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[click to enlarge]
On Mount Lyell still remain remnants
of ancient glaciers which once swept
down into the canyons of Yosemite, reshaping the mountains as they moved
ALL of the human history of Yosemite can be compressed into a trifling two or three centuries, but the drama of its creation stretches grandly over millions and multi-millions of years. All that man has done and seen in Yosemite, from Indian days until now, is but a paragraph in that more colossal story. From a handful of rocks taken from the summit and the base of a mountain and from a study of its fissures, scientists have unfolded a story of mountain-making and canyon-carving in the Sierras which staggers the imagination.
Long ago, they say, in our dim geologic past, a great block of the earth’s crust was tilted up on edge due to some tremendous internal pressure, making the Sierra Nevada, one solid granite block three hundred miles long and eighty miles wide. Like a cement sidewalk which has been upended by an earthquake, one side of this mountain rose sharp and steep, while the other sloped gradually towards the west. Accompanying this lift came a sinking and caving in the earth’s central portion, which finally settled thousands of feet below its outer edge, making the Great Basin of California.
This pushing up of a mountain range caused the rivers which had been flowing lazily across the plains to tilt too and become raging mountain torrents which cut deeply into their former channels. In this way river-beds were converted into small canyons and small canyons, in thousands of years, widened out into mountain valleys.
The two largest of these rivers in the Yosemite region were the Merced and the Tuolumne, which immediately began a race against each other and against Time. Deeper and deeper they scoured and sandpapered their way into the mountain’s side.
By the end of a hundred thousand years or so they had run their first lap and slowed down a bit. Water is always seeking its own level and eventually these two rivers had cut their way through the mountain until they were flowing more horizontal than vertical. Then Nature gave another lurch and tilted them up once more, starting the race again, setting the rivers to cutting deeper and faster through the granite walls. A third time this happened, and that final convulsion hoisted the crest of the Sierras eight thousand feet in one series of movements, twice the height of Half Dome as it towers above the floor of the Valley!
For thousands and thousands of years then the old battle for supremacy between land and water was waged in the Sierras. Rivers ground down and pulverized mountain peaks, only to carry the sediment in mountain dust down into the ocean, where, layer upon shifting layer, new mountains were slowly being built up again. Out over the ocean the rain-clouds were born which dropped their moisture in the mountains, swelling the rivers, which carried the earth into the sea again. It is a battle which goes on forever.
Then water in another form came to complete the story of Yosemite. Snows packed on the summits of the Sierras, year after year, gradually turned into ice and spread in great tongues down into the canyons, filling the channels where water once flowed with rivers of sluggishly moving ice. Ice, ice, everywhere! Like a wrinkled cloak it covered all the Sierras except the loftiest peaks, pushing and grinding down into the valleys.
Tenaya, Illilouette, and Merced canyons all had their own glaciers which met and formed one of the first Yosemite mergers, at the upper end of the Valley, packing it from wall to wall with its weight of grinding ice and embedded rock as it flowed slowly towards El Portal. And as it moved, that clumsy, frozen sea fashioned of a rugged, humpy mountain a splendor of pinnacles, spires, domes, and arches. Scraping at the base of cliffs, plucking away the less resistant quartz and slate which overlay the granite core of the world, it loosened disjointed rocks, planing and chiseling as it went until it broadened the Valley from the letter V to a U. Sloping walls were smoothed perpendicular, and cascades were turned into plunging waterfalls.
Picture the scene! Somber gray skies above a world of ice, with here and there a stark granite peak like Half Dome or El Capitan thrusting through! Ice and stone, and not a living thing, a twisted tree or a blade of grass, as far as the eye could see! That was Yosemite in the making!
Two and probably three major glaciers visited Yosemite Valley, with long intervals of milder climate between. For the seasons in Yosemite then were only two, the ice years and the nice years. One of these glaciers which filled the Valley ended in Bridal Veil Meadows, stopped by El Capitan on one side and Cathedral Rock on the other. The second pushed through these barriers almost as far as El Portal. And as they came and went they cut giant stairways in the river-beds. Vernal and Nevada Falls are a splendid example of these steps, with a hollow or tread beneath each which once held a lake as the ice melted and receded. Tenaya Canyon descended in four levels, in each of which a lake once stood, the lowest and longest being Mirror Lake, which is slowly filling in now with sediment brought down by the river. Little Yosemite Valley, like Yosemite Valley itself, was once filled to the brim with water left in the wake of the melting glaciers. The High Sierras are still dotted with glacial tarns which are slowly giving way to meadows. Infinitesimal changes become drastic when they accumulate through thousands of years. Marshes become meadows, and meadows give way to the forests, which in turn climb up the granite peaks. Time is unmeasured in the mountains, where Nature moves majestically, a hundred years to one of Man’s brief days.
Roughly speaking, then, Yosemite was formed first by an uplift which made the Sierras, then by stream erosion which cut the Valley, and finally by ice, which carved it deeper and scraped it smoother. But in its final analysis the features of Yosemite as we know them today, Cathedral Spires, Three Brothers, El Capitan, and the Royal Arches, were determined by the very substance of which they are made, their rocky structure, or “bones.”
Granite is one of the hardest substances in the world, pushed up from the very heart of the earth. Where the rock was pushed up in great solid masses, with few fissures, or cracks, it was better able to stand erosion, grinding, and weathering, and by the tearing away of less resistant rocks around them such masses as El Capitan and Sentinel Rock were etched out against the skyline.
Where those fissures or cleavage joints were vertical, great slabs of stone were more easily plucked out, forming the sheer, straight cliffs. Where the joints were curved we find the domes and spires and arches of Yosemite, which resisted the glacial plucking. Hummocks, too, have been rounded into domes by the action of water freezing in the joint cracks, causing expansion and the shaling off of thick layers of rock like a peeling.
“What happened to Half Dome?”
Everyone wonders that, of course, for there is something so fragmentary about its shape it is difficult to believe it wasn’t formed by some special cataclysm. The Indians tried to explain it through their legends. Scientists have explained it repeatedly, but for a long time they could not agree upon which explanation to accept! Rangers have even explained it to credulous tourists by telling them it was made by a glacier, which has gone back now for the other half!
But the simple explanation seems to be that Half Dome is made up of two systems of joints, one curved, the other vertical. Double-jointed! And so it presents a flat surface on one side, smoothed off by the glaciers, and a rounded one on the other, which, through long weathering, has cast off successive shells, like an onion, or an artichoke.
This process of exfoliation is going on all the time, and some day the world will awaken to discover that the overhang or eyebrow on Half Dome has vanished, for that, scientists believe, is the next scale to fall. But when this happens it will no longer concern us, or our children, and probably not even our children’s children.
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