Post 00529
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Among the interesting and influential of the secondary features of cañon scenery are the great avalanche taluses, that lean against the walls at intervals of a mile or two. In the middle Yosemite region they are usually from three to five hundred feet high, and are made up of huge, angular, well-preserved, unshifting boulders, overgrown with gray lichens, trees shrubs, and delicate flowering plants. Some of the largest of the boulders are forty or fifty feet cube, weighing from five to ten thousand tons; and where the cleavage joints of the granite are exceptionally wide apart a few blocks may be found nearly a hundred feet in diameter. These wonderful boulder piles are distributed throughout all the cañons of the range, completely choking them in some of the narrower portions, and no mountaineer will be likely to forget the savage roughness of the roads they make. Even the swift, overbearing rivers, accustomed to sweep everything out of their way, are in some places bridled and held in check by them. Foaming, roaring, in glorious majesty of flood, rushing off long rumbling trains of ponderous blocks without apparent effort, they are not able to move the largest, which, withstanding all assaults for centuries, are left at rest in the channels like islands, with gardens on their tops, fringed with foam below, with flowers above.
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.