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[woodcut of John Muir] John Muir Writings

Studies in the Sierra

by John Muir


III
Ancient Glaciers and Their Pathways

Though the gigantic glaciers of the Sierra are dead, their history is indelibly recorded in characters of rock, mountain, cañon, and forest; and, although other hieroglyphics are being incessantly engraved over these, “line upon line,” the glacial characters are so enormously emphasized that they rise free and unconfused in sublime relief, through every after inscription, whether of the torrent, the avalanche, or the restless heaving atmosphere.

In order to give the reader definite conceptions of the magnitude and aspect of these ancient ice-rivers, I will briefly outline those which were most concerned in the formation of Yosemite Valley and its cañon branches. We have seen (in the previous chapter) that Yosemite received the simultaneous thrust of the Yosemite Creek, Hoffmann, Tenaya, South Lyell, and Illilouette glaciers. These welded themselves together into one huge trunk, which swept down through the valley, receiving small affluents in its course from Pohono, Sentinel, and Indian cañons, and those on both sides of El Capitan Rock. At this period most of the upper portions of the walls of the valley were bare; but during its earliest history, the wide mouths of these several glaciers formed an almost uninterrupted covering of ice. All the ancient glaciers of the Sierra fluctuated in depth and width, and in degree of individuality, down to the latest glacial days. It must, therefore, be distinctly borne in mind that the following sketches of these upper Merced glaciers relate only to their separate condition, and to that phase of their separate condition which they presented toward the close of the period when Yosemite and its branches were works nearly accomplished.

Yosemite Creek Glacier

The broad, many-fountained glacier to which the basin of Yosemite Creek belonged, was about fourteen miles in length by four in width, and in many places was not less than a thousand feet in depth. Its principal tributaries issued from lofty amphitheatres laid well back among the northern spurs of the Hoffmann range. These at first pursued a westerly course; then, uniting with each other and absorbing a series of small affluents from the Tuolumne divide, the trunk thus formed swept round to the south in a magnificent curve, and poured its ice into Yosemite in cascades two miles wide. This broad glacier formed a kind of wrinkled ice-cloud. As it grew older, it became more regular and riverlike; encircling peaks overshadowed its upper fountains, rock islets rose at intervals among its shallowing currents, and its bright sculptured banks, nowhere overflowed, extended in massive simplicity all the way to its mouth. As the ice-winter drew near a close, the main trunk, becoming torpid, at length wholly disappeared in the sun, and a waiting multitude of plants and animals entered the new valley to inhabit the mansions prepared for them. In the meantime the chief tributaries, creeping slowly back into the shelter of their fountain shadows, continued to live and work independently, spreading moraine soil for gardens, scooping basins for lakelets, and leisurely completing the sculpture of their fountains. These also have at last vanished, and the whole basin is now full of light. Forests flourish luxuriantly over all its broad moraines, lakes and meadows nestle among its domes, and a thousand flowery gardens are outspread along its streams.

Hoffmann Glacier

The short, swift-flowing Hoffmann Glacier offered a striking contrast to the Yosemite Creek, in the energy and directness of its movements, and the general tone and tendencies of its life. The erosive energy of the latter was diffused over a succession of low boulderlike domes. Hoffmann Glacier, on the contrary, moved straight to its mark, making a descent of 5,000 feet in about five miles, steadily deepening and contracting its current, and finally thrusting itself against the upper portion of Yosemite in the form of a wedge of solid ice, six miles in length by four in width. The concentrated action of this energetic glacier, combined with that of the Tenaya, accomplished the greater portion of the work of the disinterment and sculpture of the great Half Dome, North Dome, and the adjacent rocks. Its fountains, ranged along the southern slopes of the main Hoffmann ridge, gave birth to a series of flat, wing-shaped tributaries, separated from one another by picturesque walls built of massive blocks, bedded and jointed like masonry. The story of its death is not unlike that of the Yosemite Creek, though the declivity of its channel and equal exposure to sun-heat prevented any considerable portion from passing through a torpid condition. It was first burned off on its lower course; then, creeping slowly back, lingered a while at the base of its mountains to finish their sculpture, and encircle them with a zone of moraine soil for gardens and forests.

The gray slopes of Mount Hoffmann are singularly barren in aspect, yet the traveler who is so fortunate as to ascend them will find himself in the very loveliest gardens of the Sierra. The lower banks and slopes of the basin are plushed with chaparral rich in berries and bloom—a favorite resort for bears; while the middle region is planted with the most superb forest of silver-fir I ever beheld. Nowhere are the cold footsteps of ice more warmly covered with light and life.

Tenaya Glacier

The rugged, strong-limbed Tenaya Glacier was about twelve miles long, and from half a mile to two and a half miles wide. Its depth varied from near 500 to 2,000 feet, according as its current was outspread in many channels or compressed in one. Instead of drawing its supplies directly from the summit fountains, it formed one of the principal outlets of the Tuolumne mer de glace, issuing at once from this noble source a full-grown glacier two miles wide and more than a thousand feet deep. It flowed in a general southwesterly direction, entering Yosemite at the head, between Half and North Domes. In setting out on its life-work it moved slowly, spending its strength in ascending the Tuolumne divide, and in eroding a series of parallel sub-channels leading over into the broad, shallow basin of Lake Tenaya. Hence, after uniting its main current, which had been partially separated in crossing the divide, and receiving a swift-flowing affluent from the fountains of Cathedral Peak, it set forth again with renewed vigor, pouring its massive floods over the southwestern rim of the basin in a series of splendid cascades; then, crushing heavily against the ridge of Clouds Rest, curved toward the west, quickened its pace, focalized its wavering currents, and bore down upon Yosemite with its whole concentrated energy. Toward the end of the ice-period, and while the upper tributaries of its Hoffmann companion continued to grind rock-meal for coming forests, the whole body of Tenaya became torpid, withering simultaneously from end to end, instead of dying gradually from the foot upward. Its upper portion separated into long parallel strips extending between the Tenaya basin and Tuolumne mer de glare. These, together with the shallow ice-clouds of the lake-basin melted rapidly, exposing broad areas of rolling rock-waves and glossy pavements, on whose channelless surface water ran everywhere wild and free. There are no very extensive morainal accumulations of any sort in the basin. The largest occur on the divide, near the Big Tuolumne Meadows, and on the sloping ground northwest of Lake Tenaya.* [* Because the main trunk died almost simultaneously throughout its whole extent, we, of course, find no terminal moraines curved across its channels, nor, since its banks were in most places too steeply inclined for their disposition, do we find much of the two laterals. One of the first Tenaya glacierets was developed in the shadow of Yosemite Half Dome. Others were formed along the bases of Coliseum Peak, and the long, precipitous walls extending from near Lake Tenaya to the Big Tuolumne Meadows. The latter, on account of the uniformity and continuity of their protecting shadows, formed moraines of considerable length and regularity, that are liable to be mistaken for portions of the left lateral moraine of the main glacier.]

For a distance of six miles from its mouth the pathway of this noble glacier is a simple trough from 2,000 to 3,000 feet deep, countersunk in the solid granite, with sides inclined at angles with the horizon of from thirty to fifty degrees. Above this its grand simplicity is interrupted by huge moutonéed ridges extending in the general direction of its length over into the basin of Lake Tenaya. Passing these, and crossing the bright glacial pavements that border the lake, we find another series of ridges, from 500 to 1,200 feet in height, extending over the divide to the ancient Tuolumne ice-fountain. Their bare moutonéed forms and polished surfaces indicate that they were overswept, existing at first as mere boulders beneath the mighty glacier that flowed in one unbroken current between Cathedral Peak and the southeast shoulder of the Hoffmann range.

Nevada or South Lyell Glacier

The South Lyell Glacier was less influential than the last, but longer and more symmetrical, and the only one of the Merced system whose sources extended directly to the main summits on the axis of the chain. Its numerous ice-wombs, now mostly barren, range side by side in three distinct series at an elevation above sea-level of from 10,000 to 12,000 feet. The first series on the right side of the basin extends from the Matterhorn to Cathedral Peak in a northwesterly direction a distance of about twelve miles. The second series extends in the same direction along the left side of the basin in the summits of the Merced group, and is about six miles in length. The third is about nine miles long, and extends along the head of the basin in a direction at right angles to that of the others, and unites with them at their southeastern extremities. The three ranges of summits in which these fountains are laid, and the long continuous ridge of Clouds Rest, enclose a rectangular basin, leaving an outlet near the southwest corner opposite its principal nave fountains, situated in the dark jagged peaks of the Lyell group. The main central trunk, lavishly fed by these numerous fountains, was from 1,000 to 1,400 feet in depth, from three-fourths of a mile to a mile and a half in width, and about fifteen miles in length. It first flowed in a northwesterly direction for a few miles, then curving toward the left, pursued a westerly course, and poured its shattered cascading currents down into Yosemite between Half Dome and Mount Starr King.


[Portion of the Left Bank of the Channel of the South Lyell Glacier]
Portion of the Left Bank of the Channel of the South Lyell Glacier,
near the Mouth of Cathedral Tributary.

Could we have visited Yosemite toward the close of the glacial period, we should have found its ice-cascades vastly more glorious than their tiny water representatives of the present hour. One of the most sublime of these was formed by that portion of the South Lyell current which descended the broad, rounded shoulder of Half Dome. The whole glacier resembled an oak with a gnarled swelling base and wide-spreading branches. Its banks, a few miles above Yosemite, were adorned with groups of picturesque rocks of every conceivable form and mode of combination, among which glided swift-descending affluents, mottled with black slates from the summits, and gray granite blocks from ridges and headlands. One of the most interesting facts relating to the early history of this glacier is, that the lofty cathedral spur forming the northeast boundary of its basin was broken through and overflowed by deep ice-currents from the Tuolumne region. The scored and polished gaps eroded by them in their passage across the summit of the spur, trend with admirable steadiness in a northeasterly and southwesterly direction; a fact of great importance, considered in its bearings upon questions relating to the universal ice-sheet. Traces of a similar overflow from the northeast occur on the edges of the basins of all the Yosemite glaciers.

The principal moraines of the basin occur in short, irregular sections scattered along the sides of the valleys, or spread in rough beds in level portions of their bottoms, without manifesting subordination to any system whatever. This fragmentary condition is due to interruptions caused by portions of the sides of the valleys being too precipitous for moraine matter to rest upon and to breakings and down-washings of torrents and avalanches of winter snow. The obscurity resulting from these causes is further augmented by forests and underbrush, making a patient study of details indispensable to the recognition of their unity and simple grandeur. The south lateral moraine of the lower portion of the trunk may be traced about five miles, from the mouth of the north tributary of Mount Clark to the cañon of Illilouette, though simplicity of structure has in most places been prevented by the nature of the ground and by the action of a narrow margin glacier which descended against it with variable pressure from cool, shadowy slopes above. The corresponding section of the right lateral, extending from the mouth of Cathedral tributary to Half Dome, is far more perfect in structure, because of the evenness of the ground, and because the ice-wing which curved against Clouds Rest and descended against it was fully exposed to the sun, and was, therefore, melted long before the main trunk, allowing the latter to complete the formation of this section of its moraine undisturbed. Some conception of its size and general character may be obtained by following the Clouds Rest and Yosemite trail, which crosses it obliquely, leading past several cross-sections made by small streams. A few slate boulders from the Lyell group may be seen, but the main mass of the moraine is composed of ordinary granite and porphyry, the latter having been derived from Feldspar and Cathedral valleys.

The elevation of the top of the moraine near Cathedral tributary is about 8,100 feet; near Half Dome, 7,600. It rests upon the side of the valley at angles varying from fifteen to twenty-five degrees, and in many places is straight and uniform as a railroad embankment. The greatest depth of the glacier between Clouds Rest and Mount Starr King, measuring from the highest points of its lateral moraines, was 1,300 feet. The recurrence of ridges and terraces on its sides indicate oscillations in the level of the glacier, probably caused by clusters of cooler or snowier seasons which no doubt diversified the great glacial winter, just as clusters of sunny or stormy days occasion fluctuations in the level of the streams and prevent monotony in our annual winters. When the depth of the South Lyell Glacier diminished to about 500 feet, it became torpid, on account of the retardation caused by the roughness and crookedness of its channel. But though it henceforth made no farther advance of its whole length, it possessed feeble vitality-in small sections, of exceptional slope or depth, maintaining a squirming and swedging motion, while it lay dying like a wounded serpent. The numerous fountain wombs continued fruitful long after the lower valleys were developed and vitalized with sun-heat. These gave rise to an imposing series of short residual glaciers, extending around three sides of the quadrangle basin, a distance of twenty-four miles. Most of them have but recently succumbed to the demands of the changing seasons, dying in turn, as determined by elevation, size, and exposure. A few still linger in the loftiest and most comprehensive shadows, actively engaged upon the last hieroglyphics which will complete the history of the South Lyell Glacier, forming one of the noblest and most symmetrical sheets of ice manuscripts in the whole Sierra.

Illilouette

The broad, shallow glacier that inhabited the basin of Illilouette more resembled a lake than a river, being nearly half as wide as it was long. Its greatest length was about ten miles, and its depth perhaps nowhere much exceeded 700 feet. Its chief fountains were ranged along the western side of the Merced spur at an elevation of about 10,000 feet. These gave birth to magnificent affluents, flowing in a westerly direction for several miles, in full independence, and uniting near the center of the basin. The principal trunk curved northward, grinding heavily against the lofty wall forming its left bank, and finally poured its ice into Yosemite by the South Cañon between Glacier Point and Mount Starr King. All the phenomena relating to glacial action in this basin are remarkably simple and orderly, on account of the sheltered positions occupied by its principal fountains with reference to the unifying effects of ice-currents from the main summits of the chain. A fine general view, displaying the principal moraines sweeping out into the middle of the basin from Black, Red, Gray, and Clark mountains may be obtained from the eastern base of the cone of Starr King. The right lateral of the tributary which took its rise between Red and Black mountains is a magnificent piece of ice-work. Near the upper end, where it is joined to the shoulder of Red Mountain, it is 250 feet in height, and displays three well marked terraces. From the first to the second of these, the vertical descent is eighty-five feet, and inclination of the surface fifteen degrees; from the second to the third, ninety-five feet, and inclination twenty-five degrees; and from the third to the bottom of the channel, seventy feet, made at an angle of nineteen degrees. The smoothness of the uppermost terrace shows that it is considerably more ancient than the others, many of the blocks of which it was composed having crumbled to sand.

A few miles farther down, the moraine has an average slope in front of about twenty-seven degrees, and an elevation above the bottom of the channel of six hundred and sixty-six feet. More than half of the side of the channel from the top is covered with moraine matter, and overgrown with a dense growth of chaparral, composed of manzanita, cherry, and castanopsis. Blocks of rose-colored granite, many of them very large, occur at intervals all the way from the western base of Mount Clark to Starr King, indicating exactly the course pursued by the ice when the north divide of the basin was overflowed, Mount Clark being the only source whence they could possibly have been derived.

Near the middle of the basin, just where the regular moraines flatten out and disappear, there is outspread a smooth gravel slope, planted with the olive-green Arctostaphylos glauca so as to appear in the distance as a delightful meadow. Sections cut by streams show it to be composed of the same material as the moraines, but finer and more water-worn. The main channel, which is narrow at this point, appears to have been dammed up with ice and terminal moraines, thus giving rise to a central lake, at the bottom of which moraine matter was re-ground and subsequently spread and leveled by the impetuous action of its outbreaking waters. The southern boundary of the basin is a strikingly perfect wall, extending sheer and unbroken from Black Mountain* [* This mountain occurs next south of Red Mountain, and must not be confounded with the Black Mountain six miles farther south.] to Buena Vista Peak, casting a long, cool shadow all through the summer for the protection of fountain snow. The northern rim presents a beautiful succession of smooth undulations, rising here and there to a dome, their pale gray sides dotted with junipers and silver-leafed pines, and separated by dark, feathery base-fringes of fir.

The ice-plows of Illilouette, ranged side by side in orderly gangs, have furrowed its rocks with admirable uniformity, producing irrigating channels for a brood of wild streams, and abundance of deep, rich soils, adapted to every requirement of garden and grove. No other section of the Yosemite uplands is in so high a state of glacial cultivation. Its clustering domes, sheer walls, and lofty towering peaks, however majestic in themselves, are only border adornments, submissively subordinate to their sublime garden center. The basins of Yosemite Creek, Tenaya, and South Lyell are pages of sculptured rocks embellished with gardens. The Illilouette basin is one grand garden embellished with rocks.

Nature manifests her love for the number five in her glaciers, as well as in the petals of the flowers which she plants in their pathways. These five Yosemite glaciers we have been sketching are as directly related to one another, and for as definite an object, as are the organs of a plant. After uniting in the valley, and expending the down-thrusting power with which they were endowed by virtue of the declivity of their channels, the trunk flowed up out of the valley without yielding much compliance to the crooked and comparatively small river cañon extending in a general westerly direction from the foot of the main valley. In effecting its exit a considerable ascent was made, traces of which are to be seen in the upward slope of the worn, rounded extremities of the valley walls. Down this glacier-constructed grade descend both the Coulterville and Mariposa trails; and we might further observe in this connection that, because the ice-sheet near the period of transition to distinct glaciers flowed southwesterly the south lips of all Yosemites trending east and west, other conditions being equal, are more heavily eroded, making the construction of trails on that side easier. The first trail, therefore, that was made into Yosemite, was of course made down over the south lip. The only trail entering the Tuolumne Yosemite descends the south lip, and so also does the only trail leading into the Kings River Yosemite. A large majority of deer and bear and Indian trails likewise descend the south lips of Yosemites. So extensively are the movements of men and animals controlled by the previous movements of certain snow-crystals combined as glaciers.

The direction pursued by the Yosemite trunk, after escaping from the valley, is unmistakably indicated by its immense lateral moraines extending from its lips in a west-southwesterly direction. The right moraine was disturbed by the large tributary of Cascade Creek, and is extremely complicated in structure. The left is simple until it comes under the influence of tributaries from the southeast, and both are further obscured by forests which flourish upon their mixed soil, and by the washing of rains and melting snows, and the weathering of their boulders, making a smooth, sandy, unmorainelike surface. It is, therefore, the less to be wondered at that the nature of these moraines, which represent so important a part of the chips hewn from the valley in the course of its formation, should not have been sooner recognized. Similarly situated moraines extend from the lips of every Yosemite wherever the ground admits of their deposition and retention. In Hetch-Hetchy and other smaller and younger Yosemites of the upper Merced, the ascending striae which measure the angle of ascent made by the bottom of their glaciers in their outflow are still clearly visible.


[Fig. 1]
Fig. 1
Fig. 1 is the horizontal section of the end of a Yosemite valley, showing the ordinary boat-shaped edge, and lateral moraines (M M) extending from the lips. The moraines and arrows indicate the course pursued by the outflowing ice.
[Fig. 2]
Fig. 2
Fig. 2 represents the right lip of Yosemite, situated on the upper Merced below the confluence of Cathedral tributary. The whole lip is polished and striated. The arrows indicate the direction of the striae, which measure the angle of ascent made by the outflowing ice.

In the presentation of these studies, we have proceeded thus far with the assumption that all the valleys of the region are valleys of erosion, and that glaciers were the principal eroding agents; because the intelligible discussion of these propositions requires some knowledge of the physiognomy and general configuration of the region, as well as of the history of its ancient glaciers. Our space is here available only for very brief outlines of a portion of the argument, which will be gradually developed in subsequent articles.

That fossils were created as they occur in the rocks, is an ancient doctrine, now so little believed that geologists are spared the pains of proving that nature ever deals in fragmentary creations of any sort. All of our valleys are clearly fragmentary in some degree.

[Fig. 3]
Fig. 3
[Fig. 4]
Fig. 4
Fig. 3 is a section across Yosemite Valley from Indian Cañon, which displays the stumps of slabs and columns of which the granite is here composed. Now, the complements of these broken rocks must have occupied all, or part, or more than all of the two portions of the valley, A C D and B E F. The bottom, A B, is covered with drift, but we may assume that if it were laid bare we would find it made up of the ends of slabs and columns like the sides, which filled the space A C E B; because in all valleys where the bottom is naked, the broken stumps do appear, showing that this valley was not formed by a fold in the mountain surface, or by a splitting asunder, or by subsidence, but by a breaking up and translation of rocks which occupied its place, or, in other words, by erosion.

Fig. 4 is a section across the lower portion of the valley of Illilouette south of Mount Starr King. In this case the bottom is naked, and the dotted reconstructed portions of the huge granite folds A B C D have evidently been eroded.* [* Water never erodes a wide U-shaped valley in granite, but always a narrow gorge like E F, in Fig. 4.] Even the smoothly curved trough of two rock-waves which afford sections like Fig. 5 can not be regarded as a valley originating in a fold of the surface, for we have shown in the first paper of this series that domes or extended waves, with a concentric structure like A C, may exist as concretionary or crystalline masses beneath the surface of granite possessing an entirely different structure or no determinate structure whatever, as in B.


[Fig. 5]
Fig. 5
[Fig. 6.—Illustrating Bend of Upper Tuolumne Valley]
Fig. 6.—Illustrating Bend of Upper Tuolumne Valley

The chief valley-eroding agents are water and ice. Each has been vaguely considered the more influential by different observers, although the phenomena to which they give rise are immensely different. These workmen are known by their chips, and only glacier chips form moraines which correspond in kind and quantity to the size of the valleys and condition of their surfaces. Also their structure unfolds the secret of their origin. The constant and inseparable relations of trend, size, and form which these Sierra valleys sustain to the ice-fountains in which they all head, as well as their grooved and broken sides, proclaim the eroding force to be ice. We have shown in the second chapter that the trend of Yosemite valleys is always a direct resultant of the forces of their ancient glaciers, modified by obvious peculiarities of physical structure of their rocks. The same is true of all valleys in this region. We give one example, the upper Tuolumne Valley, which is about eight miles long, and from 2,000 to 3,000 feet deep, and trends in a generally northerly direction. If we go to its head on the base of Mount Lyell, and follow it down, we find that after trending steadily about two miles it makes a bend of a few degrees to the left (A, Fig. 6). Looking for the cause, we perceive a depression on the opposite or right wall; ascending to it, we find the depression to be the mouth of a tributary valley which leads to a crater-shaped ice-fountain (B) which gave rise to the tributary glacier that, in thrusting itself into the valley trunk, caused the bend we are studying. After maintaining the new trend thus acquired for a distance of about a mile and a half, the huge valley swerves lithely to the right at C. Looking for the cause, we find another tributary ice-grooved valley coming in on the left, which like the first conducts back to an ice-womb (D) which gave birth to a glacier that in uniting with the trunk pushed it aside as far as its force, modified by the direction, smoothness, and declivity of its channel, enabled it to do. Below this, the noble valley is again pushed round in a curve to the left by a series of small tributaries which, of course, enter on the right, and with each change in trend there is always a corresponding change in width or depth, or in both. No valley changes its direction without becoming larger. On nearing the Big Meadows it is swept entirely round to the west by huge glaciers, represented by the large arrows, which descended from the flanks of Mounts Dana, Gibbs, Ord, and others to the south. For thirty miles farther, we find everywhere displayed the same delicate yielding to glacial law, showing that, throughout the whole period of its formation, the huge granite valley was lithe as a serpent, and winced tenderly to the touch of every tributary. So simple and sublime is the dynamics of the ancient glaciers.

Every valley in the region gives understandable evidence of having been equally obedient and sensitive to glacial force, and to no other. The erosive energy of ice is almost universally underrated, because we know so little about it. Water is our constant companion, but we cannot dwell with ice. Water is far more human than ice, and also far more outspoken. If glaciers, like roaring torrents, were endowed with voices commensurate with their strength, we would be slow to question any ascription of power that has yet been bestowed upon them. With reference to size, we have seen that the greater the ice-fountains the greater the resulting valleys; but no such direct and simple proportion exists between areas drained by water streams and the valleys in which they flow. Thus, the basin of Tenaya is not one-fourth the size of the South Lyell, although its cañon is much larger. Indeed, many cañons have no streams at all, whose topographical circumstances are also such as demonstrate the impossibility of their ever having had any. This state of things could not exist if the water streams which succeeded the glaciers could follow in their tracks, but the mode and extent of the compliance which glaciers yield to the topography of a mountainside, is very different from that yielded by water streams; both follow the lines of greatest declivity, but the former in a far more general way. Thus, the greater portion of the ice-current which eroded Tenaya Cañon flowed over the divide from the Tuolumne region, making an ascent of over 500 feet. Water streams, of course, could not follow; hence the dry channels, and the disparity, to which we have called attention, between Tenaya Cañon and its basin.

Anyone who has attentively observed the habits and gestures of the upper Sierra streams, could not fail to perceive that they are young, and but little acquainted with the mountains; rushing wildly down steep inclines, whirling in pools, sleeping in lakes, often halting with an embarrassed air and turning back, groping their way as best they can, moving most lightly just where the glaciers bore down most heavily. With glaciers as a key the secrets of every valley are unlocked. Streams of ice explain all the phenomena; streams of water do not explain any; neither do subsidences, fissures, or pressure plications.

We have shown in the previous paper that post-glacial streams have not eroded the 500,000th part of the upper Merced cañons. The deepest water gorges with which we are acquainted are between the upper and lower Yosemite falls, and in the Tenaya Cañon about four miles above Mirror Lake. These are from twenty to a hundred feet deep, and are easily distinguished from ice-eroded gorges by their narrowness and the ruggedness of their washed and pot-holed sides.

The gorge of Niagara River, below the falls, is perhaps the grandest known example of a valley eroded by water in compact rock; yet, comparing equal lengths, the glacier-eroded valley of Yosemite is a hundred times as large, reckoning the average width of the former 900 feet, and depth 200. But the erosion of Yosemite Valley, besides being a hundred times greater, was accomplished in hard granite, while the Niagara was in shales and limestones. Moreover, Niagara cañon, as it now exists, expresses nearly the whole amount of erosion effected by the river; but the present Yosemite is by no means an adequate expression of the whole quantity of glacial erosion effected there since the beginning of the glacial epoch, or even from that point in the period when its principal features began to be developed, because the walls were being cut down on the top simultaneously with the deepening of its bottom. We may fairly ascribe the formation of the Niagara gorge to its river, because we find it at the upper end engaged in the work of its further extension toward Lake Erie; and for the same reason we may regard glaciers as the workmen that excavated Yosemite, for at the heads of some of its branches we find small glaciers engaged in the same kind of excavation. Merced cañons may be compared to mortises in the ends of which we still find the chisels that cut them, though now rusted and worn out. If Niagara River should vanish, or be represented only by a small brook, the evidence of the erosion of its gorge would still remain in a thousand water-worn monuments upon its walls. Nor, since Yosemite glaciers have been burned off by the sun, is the proof less conclusive that in their greater extension they excavated Yosemite, for, both in shape and sculpture, every Yosemite rock is a glacial monument.

When we walk the pathways of Yosemite glaciers and contemplate their separate works-the mountains they have shaped, the cañons they have furrowed, the rocks they have worn, and broken, and scattered in moraines—on reaching Yosemite, instead of being overwhelmed as at first with its uncompared magnitude, we ask, Is this all? wondering that so mighty a concentration of energy did not find yet grander expression.


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