observing again and again; measuring the heights of taluses throughout the range on both flanks, and the variations in the angles of their surface slopes; studying the way their boulders were assorted and related and brought to rest, and the cleavage joints of the cliffs from whence they were derived, cautious about making up my mind. Only after I had seen one made did all doubt as to their formation vanish.
In Yosemite Valley, one morning about two o’clock, I was aroused by an earthquake; and though I had never before enjoyed a storm of this sort, the strange, wild thrilling motion and rumbling could not be mistaken, and I ran out of my cabin, near the Sentinel Rock, both glad and frightened, shouting, “A noble earthquake!” feeling sure I was going to learn something. The shocks were so violent and varied, and succeeded one another so closely, one had to balance in walking as if on the deck of a ship among the waves, and it seemed impossible the high cliffs should escape being shattered. In particular, I feared that the sheer-fronted Sentinel Rock, which rises to a height of three thousand feet, would be shaken down, and I took shelter back of a big pine, hoping I might be protected from outbounding boulders, should any come so far. I was now convinced that an earthquake had been the maker of the taluses, and positive proof soon came. It was a calm moonlight night, and no sound was heard for the first minute or two save a low muffled underground rumbling and a slight rustling of the agitated trees, as if, in wrestling with the mountains, Nature were holding her breath. Then, suddenly, out of the strange silence and strange motion there came a tremendous roar. The Eagle Rock, a short distance up the valley, had given way, and I saw it falling in thousands of the great boulders I had been studying so long, pouring to the valley floor in a free curve luminous from friction, making a terribly sublime and beautiful spectacle,—an arc of the fifteen hundred feet span, as true in form and as steady as a rainbow, in the midst of the stupendous roaring rock storm. The sound was inconceivably deep and broad and earnest, as if the whole earth, like a living creature, had at last found a voice and were calling to her sister planets. It seemed to me that if all the thunder I ever heard were condensed into one roar it would not equal this rock roar at the birth of a mountain talus. Think, then, of the roar that arose to heaven when all the thousands of ancient cañon taluses throughout the length and breadth of the range were simultaneously given birth.
The main storm was soon over, and, eager to see the new-born talus, I ran up the valley in the moonlight and climbed it before the huge blocks, after their wild fiery flight, had come to complete rest. They were slowly settling into their places, chafing, grating against one another, groaning, and whispering; but no motion was visible except in a stream of small fragments pattering down the face of the cliff at the head of the talus. A cloud of dust particles, the smallest of the boulders, floated out across the whole breadth of the valley and formed a ceiling that lasted until after sunrise; and the air was loaded with the odor of crushed Douglas spruces, from a grove that had been mowed down and mashed like weeds.