per season, on the melting of the winter snow, when all weakly constructed dams and drift-heaps are broken up and reformed. It is a fact of great geological interest that only that portion of the general detritus of post-glacial denudation—that is, in the form of mud, sand, fine gravel, and matter held in solution -has ever at any time been carried entirely out of the range into the plains or ocean. In the cañon of the Tuolumne River, we find that the chain of lake basins which stretch along the bottom from the base of Mount Lyell to the Hetch-Hetchy Valley are filled with detritus, through the midst of which the river flows; but the washed boulders, which form a large portion of this detritus, instead of being constantly pushed forward from basin to basin, lie still for centuries at a time, as is strikingly demonstrated by an undisturbed growth of immense sugar-pines and firs inhabiting the river-banks. But the presence of these trees upon water-washed boulders only shows that no displacement has been effected among them for a few centuries. They still must have been swept forward and outspread in some grand flood prior to the planting of these trees. But even this grand old flood of glacial streams, whose magnificent traces occur everywhere on both flanks of the range, did not remove a single boulder from the higher to the lower Sierra in that section of the range drained by the Tuolumne and Merced, much less into the ocean, because the lower portion of the Hetch-Hetchy basin, situated about half-way down the western flank, is still in process of filling up, and as yet contains only sand and mud to as great a depth as observation can reach in river sections. The river flows slowly through this alluvial deposit and out of the basin over a lip of solid bed-rock, showing that not a single high Sierra boulder ever passed it since the close of the glacial period; and the same evidence is still more strikingly exhibited in similarly situated basins in the Merced Valley. Frost plays a very inferior part in Sierra degradation. The lower half of the range is almost entirely exempt from its disruptive effects, while the upper half is warmly snow-mantled throughout the winter months. At high elevations of from ten to twelve thousand feet, sharp frosts occur in the months of October and November, before much snow has fallen; and where shallow water-currents flow over rocks traversed by open divisional joints, the freezing that ensues forces the blocks apart and produces a ruinous appearance, without effecting much absolute displacement. The blocks thus loosened are, of course, liable to be moved by flood-currents. This action, however, is so limited in range, that the general average result is inappreciable. Atmospheric weathering has, after all, done more to blur and degrade the glacial features of the Sierra than all other agents combined, because of the universality of its scope. No mountain escapes its decomposing and mechanical effects. The bases of mountains are mostly denuded by streams of water, their summits by streams of air. The winds that sweep the jagged peaks assume magnificent proportions, and effect changes of considerable importance. The smaller particles of disintegration are rolled or shoved to lower levels just as they are by water currents, or they are caught up bodily in strong, passionate gusts, and hurled against trees or higher portions of the surface. The manner in which exposed tree-trunks are thus wind-carved and boulders polished will give some conception of the force with which this agent moves. Where boulders of a form fitted to shed off snow and rain have settled protectingly upon a polished and striated surface, then the protected portion will, by the erosion and removal of the unprotected surface around it, finally come to form a ped nudation at that point. These boulder pedestals, furnishing so admirable a means of gauging atmospheric erosion, occur throughout the middle granitic region in considerable numbers: some with their protecting boulders still poised in place, others naked, their boulders having rolled off on account of the stool having been eroded until too small for them to balance upon. It is because of this simple action that all very old, deeply weathered ridges and slopes are boulderless, Nature having thus leisurely rolled them off, giving each a whirling impulse as it fell from its pedestal once in hundreds or thousands of years. Moutonnéed rock forms shaped like Figure 3 are abundant in the middle granitic region. They frequently wear a single pine, jauntily wind-slanted, like a feather in a cap, and a single large boulder, poised by the receding ice - sheet, that often produces an impression of having been FIG. 3. thus placed artificially, exciting the curiosity of the most apathetic mountaineer. Their occurrence always shows that the surfaces they are resting upon are not yet deeply eroded. Ice - planed veins of quartz and feldspar are frequently weathered into relief by the superior resistance they offer to erosion, but they seldom attain a greater height than three or four inches ere they become weather-cracked and lose their glacial polish, thus becoming useless as means of gauging denudation. Ice-burnished feldspar crystals are brought into relief in the same manner to the height of about an inch, and are available to this extent in determining denudation over large areas in the upper portion of the middle region. This brief survey of the various forces incessantly or occasionally at work wasting the Sierra surface would at first lead us to suppose that the sum total of the denudation must be enormous; but, on the contrary, so indestructible are the Sierra rocks, and so brief has been the period through which they have been exposed to these agents, that the general result is found to be comparatively insignificant. The unaltered polished areas constituting so considerable a portion of the upper and middle regions have not been denuded the one-hundredth part of an inch. Farther down measuring tablets abound bearing the signature of the ice. The amount of torrential and avalanchial denudation is also certainly estimated within narrow limits by measuring down from the unchanged glaciated surfaces lining their banks. Farther down the range, where the polished surfaces disappear, we may still reach a fair approximation by the height of pot-holes drilled into the walls of gorges, and by the forms of the bottoms of the valleys containing these gorges, and by the shape and condition of the general features. Summing up these results, we find that the average quantity of post-glacial denudation in the upper half of the range, embracing a zone twenty-five or thirty miles wide, probably does not exceed a depth of three inches. That of the lower half has evidently been much greater-probably several feet-but certainly not so much as radically to alter any of its main features. In that portion of the range where* the depth of glacial denudation exceeds a mile, that of post-glacial denudation is less than a foot. From its warm base to its cold summit, the physiognomy of the Sierra is still strictly glacial. Rivers have only traced shallow wrinkles, avalanches have made scars, and winds and rains have blurred it, but the change, as a whole, is not greater than that effected on a human countenance by a single year of exposure to common alpine storms. See study No. IV, in SIERRA CLUB BULLETIN of January, 1918. Founded 1892 402 MILLS BUILDING, SAN FRANCISCO, CALIFORNIA THE PURPOSES OF THE CLUB ARE: To explore, enjoy, and render accessible the mountain regions of the Pacific Coast; to publish authentic information concerning them; to enlist the support and co-operation of the people and the Government in preserving the forests and other natural features of the Sierra Nevada. JOHN MUIR, President 1892 to 1914 OFFICERS AND COMMITTEES FOR THE YEAR 1918-1919 BOARD OF DIRECTORS WM. E. COLBY, San Francisco.. VERNON L. KELLOGG, Stanford University. J. N. LE CONTE, Berkeley .President Vice-President .Treasurer Secretary WILLIAM FREDERIC BADÈ, Berkeley ALBERT H. ALLEN, Berkeley WALTER L. HUBER, San Francisco ROBERT M. PRICE, Reno, Nevada CLAIR S. TAPPAAN, Los Angeles HONORARY VICE-PRESIDENTS JAMES BRYCE, London, England ROBERT UNDERWOOD JOHNSON, New York City COMMITTEES Outing Committee: WILLIAM E. COLBY (Chairman and Manager), CLAIR CONTE (Chairman), MARION Randall PARSONS, ROBERT M. Price. Auditing Committee: WILLIAM F. BADÈ (Chairman), WALTER L. HUBER, ROBERT M. PRICE. Librarian: WALLACE BRADFORD. Southern California Section Executive Committee: PHIL S. BERNAYS (Chairman, 315 W. 3rd Street, Los Angeles), CHAS. J. Fox (Secretary), BENJAMIN W. FENTON (Treasurer), MABelle McCalla (Assistant Secretary), MRS. HENRY BRAUN, MARY F. KELLOGG, Ernest DAWSON, CARRIE E. TRACY, LLOYD B. AUSTIN. .Associate Editor and Book Reviews Notes and Correspondence ...Forestry Notes ALBERT H. ALLEN, FRANCIS P. FARQUHAR WALTER L. HUBER, WILLIAM T. GOLDSBOROUGH, JOSEPH N. LE CONTE, ELLIOTT MCALLISTER, ELIZABETH M. BADÈ |