Studies in the Sierra, No. IV. - Glacial Denudation.
. \ \ 0 " \ 1 174 '. STUDIES IN THE SIERRA. [Aug. I have discovered that Mademoiselle Ida Gilmore was an inmate of the pension of a Madame Ganil, and that you' (here I put my hand in the drawer and grasped the skull)—'and that you, her most miserable son, are the sham vicomte and...
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Scholarly Commons
1874
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| Summary: | . \ \ 0 " \ 1 174 '. STUDIES IN THE SIERRA. [Aug. I have discovered that Mademoiselle Ida Gilmore was an inmate of the pension of a Madame Ganil, and that you' (here I put my hand in the drawer and grasped the skull)—'and that you, her most miserable son, are the sham vicomte and sham Duplessis that you are Miss Gilmore's murderer, and that this is the skull of your victim!' "I dashed the white thing down in front of him, and leaped up. He eyed it for a moment as if fascinated, then, with a shudder and a howl, he too leaped to his feet, seized a chair and swung it aloft. ."'Curse you, American devil!' he yelled, 'you shall never live to say you have caught me!' "At me he leaped, and that so suddenly that I should not now be writing this, but that in the same moment the chair was seized from behind, and a quiet voice said, ' Doucement, douce- ment, mon cheri1 In that same moment M. Renard had slipped a pair of steel bracelets on the trembling wrists of the white-faced wretch. In that same moment, too, the door was burst, open and a distracted, weeping little woman rushed in. It was Madame Ganil! She comprehended the scene in a moment, and flinging herself at her son's feet she moaned out, 'Oh, monfils/ monfiIs! why did you not tell me you were here ? I then might have saved you, might have saved you, mouflls/'" Let the doctor's letter end here; there is little more to say. Mother and son were torn apart, for the mother's guilt ended with connivance at the false marriage. The son confessed his greater crime — not differing after all, except in details, from the semi-fictitious description given him by Dr. Andros. The tooth of the platinum filling had found a mighty tongue to cry aloud for vengeance; and the slow, terrible sword of Justice is at last laid bare — let us hope, never to find its scabbard till judgment to the last awful jot and tittle be executed on the murderer of Ida Gilmore. I STUDIES IN THE SIERRA. NO. IV. —GLACIAL DENUDATION. GLACIAL denudation is one of the ' noblest and simplest manifestations of sun-power. Ocean water ife lifted infra vapor, crystallized into snow, and sown broadcast upon the mountains. Thaw and frost, combined with the pressure of its own weight, changes it to ice, which, although in appearance about as hard and inflexible as glass, immediately begins to flow back toward the sea whence it came, and at a rate of motion about equal to that of the hour - hand of a watch. This arrangement is illustrated in Fig. i, wherein a wheel constructed of watervapor, snow, and ice, and as irregular in shape as in motion, is being sun-whirled against a mountain-side with a mechanical wearing action like that of an ordinary grindstone. In north Greenland the snow supply and general climatic conditions are such that "its glaciers discharge directly into the sea, and so perhaps did all first-class glaciers when in their prime; but now the world is so warm, and the snow-crop so scanty, most glaciers melt long before reaching the ocean. Schlagenweit tells us those of Switzerland melt on the average at an elevation of about 1874-3 STUDIES IN THE SIERRA. 175 Fig. 7,400 feet above sea-level; the Himalaya glacier, in which the Ganges takes its rise, does not descend below 12,914- feet;* while those of our Sierra melt at an average elevation of about 11,000 feet. In its progress down a mountainside a glacier follows the directions of greatest declivity, a law subject to very important modifications in its general application. Subordinate ranges many hundred feet in height are frequently overswept smoothly and gracefully without any visible manifestation of power. Thus, the Tenaya outlet of the ancient Tuolumne mer de glace glided over the Merced divide, which is more than 500 feet high, impelled by the force of that portion of the glacier which was descending the higher slopes of mounts Dana, Gibbs, and others, at a distance of ten miles. The deeper and broader the glacier, the greater the horizontal distance over which the impelling force may be transmitted. No matter how much the courses of glaciers are obstructed by inequalities of surface such as ridges and canons, if they are deep enough and wide enough, and the general declivity be * According to Captain Hodgson. sufficient, they will flow smoothly over them all just as calm water-streams flow over the stones and wrinkles of their channels. PRESENT CONDITION OF THE SIERRA CONSIDERED WITH REFERENCE TO GLACIAL ACTION. The most important glacial phenomena presented in the Sierra are: First, polished, striated, scratched, and grooved surfaces, produced by the glaciers slipping over and past hard rocks in their pathways. Secondly, moraines, or accumulations of mud, dust, sand, gravel, and blocks of various dimensions, deposited by the glaciers in their progress, in certain specific methods. Thirdly, sculpture in general, chiefly as seen in canons, lake-basins, hills, ridges, and separate rocks, whose forms, trends, distribution, etc., are the peculiar offspring of glaciers. In order that my readers may have clear conceptions of the distribution and comparative abundance of the above phenomena, I will give here a section of the west flank from summit to base between the Tuolumne and Merced rivers, which, though only a rough approxima- ' I ' 176 STUDIES IN THE SIERRA. [Aug. /YK /aah: /WO ~./-)3i'y-rr*r 5 -7.aXA - -sii; tion, is sufficiently accurate for our purpose. The summit region from D to C (Fig. 2) is composed of Jiig-hly metamor- phic slates, so also is most of the lower region, B to A. The middle region is granite, with the exception of a few small slate -cappings upon summits of the Merced and Hoffmann spurs. With regard to the general topography of the section, which may be taken as fairly characteristic of the greater portion of the range, the summit forms are sharp and angular, because they have been down-Rowed; all the middle and lower regions comprising the bulk of the range have rounded forms, because they have been overflowed. In the summit region all the glacial phenomena mentioned above are found in a fresh condition, simply on account of their youthfulness. Scores of small glaciers still exist here where we can watch their actions. But the middle region is the most interesting, because, though older, it contains all the phenomena, and on a far grander scale, on account of the superior physical structure of granite for the reception of glacial history. Notwithstanding the grandeur of canons and moraines, with their glorious adornments, stretching in sublime simplicity delicately compliant to' glacial law, and the endless variety of picturesque rocks rising in beautiful groups out of the dark forests, by far the most striking of all the ice phenomena .are the polished surfaces, the beauty and mechanical excellence of which no words will describe. They occur in large irregular patches many acres in extent in the summit and upper half of the middle region, bright and stainless as the untrodden sky. They reflect the sunbeams like glass, and though they have been subjected to the corroding influences of the storms of ten thousands of years, to frosts, rains, dews, yet are they in many places unblurred, undimmed, as if finished but yesterday. The attention of the mountaineer is seldom arrested by moraines however conspicuously regular and artificial in form, or by cafions however deep, or rocks however noble, but he stoops and rubs his hands' admiringly on these shining surfaces, and tries hard to account for their mysterious smoothness. He has beheld the summit snows descending in booming avalanches, but he concludes that it can not be* the work of snow, because he finds them far beyond the reach of avalanches neither can water be the agent, he says, for he finds them on the tops of the loftiest domes. Only the winds seem capable of following and flowing in the' paths indicated by their scratches and grooves, and some observers have actually ascribed the phenomenon to this cause. Even horses and dogs gaze wonderingly at the strange brightness of the ground, and smell it, and place their feet upon it cautiously; only the wild mountain sheep seems to move wholly at ease upon these glistening pavements. This polish is produced by glaciers slipping with enormous pressure over hard, close-grained slates or granite. The fine striations, so small as to be scarcely visible, are evidently caused by grains of sand imbedded in the bottom of the ice; the scratches and smaller grooves', by stones with sharp graving edges. Scratches are therefore most abundant and roughest in the region of I874-] STUDIES IN THE SIERRA. 177 metamorphic slates, which break up by the force of the overflowing currents into blocks with hard cutting angles, and gradually disappear where these graving tools, having been pushed so far, have had their edges worn off. The most extensive areas of polished surfaces are found in the upper half of the middle region, where the granite is most solid in structure and contains the greatest quantity of silex. They are always brighter, and extend farther down from the axis of the range, on the north sides of cafions that trend in a westerly direction, than on the south sides; because, when wetted by corroding rains and snows, they are much sooner dried again, the north sides receiving 4ireet sunshine, while the south walls are mostly in shadow and remain longest' wet, 178 STUDIES IN THE SIERRA. ' [Aug. (J-v-fv- 'X summit of the Sierra page is clear ;\the farther we descend, the moreits inscriptions are crossed and recrossed/ Dews have dimmed it, torrents have scrawled it, and the earthquake and avalanche have erased many a delicate line. Groves and meadows, forests and fields, darken and confuse its more enduring characters along the bottom, until only the laborious student can decipher even the most emphasized passages of the original manuscript. METHODS OF GLACIAL DENUDATION. All geologists recognize the fact that glaciers wear away the rocks over which they move, but great vagueness prevails as to the size of the fragments, their abundance, and the way in which the glacial energy expends itself in detaching and carrying them away. And, if possible, still greater vagueness prevails as to the forms of the rocks and valleys resulting from this erosion. This is not to be wondered at, when we consider how recently glacial history has been studied, and how profound the silence and darkness under which glaciers prosecute their works. In this article, I can do little more for my readers than indicate methods of study, and results which may be obtained by those who desire to study the phenomena for themselves. In the first place, we may go to the -hVifig glaciers and learn what we can of their weight, motions, and general habits*—how they detach, transport, and accumulate rocks from various sources. Secondly, we may follow in the tracks of the ancient glaciers, and study their denuding power from the forms of their channels, and from the fragments composing the moraines, and the condition of the surfaces from which they were derived, and whether these fragments were rubbed, split,, or broken off. * Here I would refer my readers to the excellent elementary works of Agassiz, Tyndall, and Forbes. The waters which rush out from beneath all living glaciers are turbid, and if we follow them to their resting-places in pools we shall find them depositing fine mud, which when rubbed between the thumb and finger is smooth as flour. This mud is ground off from the bed of the glacier by a smooth, slipping motion accompanied with immense pressure, giving rise to the polished surfaces we have already noticed. These mud particles are the smallest chips which glaciers make in the degradation of mountains. Toward the end of the summer, when the winter snows are melted, particles of dust and sand are seen scattered over the surfaces of the Sierra glaciers in considerable quantities, together with angular masses of rock derived from the shattered storm-beaten cliffs that tower above their heads. The separation of these masses, which vary greatly in size, is due only in part to the action of the glacier, although they all are borne down like drift on the surface of a river and deposited together in moraines. The winds scatter down most of the sand and dust. Some of the larger fragments are set free by the action of frost, rains, and general weathering agencies; while considerable quantities are borne down in avalanches of snow, and hurled down by the shocks of earthquakes. Yet the glacier performs an important part in the production of these superficial effects, by undermining the cliffs whence the fragments fall. During my Sierra explorations in the summers of 1872 and 1873, almost every glacier I visited offered illustrations of the special action of earthquakes in this connection, the earthquake of March, 1872, having just finished shaking the region with considerable violence, leaving the rocks which it hurled upon the ice fresh and nearly unchanged in position. 'X-.In all moraines we find stones, which, from their shape and composition, and J\ 1874-3 STUDIES IN THE SIERRA. 179 the finish of their surfaces, we know were not thus derived from the summit peaks overtopping the glaciers, but from the rocks past whjch and over which they flowed. I have seen the north Mount Ritter glacier in the act of grinding the side of its channel, and breaking off fragments and rounding their angles, by crushing and rolling them between the wall and ice. In all the pathways of the ancient glaciers, also, there remain noble illustrations of the power of ice, not only in wearing away the sides of their channels in the form of mud, but in breaking them up into huge blocks. Explorers into the upper portion of the middle granite region will frequently come upon blocks of great size and regularity of form, possessing some character of color or composition which enables them to follow back on their trail and discover the rock or mountain-side from whence they were torn. The size of the blocks, their abundance"' along the line of dispersal, and the probable rate of motion of the glacier which quarried and transported them, form data by which a-sirght approximation to the rate of block denudation may be reached. Fig. 3 is a rock about two miles west of Lake Tenaya, with a train of bowlders derived from it. The bowlders are scattered along a level ridge, where they have not been disturbed in any appreciable degree since they came to rest toward the close of the glacial period. An examination of the rock proves conclusively that not only were these blocks—many of which are twelve feet in diameter — derived from it, but that they were torn off its side by the direct mechanical action of the glacier, For had they sim-" ply fallen-upon the surface of the glacier from above, then the rock would present a crumbling, ruinous condition—which it does not—and a talus of similar blocks would have accumulated at its base after there was no glacier to remove them as they fell; but no such talus exists, i8o STUDIES IN THE SIERRA. [Aug. iK the rock remaining compact, as if it had scarcely felt the touch of a single storm. Yet, what countless seasons of weathering, combined with earthquake violence, could not accomplish, was done by the Tenaya Glacier, as it swept past on its way to Yosemite. A still more striking and instructive example of side-rock erosion may be found about a mile north of Lake Tenaya. Here the glaciated pavements are more perfectly preserved than elsewhere in the Merced basin. Upon them I found a train of .solid granite blocks, which attracted my attention from their isolated position, and the uniformity of their mechanical characters. I determined to seek their fountain quarry, knowing it must be neaiy-beGause their angles were unworn. Their source proved to be the sideof one of the lofty elongated ridges stretching toward the Big Tuolumne Meadows. They had been quarried from the base of the ridge, which is ice-polished and undecayed to the summit. The reason that only this particular portion of the ridge afforded blocks of this kind, and so abundantly as to bevtraceable, is that the cleavage planes here separated the rock into par- allelopipeds which sloped forward obliquely into the side of the glacier, which was thus enabled to grasp them firmly and strip them off, just as the spikelets of an ear of wheat are stripped off by Fig. 4. running the fingers down from the top toward the base. An instance where the structure has an exactly opposite effect upon the erodibility of the side of a rock is given in Fig. 4, where the cleavage planes separate it into immense slabs which overlap each other with reference to the direction of the glacier's motion, like the shingles of a roof. Portions of the sides of rocks or cafion walls whose structure is of the latter character always project, because of the greater resistance they have been able to offer to the action of the past-flowing glacier, while those portions whose structure is similar to that of the former example always recede. Fig. 5. Fig. 5 is a profile view of a past-flowed glacier rock, about 1,500 feet high, forming part of the north wall of Little Yosemite Valley near the head. Its grooved, polished, and fractured surface bears witness in unmistakable terms to the enormous pressure it has sustained from that portion of the great South Lyell Glacier which forced its way down through the valley, and to the quantity, and size, and kind of fragments which have been removed from it, as a necessary result of this glacial action. The dotted lines give an approximate reconstruction of the rock as far as to the outside layer at A. Between A and B the broken ends of concentric layers, of which the whole rock seems to be built, give some idea of the immense size of some of the chips. The reason for the 1874-3 STUDIES IN THE SIERRA. 181 greater steepness of the front from A to B than from B to C will be perceived at a glance; and, since the cleavage planes and other controlling elements in its structure are evidently the same throughout the greater portion of its mass as those which determined its present condition, if the glacial winter had continued longer its more characteristic features would probably have remained essentially the same until the rock was nearly destroyed. Fig. 6. The section given in Fig. 6 is also taken from the north side of the same valley. It is inclined at an angle of about twenty-two degrees, and therefore has been more flowed over than flowed past. The whole surface, excepting the vertical portion at A, which is forty feet high, is polished and striated. The arrows indicate the direction of the striae. At A a few incipient cleavage planes are beginning to appear, which show the sizes of some of the chips which the glacier would have broken or split off had it continued longer at work. The whole of the missing layer which covered the rock at B, was evidently detached and carried off in this way. The abrupt transition from the polished surface to the split angular front at A, shows in a most unequivocal manner that glaciers erode rocks in at least two very different modes—first, by grinding them into mud; second, by breaking and splitting them into blocks, whose sizes are measured by the divisional planes they possess and the intensity and direction of application of the force brought to bear upon them. That these methods prevail in the denudation of overflowed as well as past-fiov/ed rocks, is shown by the condition of every cafion of the region. For if mud particles only were detached, then all the bottoms would be smooth grooves, interrupted only by flowing undulations; but, instead of this condition, we find that every canon bottom abounds in steps sheer-fronted and angular, and some of them hundreds of feet in height, though ordinarily from one to ten or twelve feet. These step- fronts in most cases measure the size of the chips of erosion in—one direction. Many of these interesting ice-chips may be seen in their tracks removed to great distances or only a few feet, when the melting of the glaciers at the close of the period put a stop to their farther progress, leaving them as lessons of the simplest kind. Fig. 7. Fig. 7, taken from the Hoffmann fork of Yosemite Creek basin, shows the character of some of these steps. This one is fifteen feet high at the highest place, and the surface, both at top and bottom, is ice-polished, indicating that no disturbing force has interfered with the phenomena since the termination of the glacial period. Fig. 8'is a dome on the upper San 182 STUDIES IN THE SIERRA. [Aug. naya, above Mirror Lake. The edges of unremoved layers are visible at B and C. This rock is an admirable illustration of the manner in which a broad deep glacier clasps and denudes a dome. When we narrowly inspect it, and trace the striae, we perceive that it has been eroded at once in front, back, and sides, and none of the fragments thus removed Joaquin, the top of which is about 7,700 feet above sea-level. The arrow indicates the direction of application of the ice-force, which is seen to coincide with the position of remaining fragments of layers, the complements of which have been eroded away. Similar fragments occur on the stricken side of all domes whose structure and position were favorable for their formation and preservation. Fig. 9 is a fragmentary dome, situated on the south side of the Mono trail, near the base of Mount Hoffmann. Remnants of concentric shells of granite from five to ten feet thick are seen on the up-stream side at A, where it received the thrust of the Hoffmann Glacier, when on its way to join the Te- FlG. 9. are to be found around its base. Here I would direct special attention to the fact that it is on the upper side of this rock at A, just where the pressure was greatest, that the erosion has been least; because there the layers were pressed against one another, instead of away from one another, as on the sides and tack, and could not, therefore, be so easily broken up. QUANTITY OF GLACIAL DENUDATION. These simple observations we have been making plainly indicate that the Sierra, from summit to base, was covered by a sheet of crawling ice, as it is now covered by the atmosphere. Its crushing currents slid over the highest domes, as well as along the deepest canons, wearing, breaking, and degrading every portion of the surface, however resisting. The question, therefore, arises, What is the quantity of this degradation ? As far as its limit is concerned, it is clear that, inasmuch as glaciers can not move without in some way and at some rate lowering the surfaces they are in contact with, a mountain range 1874-3 STUDIES IN THE SIERRA. 183 may be denuded until the declivity becomes so slight that the glaciers come to rest; or are melted, as was the case with those concerned in the degradation of the Sierra. However slow the rate of wear, given a sufficient length of time, and any thickness of rock, whether a foot or hundreds of thousands of feet, will be removed. No student pretends to give an arithmetical expression to the glacial epoch, though it is universally admitted that it extended through thousands or millions of years. Nevertheless, geologists are found who can neither give Nature time enough for her larger operations, or for the erosion of a mere canon furrow, without resorting to sensational cataclysms for an explanation of the phenomena. If the Sierra were built of one kind of rock, homogeneous in structure throughout its sections, then perhaps we would be unable to produce any plain, printa- ~ble evidence relative to the amount of denudation effected; but, fortunately for the geologist, this is not the case. The summits of the range in the section under special consideration are capped with slates; so are several'peaks of outlying spurs, as those of the Merced and Hoffmann; and all the base is slate-covered. The circumstances connected with their occurrence in these localities and absence in others, furnish proof little short of demonstration that they once covered all the range, and, from their known thickness in the places where they occur, we may approximate to the quantity removed where they are less abundant or wanting. Moreover, we have seen in study No. 3 that the physical structure of granite is such that we may know whether or not its forms are broken. The opposite sides of valley walls exhibiting similar fragmentary sections often demonstrate that the valleys, were formed by the removal of an amount of rock equal in depth to that of the valleys. •k 0$'t CAMMA. II [Aug. „y Fig. 10 is an Kteql section across the range from base to summit. That slates covered the whole granitic region between B and D, is shown by the fact that slates cap the summits of spurs in the denuded gap where they are sufficiently high, as at C. Also, where the granite comes in contact with the slates, and for a considerable depth beneath the line of contact, it partakes, in a greater or less degree, of the physical structure of slates, enabling us to determine the fact that in many places slates have covered the granite where none are now visible for miles, and also furnishing data by which to approximate the depth at which these surfaces lie beneath the original summit of the granite. Phenomena relating to this portion of the argument abound in the upper basins of the tributary streams of the Tuolumne and Merced; for their presentation, however, in detail, we have no space in these brief outlines. If, therefore, we would restore this section of the range to its unglaciated condition, we would have, first, to fill up all the valleys and canons. Secondly, all the granite domes and peaks would have to be buried until the surface reached the level of the line of contact with the slates. Thirdly, in the yet grander restoration of the missing portions of both granite and slates up to the line between the summit slates and those of the base, as indicated in Fig. 10 by the dotted line, the maximum thickness of the restored rocks in the middle region would not be less than a mile and a half, and average a mile. But, because the summit peaks are only sharp residual fragments, and the foot-hills roimded residual fragments, when all the intervening region is restored up to the dotted line in the figure, we still have only partially reconstructed the range, for the unglaciated' summits may have towered many thousands of feet above their present heights. And when we consider that living glaciers are still engaged in lowering the summits which are already worn to mere blades and pinnacles, it will not seem improbable that the whole quantity of glacial denudation in the middle region of the western flank of the Sierra considerably exceeds a mile in average depth. So great was the amount of chipping required to bring out the present architecture of the Sierra. *• CAMMA.* Here again? Then lA him enter; long enough have I denied him Who hath sought'me late and early; truly, grief must have an end. I have sorrowed long and deeply he is worthy, I have tried him; Tell him Camma weeps no longer, that she waits to greet her friend. Chaste Diana, look and listen.' Low I bow before thine altar; Only thou art left to help me, all my strength must come from thee: Thou art pure, and strong, and holy.— do not blame me if I falter: Give me strength to smile and slay him — let thy spirit enter me. *Camma, a Gaulish lady, was much persecuted by the advances of a powerful Roman noble. As, in spite of all his entreaties, however, she remained faithful to her marriage vow, he caused her husband to be assassinated. She then took refuge in the temple of Diana, and enrolled herself among thepriestesses. Her lover sent noble after noble to beg an interview, and was at last admitted. The Lady Camma professed to be favorable to his wishes, but insisted on first pouring out a libation to the goddess. Having drank, she handed him the cup, which was poisoned, and both died in the temple.—Plutarch, De Mill. Virt. https://scholarlycommons.pacific.edu/jmb/1082/thumbnail.jpg |
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