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The Josemite By John Muir

Chapter 11
The Ancient Yosemite Glaciers:
How the Valley Was Formed

All California has been glaciated, the low plains and valleys as well as the mountains. Traces of an ice-sheet, thousands of feet in thickness, beneath whose heavy folds the present landscapes have been molded, may be found everywhere, though glaciers now exist only among the peaks of the High Sierra. No other mountain chain on this or any other of the continents that I have seen is so rich as the Sierra in bold, striking, well-preserved glacial monuments. Indeed, every feature is more or less tellingly glacial. Not a peak, ridge, dome, cañon, yosemite, lake-basin, stream or forest will you see that does not in some way explain the past existence and modes of action of flowing, grinding, sculpturing, soil-making, scenery-making ice. For, notwithstanding the post-glacial agents--the air, rain, snow, frost, river, avalanche, etc.--have been at work upon the greater portion of the Range for tens of thousands of stormy years, each engraving its own characters more and more deeply over those of the ice, the latter are so enduring and so heavily emphasized, they still rise in sublime relief, clear and legible, through every after-inscription. The landscapes of North Greenland, Antarctica, and some of those of our own Alaska, are still being fashioned beneath a slow-crawling mantle of ice, from a quarter of a mile to probably more than a mile in thickness, presenting noble illustrations of the ancient condition of California, when its sublime scenery lay hidden in process of formation. On the Himalaya, the mountains of Norway and Switzerland, the Caucasus, and on most of those of Alaska, their ice-mantle has been melted down into separate glaciers that flow river-like through the valleys, illustrating a similar past condition in the Sierra, when every cañon and valley was the channel of an ice-stream, all of which may be easily traced back to their fountains, where some sixty-five or seventy of their topmost residual branches still linger beneath protecting mountain shadows.

The change from one to another of those glacial conditions was slow as we count time. When the great cycle of snow years, called the Glacial Period, was nearly complete in California, the ice-mantle, wasting from season to season faster than it was renewed, began to withdraw from the lowlands and gradually became shallower everywhere. Then the highest of the Sierra domes and dividing ridges, containing distinct glaciers between them, began to appear above the icy sea. These first river-like glaciers remained united in one continuous sheet toward the summit of the Range for many centuries. But as the snow-fall diminished, and the climate became milder, this upper part of the ice-sheet was also in turn separated into smaller distinct glaciers, and these again into still smaller ones, while at the same time all were growing shorter and shallower, though fluctuations of the climate now and then occurred that brought their receding ends to a standstill, or even enabled them to advance for a few tens or hundreds of years.

Meanwhile, hardy, home-seeking plants and animals, after long waiting, flocked to their appointed places, pushing bravely on higher and higher, along every sun-warmed slope, closely following the retreating ice, which, like shreds of summer clouds, at length vanished from the new-born mountains, leaving them in all their main, telling features nearly as we find them now.

Tracing the ways of glaciers, learning how Nature sculptures mountain-waves in making scenery-beauty that so mysteriously influences every human being, is glorious work.

The most striking and attractive of the glacial phenomena in the upper Yosemite region are the polished glacier pavements, because they are so beautiful, and their beauty is of so rare a kind, so unlike any portion of the loose, deeply weathered lowlands where people make homes and earn their bread. They are simply flat or gently undulating areas of hard resisting granite, which present the unchanged surface upon which with enormous pressure the ancient glaciers flowed. They are found in most perfect condition in the subalpine region, at an elevation of from eight thousand to nine thousand feet. Some are miles in extent, only slightly interrupted by spots that have given way to the weather, while the best preserved portions reflect the sunbeams like calm water or glass, and shine as if polished afresh every day, notwithstanding they have been exposed to corroding rains, dew, frost, and snow measureless thousands of years.

The attention of wandering hunters and prospectors, who see so many mountain wonders, is seldom commanded by other glacial phenomena, moraines however regular and artificial-looking, cañons however deep or strangely modeled, rocks however high; but when they come to these shining pavements they stop and stare in wondering admiration, kneel again and again to examine the brightest spots, and try hard to account for their mysterious shining smoothness. They may have seen the winter avalanches of snow descending in awful majesty through the woods, scouring the rocks and sweeping away like weeds the trees that stood in their way, but conclude that this cannot be the work of avalanches, because the scratches and fine polished strife show that the agent, whatever it was, moved along the sides of high rocks and ridges and up over the tops of them as well as down their slopes. Neither can they see how water may possibly have been the agent, for they find the same strange polish upon ridges and domes thousands of feet above the reach of any conceivable flood. Of all the agents of whose work they know anything, only the wind seems capable of moving across the face of the country in the directions indicated by the scratches and grooves. The Indian name of Lake Tenaya is "Pyweak"--the lake of shining rocks. One of the Yosemite tribe, Indian Tom, came to me and asked if I could tell him what had made the Tenaya rocks so smooth. Even dogs and horses, when first led up the mountains, study geology to this extent that they gaze wonderingly at the strange brightness of the ground and smell it, and place their feet cautiously upon it as if afraid of falling or sinking.

In the production of this admirable hard finish, the glaciers in many places flowed with a pressure of more than a thousand tons to the square yard, planing down granite, slate, and quartz alike, and bringing out the veins and crystals of the rocks with beautiful distinctness. Over large areas below the sources of the Tuolumne and Merced the granite is porphyritic; feldspar crystals in inch or two in length in many places form the greater part of the rock, and these, when planed off level with the general surface, give rise to a beautiful mosaic on which the happy sunbeams plash and glow in passionate enthusiasm. Here lie the brightest of all the Sierra landscapes. The Range both to the north and south of this region was, perhaps, glaciated about as heavily, but because the rocks are less resisting, their polished surfaces have mostly given way to the weather, leaving only small imperfect patches. The lower remnants of the old glacial surface occur at an elevation of from 3000 to 5000 feet above the sea level, and twenty to thirty miles below the axis of the Range. The short, steeply inclined cañons of the eastern flank also contain enduring, brilliantly striated and polished rocks, but these are less magnificent than those of the broad western flank.

One of the best general views of the brightest and best of the Yosemite park landscapes that every Yosemite tourist should see, is to be had from the top of Fairview Dome, a lofty conoidal rock near Cathedral Peak that long ago I named the Tuolumne Glacier Monument, one of the most striking and best preserved of the domes. Its burnished crown is about 1500 feet above the Tuolumne Meadows and 10,000 above the sea. At first sight it seems inaccessible, though a good climber will find it may be scaled on the south side. About half-way up you will find it so steep that there is danger of slipping, but feldspar crystals, two or three inches long, of which the rock is full, having offered greater resistance to atmospheric erosion than the mass of the rock in which they are imbedded, have been brought into slight relief in some places, roughening the surface here and there, and affording helping footholds.

The summit is burnished and scored like the sides and base, the scratches and strife indicating that the mighty Tuolumne Glacier swept over it as if it were only a mere boulder in the bottom of its channel. The pressure it withstood must have been enormous. Had it been less solidly built it would have been carried away, ground into moraine fragments, like the adjacent rock in which it lay imbedded; for, great as it is, it is only a hard residual knot like the Yosemite domes, brought into relief by the removal of less resisting rock about it; an illustration of the survival of the strongest and most favorably situated.

Hardly less wonderful is the resistance it has offered to the trying mountain weather since first its crown rose above the icy sea. The whole quantity of post-glacial wear and tear it has suffered has not degraded it a hundredth of an inch, as may readily be shown by the polished portions of the surface. A few erratic boulders, nicely poised on its crown, tell an interesting story. They came from the summit-peaks twelve miles away, drifting like chips on the frozen sea, and were stranded here when the top of the monument merged from the ice, while their companions, whose positions chanced to be above the slopes of the sides where they could not find rest, were carried farther on by falling back on the shallowing ice current.



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