STUDIES IN THE SIERRA*

BY JOHN MUIR

NO. V. POST-GLACIAL DENUDATION

WHEN

HEN Nature lifted the ice-sheet from the mountains she may well be said not to have turned a new leaf, but to have made a new one of the old. Throughout the unnumbered seasons of the glacial epoch the range lay buried, crushed, and sunless. In the stupendous denudation to which it was then subjected, all its pre-glacial features disappeared. Plants, animals, and landscapes were wiped from its flanks like drawings from a blackboard, and the vast page left smooth and clean, to be repictured with young life and the varied and beautiful inscriptions of water, snow, and the atmosphere.

The variability in hardness, structure, and mineralogical composition of the rocks forming the present surface of the range has given rise to irregularities in the amount of postglacial denudation effected in different portions, and these irregularities have been greatly multiplied and augmented by differences in the kind and intensity of the denuding forces, and in the length of time that different portions of the range have been exposed to their action. The summits have received more snow, the foothills more rain, while the middle region has been variably acted upon by both of these agents. Again, different portions are denuded in a greater or less degree according to their relations to level. The bottoms of trunk valleys are swept by powerful rivers, the branches by creeks and rills, while the intervening plateaus and ridges are acted upon only by thin, feeble currents, silent and nearly invisible. Again some portions of the range are subjected every winter to the scouring action of avalanches, while others are entirely beyond the range of such action. But the most influential of the general causes that have conspired to produce ir

* Reprinted, as revised by the author, from the Overland Monthly of November, 1874.

TEN LAKE BASIN, YOSEMITE NATIONAL PARK

"Ten lovely lakelets lying near together in one general hollow, like eggs in a nest."--JOHN MUIR Photo by Wm. E. Colby

regularity in the quantity of post-glacial denudation is the difference in the length of time during which different portions of the range have been subjected to denuding agents. The icesheet melted from the base of the range tens of thousands of years ere it melted from the upper regions. We find, accordingly, that the foothill region is heavily weathered and blurred, while the summit, excepting the peaks, and a considerable portion of the middle region remain fresh and shining as if they had never suffered from the touch of a single storm.

Perhaps the least known among the more outspoken agents of mountain degradation are those currents of eroding rock called avalanches. Those of the Sierra are of all sizes, from a few sand-grains or crystals worked loose by the weather and launched to the bottoms of cliffs, to those immense earthquake avalanches that thunder headlong down amid fire and smoke and dust, with a violence that shakes entire mountains. Many avalanche-producing causes, as moisture, temperature, winds, and earthquakes, are exceedingly variable in the scope and intensity of their action. During the dry, equable summers of the middle region, atmospheric disintegration goes silently on, and many a huge mass is made ready to be advantageously acted upon by the first winds and rains of winter. Inclined surfaces are then moistened and made slippery, decomposed joints washed out, frost-wedges driven in, and the grand avalanche storm begins. But though these stone-storms occur only in winter, the attentive mountaineer may have the pleasure of witnessing small avalanches in every month of the year. The first warning of the bounding free of a simple avalanche is usually a dull muffled rumble, succeeded by a ponderous crunching sound; then perhaps a single huge block weighing a hundred tons or more may be seen wallowing down the face of a cliff, followed by a train of smaller stones, which are gradually left behind on account of the greater relative resistance they encounter as compared with their weight. The eye may therefore follow the large block undisturbed, noting its awkward, lumbering gestures as it gropes its way through the air in its first wild journey, and how it is made to revolve like a star upon its axis by striking on projecting portions of the walls while it pursues the grand smooth curves of general

descent. Where it strikes a projecting boss it gives forth an intense gasping sound, which, coming through the darkness of a storm-night, is indescribably impressive; and when at length it plunges into the valley, the ground trembles as if shaken by an earthquake.

On the 12th of March, 1873, I witnessed a magnificent avalanche in Yosemite Valley from the base of the second of the Three Brothers. A massive stream of blocks bounded from ledge to ledge and plunged into the talus below with a display of energy inexpressibly wild and exciting. Fine gray foamdust boiled and swirled along its path, and gradually rose above the top of the cliff, appearing as a dusky cloud on the calm sky. Unmistakable traces of similar avalanches are visible here, probably caused by the decomposition of the feldspathic veins with which the granite is interlaced.

Earthquakes, though not of frequent occurrence in the Sierra, are powerful causes of avalanches. Many a lofty tower and impending brow stood firm through the storms of the first post-glacial seasons. Torrents swept their bases, and winds and snows slipped glancingly down their polished sides, without much greater erosive effect than the passage of cloudshadows. But at length the new-born mountains were shaken by an earthquake-storm, and thousands of avalanches from cañon walls and mountain sides fell in one simultaneous crash. The records of this first post-glacial earthquake present themselves in every cañon and around the bases of every mountain summit that I have visited; and it is a fact of great geological interest that to it alone more than nine-tenths of all the cliff taluses which form so striking a characteristic of cañon scenery are due. The largest of these earthquake taluses are from 500 to 1000 feet in height, and are timbered with spruce, pine, and live-oak over their entire surfaces, showing that they have not been disturbed since their formation, either by denudation or accessions of fresh material.

The earthquake which destroyed the village of Lone Pine, in March, 1872, shook the Sierra with considerable violence, giving rise to many new taluses, the formation of one of which I was so fortunate as to witness.

The denuding action of avalanches is not unlike that of