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Studies in the Sierra

by John Muir

V
Post-Glacial Denudation

When 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 post-glacial 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 irregularity 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 ice-sheet 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 distintegration 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 foam-dust 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 cloud-shadows. 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 strikingly a characteristic of cañon scenery are due. The largest of these earthquake taluses are from 500 to 1,000 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 water-torrents. They are frequently seen descending the summit peaks, flowing in regular channels, the surfaces of which they erode by striking off large chips and blocks, as well as by wearing off sand and dust.

A considerable amount of grinding also goes on in the body of the avalanche itself, reducing the size of the masses, and preparing them for the action of other agents. Some avalanches hurl their detritus directly into the beds of streams, thus bringing it under the influence of running water, by which a portion of it is carried into the ocean.

The range of rock avalanches, however produced, is restricted within comparatively narrow bounds. The shattered peaks are constant fountains, but the more powerful mountain-shaking avalanches are confined to the edges of deep cañons in a zone twelve or fifteen miles wide, and gradually merge into land-slips along their lower limits.

Large rock avalanches pour freely through the air from a height of hundreds or thousands of feet, and on striking the bottom of the valley are dashed into a kind of coarse stone foam. Or, they make the descent in several leaps, or rumble over jagged inclines in the form of cascades. But in any case they constitute currents of loose-flowing fragments. Landslips, on the contrary, slip in one mass, and, unless sheer cliffs lie in their paths, may come to rest right-side up and undivided. There is also a marked difference in their geographical distribution, land-slips being restricted to deeply eroded banks and hillsides of the lower half of the range, beginning just where rock avalanches cease. Again, the material of land-slips is chiefly fine soil and decomposing boulders, while that of rock avalanches is mostly of unweathered angular blocks.


[Fig. 1]
Fig. 1

Let Figure 1 represent a section across a valley in which moraine matter, A, is deposited upon the inclined bed-rock, B B B. Now, strong young moraine material deposited in this way, in a kind of rude masonry, always rests, or is capable of resting, at a much steeper angle than the same material after it has grown old and rotten. If a poultice of acid mud be applied to a strong boulder, it will not be much affected in an hour or day, but if kept on for a few thousands or tens of thousands of years, it will at length soften and crumble. Now, Nature thus patiently poultices the boulders of the moraine banks under consideration. For many years subsequent to the close of the ice period very little acid for this purpose was available, but as vegetation increased and decayed, acids became more plentiful, and boulder decomposition went on at an accelerated rate, until a degree of weakness was induced that caused the sheerest portions of the deposits, as A B D (Fig. 1), to give way, perhaps when jarred by an earthquake, or when burdened with snow or rain, or partially undermined by the action of a stream.

It appears, therefore, that the main cause of the first post-glacial landslips is old age. They undoubtedly made their first appearance in moraine banks at the foot of the range, and gradually extended upward to where, we now find them, at a rate of progress measured by that of the recession of the ice-sheet, and by the durability of moraines and the effectiveness of the corroding forces brought into action upon them. In those portions of the Sierra where the morainal deposits are tolerably uniform in kind and exposure, the upper limits of the land-slip are seen to stretch along the range with as great constancy of altitude as that of the snow-line.

The above-described species of land-slip is followed up the range by another of greater size, just as the different forest trees follow one another in compliance with conditions of soil and climate. After the sheer end the deposit (A B D, Fig. 1 ) has slipped, the whole mass may finally slip on the bed-rock by the further decomposition, not only of the deposit itself, but of the bed-rock on which it rests. Bed-rocks are usually more or less uneven. Now, it is plain that when the inequalities B B B crumble by erosion, the mass of the deposit will not be so well supported; moreover, the weight of the mass will continue to increase as its material is more thoroughly pulverized, because a greater quantity of moisture will be required to saturate it. Thus it appears that the support of moraine deposits diminishes, just as the necessity for greater support increases, until a slip is brought on.

Slips of this species are often of great extent, the surface comprising several acres overgrown with trees, perhaps moving slowly and coming to rest with all their load of vegetation uninjured, leaving only a yawning rent to mark their occurrence. Others break up into a muddy disorderly flood, moving rapidly until the bottom of the wall is reached. Land-slides occur more frequently on the north than on the south sides of ridges because of the greater abundance of weight-producing and decomposing moisture. One of the commonest effects of land-slips is the damming of streams, giving rise to large accumulations of water, which speedily burst the dams and deluge the valleys beneath, sweeping the finer detritus before them to great distances, and at first carry boulders tons in weight.

The quantity of denudation accomplished by the Sierra land-slips of both species is very small. Like rock-falls, they erode the surface they slip upon in a mechanical way, and also bring down material to lower levels, where it may be more advantageously exposed to the denuding action of other agents, and open scars whereby rain-torrents are enabled to erode gullies; but the sum of the areas thus affected bears an exceedingly small proportion to the whole surface of the range.

The part which snow avalanches play in the degradation of mountains is simpler than that of free-falling or cascading rocks, or either species of land-slip; these snow avalanches being external and distinct agents. Their range, however, is as restricted as that of either of the others, and like them they only carry their detritus a short distance and leave it in heaps at the foot of cliffs and steep inclines. There are three well-marked and distinct species of snow avalanche in the upper half of the Sierra, differing widely in structure, geographical distribution, and in the extent and importance of the geological changes they effect. The simplest and commonest species is formed of fresh mealy snow, and occurs during and a short time after every heavy snow-fall wherever the mountain slopes are inclined at suitable angles. This species is of frequent occurrence throughout all the steep-flanked mountains of the summit of the range, where it reaches perfection, and is also common throughout the greater portion of the middle region. Avalanches are the feeders of the glaciers, pouring down their dry mealy snow into the womb-amphitheaters, where it is changed to névé and ice. Unless distributed by storm-winds, they cascade down the jagged heights in regular channels, and glide gracefully out over the glacier slopes in beautiful curves; which action gives rise in summer to a most interesting and comprehensive system of snow-sculpture. The detritus discharged upon the surface of the glaciers forms a kind of stone-drift which is floated into moraines like the straws and chips of rivers.

Few of the defrauded toilers of the plain know the magnificent exhilaration of the boom and rush and outbounding energy of great snow avalanches. While the storms that breed them are in progress, the thronging flakes darken the air at noonday. Their muffled voices reverberate through the gloomy cañons, but we try in vain to catch a glimpse of their noble forms until rifts appear in the clouds, and the storm ceases. Then in cliff-walled valleys like Yosemite we may witness the descent of half a dozen or more snow avalanches within a few hours.

The denuding power of this species of avalanche is not great, because the looseness of the masses allows them to roll and slip upon themselves. Some portions of their channels, however, present a roughly scoured appearance, caused by rocky detritus borne forward in the under portion of the current. The avalanche is, of course, collected in a heap at the foot of the cliff, and on melting leaves the detritus to accumulate from year to year. These taluses present striking contrasts to those of rock avalanches caused by the first great pre-glacial earthquake. The latter are gray in color, with a covering of slow-growing lichens, and support extensive groves of pine, spruce, and live-oak; while the former, receiving additions from year to year, are kept in a raw formative state, neither trees nor lichens being allowed time to grow, and it is a fact of great geological significance that no one of the Yosemite snow avalanches, although they have undoubtedly flowed in their present channels since the close of the glacial period, has yet accumulated so much débris as some of the larger earthquake avalanches which were formed in a few seconds.

The next species of avalanche in natural order is the annual one, composed of heavy crystalline snows which have been subjected to numerous alternations of frost and thaw. Their development requires a shadowed mountain side 9,000 or 10,000 feet high, inclined at such an angle that loose fresh snow will lodge and remain upon it, and bear repeated accessions throughout the winter without moving; but which, after the spring thaws set in, and the mountain side thus becomes slippery, and the nether surface of the snow becomes icy, will then give way.

One of the most accessible of the fountains of annual avalanches is the northern slope of Cloud's Rest, above the head of the Yosemite Valley. Here I have witnessed the descent of three within half an hour. They have a vertical descent of nearly a mile on a smooth granite surface. Fine examples of this species of avalanche may also be observed upon the north side of the dividing ridge between the basins of Ribbon and Cascade creeks, and in some portions of the upper Nevada Cañon. Their denuding power is much greater than that of the first species, on account of their greater weight and compactness. Where their pathways are not broken by precipices, they descend all or part of their courses with a hard snout kept close down on the surface of the rock, and because the middle of the snout is stronger, the detritus heaps are curved after the manner of terminal moraines. These detritus heaps also show an irregularly corrugated and concentric structure. An examination of the avalanche pathways shows conclusively that the annual accretions of detritus, scraped from their surfaces, are wholly insufficient to account for the several large concentric deposits. But when, after the detritus of many years has been accumulated by avalanches of ordinary magnitude, a combination of causes, such as rain, temperature, and abundant snow-fall, gives rise to an avalanche of extraordinary size, its superior momentum will carry it beyond the limits attained by its predecessors, and sweep forward the accumulations of many years concentric with others of like magnitude into a single mass. A succession of these irregularities will obviously produce results corresponding in every particular with the observed phenomena.

What we may call century avalanches, as distinguished from annual, are conceived and nourished on cool mountain sides 10,000 or 12,000 feet in height, where the snow falling from winter to winter will not slip, and where the exposure and temperature are such that it will not always melt off in summer. Snow accumulated under these conditions may linger without seeming to greatly change for years, until some slowly organized group of causes, such as temperature, abundance of snow, condition of snow, or the mere occurrence of an earthquake, launches the grand mass. In swooping down the mountain flanks they usually strip off the forest trees in their way, as well as the soil on which they were growing.

Some of these avalanche pathways are 200 yards wide, and extend from the upper limit of the tree-line to the bottom of the valleys. They are all well "blazed" on both sides by descending trunks, many of which carry sharp stones clutched in their up-torn roots. The height of these "blazes" on the trees bordering the avalanche gap measures the depth of the avalanche at the sides, while in rare instances some noble silver-fir is found standing out in the channel, the only tree sufficiently strong to withstand the mighty onset; the scars upon which, or its broken branches, recording the depth of the current. The ages of the trees show that some of these colossal avalanches occur only once in a century, or at still wider intervals. These avalanches are by far the most powerful of the three species, although from the rarity of their occurrence and the narrowness of the zone in which they find climatic conditions suited to their development, the sum of the denudation accomplished by them is less than that of either of the others.

We have seen that water in the condition of rain, dew, vapor, and melting snow, combined with air, acts with more or less efficiency in corroding the whole mountain surface, thus preparing it for the more obviously mechanical action of winds, rivers, and avalanches. Running water is usually regarded as the most influential of all denuding agents. Those regions of the globe first laid bare by the melting of the ice-sheet present no unchanged glaciated surfaces from which, measuring down, we may estimate the amount of post-glacial denudation. The streams of these old eroded countries are said by the poets to "go on forever," and the conceptions of some geologists concerning them are scarcely less vague.

Beginning at the foot of the Sierra glaciers, and following the torrents that rush out from beneath them down the valleys, we find that the rocks over which they flow are weathered gradually, and increasingly, the farther we descend; showing that the streams in coming into existence grew like trees from the foot of the range upward, gradually ramifying higher and wider as the ice-sheet was withdrawn--some of the topmost branchlets being still in process of formation.

Rivers are usually regarded as irregular branching strips of running water, shaped somewhat like a tree stripped of its leaves. As far as more striking features and effects are concerned, the comparison is a good one; for in tracing rivers to their fountains we observe that as their branches divide and redivide, they speedily become silent and inconspicuous, and apparently channelless; yet it is a mistake to suppose that streams really terminate where they become too small to sing out audibly, or erode distinct channels. When we stoop down and closely examine any portion of a mountain surface during the progress of a rain-storm, we perceive minute water-twigs that continue to bifurcate until like netted veins of leaves the innumerable currentlets disappear in a broad universal sheet.

It would appear, therefore, that rivers more nearly resemble certain gigantic algae with naked stalks, and branches webbed into a flat thallus. The long unbranched stalks run through the dry foothills; the webbed branches frequently overspread the whole surface of the snowy and rainy alpine and middle regions, as well as every moraine, bog, and névé bank. The gently gliding rain-thallus fills up small pits as lakelets and carries away minute specks of dust and mica. Larger sand-grains are overflowed without being moved unless the surface be steeply inclined, while the rough grains of quartz, hornblende, and feldspar, into which granite crumbles, form obstacles around which it passes in curves. Where the currentlets concentrate into small rills, these larger chips and crystals are rolled over and over, or swept forward partly suspended, just as dust and sand-grains are by the wind.

The transporting power of steeply inclined torrents is far greater than Is commonly supposed. Stones weighing several tons are swept down steep cañon gorges and spread in rugged deltas at their mouths, as if they had been floated and stranded like blocks of wood. The denudation of gorges by the friction of the boulders thus urged gratingly along their channels is often quite marked.

Strong torrents also denude their channels by the removal of blocks made separable from the solid bed-rock by the development of cleavage planes. Instructive examples of this species of denudation may be studied m the gorges between the upper and lower Yosemite falls and the Tenaya Cañon, four miles above Mirror Lake. This is the most rapid mode of torrent denudation I have yet observed, but its range is narrowly restricted, and its general denuding effects inappreciable.

Water-streams also denude mountains by dissolving them and carrying them away in solution, but the infinite slowness of this action on hard porphyritic granite is strikingly exemplified by the fact that in the upper portion of the middle region granite ice-planed pavements have been flowed upon incessantly since they were laid bare on the breaking up of the glacial winter without being either decomposed, dissolved, or mechanically eroded to the depth of the one-hundredth part of an inch.

Wind-blown dust, mica flakes, sand, and crumbling chips are being incessantly moved to lower levels wherever wind or water flows. But even in the largest mountain rivers the movement of large boulders is comparatively a rare occurrence. When one lies down on a river-bank opposite a boulder-spread incline and listens patiently for a day or two, a dull thumping sound may occasionally be heard from the shifting of a boulder, but in ordinary times few streams do much boulder work; all the more easily moved blocks having been adjusted and readjusted during freshets, when the current was many times more powerful. All the channels of Sierra streams are subjected to the test action of at least one freshet per season, on the melting of the winter snow, when all weakly constructed dams and drift-heaps are broken up and re-formed.

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 dose 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 pedestal for the stone which saved it. Figure 2 shows where a boulder, B. has settled upon and protected from erosion a portion of the original glaciated surface until the pedestal, A, has been formed, the height of which is of course the exact measure of the whole quantity of post-glacial denudation 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 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.


[Fig. 2]
Fig. 2.
[Fig. 3]
Fig. 3.

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.


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