Studies in the Sierra. No. VI.-Formation of Soils.

68 Sierra Club Bulletin could not expect to live more than ten years, because of some forecast by an eminent man as to seventy-five years being his limit of age. It came as no surprise to hear that this master in the realm of geology and physiography, this teacher revered and beloved by his disciples, this man of the broad sympathies and of the great heart, had passed away shortly after believing that at a time when his maximum effort was needed in the cause of humanity he had passed his zenith and had entered instead upon his days of splendid decadence. . Geological Survey, Department of Mines, Sydney, N. S. W. The mind of ens with the enlarging day. It is said the sand-hills of the desert under the noonday sun emit strange sounds; that the rocky valleys are vocal; the primeval for est speaks in its depths; hollow ocean sends a muttering to the becalmed vessel; and up in the mountains the bound words are set loose. Of old times the huntsmen in our own woods met the noon-day spirit under the leafy canopy. RICHARD JEFFERIES STUDIES IN THE SIERRA* By John Muir NO. VI. FORMATION OF SOILS NATURE has plowed the Sierra flanks more than a mile deep through lava, slate, and granite, thus giving rise to a most lavish abundance of fruitful soils. The various methods of detachment of soil-fragments from the solid rocks have been already considered in the foregoing studies on glacial and post-glacial denudation. It now remains to study the formation of the variously eroded fragments into beds available for the uses of vegetable life. If all the soils that now mantle the Sierra flanks were spread out in one sheet of uniform thickness, it would measure only a few feet in depth, and its entire removal would not appreciably affect the configuration of any portion of the range. The largest beds rarely exceed a hundred feet in average thickness, and a very considerable proportion of the whole surface is naked. But we have seen that glaciers alone have ground the west flank of the range into soil to a depth of more than a mile, without taking into account the work of other soil-producing agents, as rains, avalanches, torrents, earthquakes, etc. It appears, therefore, that not the one-thousandth part of the whole quantity of soil eroded from the range since the beginning of the glacial epoch is now left upon its flanks. The cause of this comparative scantiness of the Sierra soil- beds will be readily apprehended when we reflect that the glacier, which is the chief soil-producing agent, no sooner detaches a soil-fragment than it begins to carry it away. During the long glacial winter, soil-material was poured from the range as from a fountain, borne outward by the mighty currents of the ice-sheet to be deposited in its terminal moraines. The only one of these ancient ice-sheet moraines which has re- * Reprinted from the Overland Monthly of December, 1874. The author\u27s revisions and corrections have been incorporated from a copy of the article found among his papers.ΓÇöThe Editors. 7o Sierra Club Bulletin tained its principal characteristics unaltered down to the present time is that magnificent belt of soil upon which all the majestic forests of the Sierra are growing. It stretches along the west flank of the range like a smooth-flowing ribbon, waving compliantly up and down over a thousand hills and hollows, at an elevation of from four to seven thousand feet above the level of the sea. In some places it is more than a hundred feet deep and twenty miles wide, but it is irregular as a sun-wasted snow-wreath both in width and in depth, on account of the configuration of the surface upon which it rests, and the varying thickness and declivity of the ice-sheet at the period of its deposition. The long weathering and the multitude of storm- washings to which it has been subjected have made its outlines still more indefinite and variable. Furthermore, its continuity is interrupted at intervals of fifteen or twenty miles by the river canons which cross it nearly at right angles. For, at the period of the deposition of the main soil-belt as a terminal moraine of the ice-sheet, long finger-like glaciers extended down every one of these canons, thus effectually preventing the-continuance of the main terminal moraine across the canon channels. The method of the deposition of broad belts of terminal - moraine soil will be made plain by reference to Figure I, which represents a deposit of this kind lying at the foot of Moraine Lake, made by the Bloody Canon glacier in its recession toward the period of its extinction. A A are the main lateral moraines extending Γûá from the jaws of the canon out into the Mono Plain; I, 2, 3, 4, 5, 6 are concentric belts of terminal - moraine soil deposited by the glacier in its retreat. These soil-belts, or fur- Studies in the Sierra 7i Fig. i. rows, are twenty or thirty yards apart. After belt number i was laid down, the glacier evidently withdrew at a faster rate, until a change of climate as regards heat or cold, or the occurrence of a cluster of snowier years, checked its backward motion sufficiently to afford it time to deposit belt number 2, and so on; the speed of the dying glacier\u27s retreat being increased and diminished in rhythmic alternations of frost and thaw, sunshine and snow, all of which found beautiful and enduring expression in its ridged moraines. The promontories P P are portions of a terminal soil-belt, part of which is covered by the lake. Similar fields of corrugated moraine matter occur farther down, marking lingering and fluctuating periods in the recession of the glacier similar to the series we have been studying. Now, it is evident that if, instead of thus dying a lingering death, the glacier had melted suddenly while it extended into the Mono plain, these wide soil-fields could not have been made. Neither could the grand soil-belt of the western flank have existed if the ice-sheet had melted in one immense thaw while it extended as a seamless mantle over all the western flank. Fortunately for Sierra vegetation and the life dependent upon it, this was not the case; instead of disappearing suddenly, like a sun-stricken cloud, it withdrew from the base of the great soil- belt upward, in that magnificently deliberate way so characteristic of natureΓÇöadding belt to belt in beautiful order over lofty plateaus and rolling hills and valleys, wherever soil could be made to lie. Winds and rains, acting throughout the ample centuries, smooth rough glacial soils like harrows and rollers. But this culture is carried on at an infinitely slow rate, as we measure time. Comparing the several moraine-fields of Bloody Canon, we observe that the ridged concentric structure (Fig. i) becomes gradually less distinct the farther we proceed out into the plain, just as the plow-ridges in a farmer\u27s field become less distinct the more they are harrowed. Now, the difference in time between the deposition of contiguous moraine-fields in Bloody Canon is probably thousands of years, yet the difference as regards smoothness and freshness of aspect corresponding to this difference in time is in some instances scarcely discernible. In 72 Sierra Club Bulletin Studies in the Sierra 73 the field represented in Figure I these leveling operations may be studied to excellent advantage. The furrows between the several ridges are leisurely filled up by the inblowing and washing of leaves and the finer material of the adjacent ridges. As the weathering of the surface boulders goes on, the crumbling material which falls from them collects about their bases, thus tending to bury them, and produce that smoothness of surface which characterizes all the more ancient moraine-fields of the Sierra. The great forest soil-belt of the west flank has not been hitherto recognized as a moraine at all, because not only is it so immensely extended that general views of it can not be easily obtained, but it has been weathered until the greater portion of its surface presents as smooth an appearance as a farmer\u27s wheat-field. It may be urged against the morainal origin of the forest belt that its sections exposed by freshet streams present a quite different appearance from similar sections of more recent moraine-beds unmistakably such; but careful inspection shows the same gradual transition from the boulder roughness of the one to the crumbled earthiness of the other that we have already traced between the superficial roughness and smoothness of moraines according to age. Under certain conditions moraine boulders decompose more rapidly beneath than upon the surface. Almost every section of the forest belt presents specimens in every stage of decay, and, because those that are water-rounded and polished are more enduring than others, they occur in comparatively greater abundance as the soil becomes more ancient. The position of the soil-belt is given in the ideal cross-section of the range (Fig. 2)1. Its upper limit nearly coincides with the edge of a comparatively level bench, A B, extending back to the summit peaks. Upon this lofty, gently inclined bed the waning ice- sheet lay nearly motionless, shallowing simultaneously across its whole breadth, and finally broke up into distinct ice-streams which occupied the present river canons. These have left their lateral moraines in the form of long branching ridges of soil, several miles apart, extending from the summit ice-wombs down to the main soil-belt, into which they blend and disappear. But if the ice-sheet had maintained its continuity to the very end of the glacial epoch, soil would evidently have been laid down in one continuous bed all the way back to the summit, because under these conditions every portion of the surface in succession would have been loaded with terminal moraine-belts pressed one against the other like plow-ridges. Under the conditions which prevailed toward the close of the great winter, the separate glaciers as well as the ice-sheet shallowed, became torpid, and died away simultaneously throughout all this upper region; no terminal moraines are therefore to be met until we reach those of the small residual glaciers which took shelter in the loftiest and coolest shadows of the summit peaks. Nor will this state of things be wondered at, when we consider how slight is the difference in elevation, and consequently in climate, between the upper and lower limits (A and B, Fig. 2) of this bare alpine bench, as compared with that of the slope (C A) beneath it, upon which the soil-belt lies. The effect of shadows in determining the formation, size, and distribution of glacial soil-beds must not be overlooked. When the seasons grew warm and the long crooked glaciers were driven from the sun-beaten summit bench, thousands of small residual glaciers, from half a mile to two or three miles in length, lingered on through many a century in the shelter of frosty shadows. Accordingly, we find the moraines of these hiding glaciers in the highest and coolest recesses, shaped and measured with strict reference to their adjacent shadows. A considerable number of these interesting shadow-moraines are still in process of formation, presenting a raw and rubbish-like appearance, as if the boulders, mud, and sand of which they are composed had been newly mined from the mountain\u27s flank, and dumped loosely from a car. Ancient shadow-moraines, 74 Sierra Club Bull delightfully gardened and forested, occur in all deep Yosemitic canons trending in an east and west direction; but their first forms are so heavily obscured by thousands of years of weathering that their shadow-glacial origin would scarcely be suspected. In addition to these broad zones and fields and regularly deposited moraine ridges, glacial soil occurs in isolated strips and patches upon the wildest and most unlikely placesΓÇöaloft on jutting crags, and along narrow horizontal benches ranged one above another, on sheer-fronted precipices, wherever the strong and gentle glaciers could get a boulder to lie. To these inaccessible soil-beds companies of pines and alp-loving flowers have found their way, and formed themselves into waving fringes and rosettes, whose beauty is strikingly relieved upon the massive ice-sculptured walls. Nothing in the history of glacial soil-beds seems more remarkable than their durability in the forms in which they were first laid down. The wild violence of mountain storms would lead one to fancy that every moraine would be swept from plateau and ridge in less than a dozen seasons, yet we find those of the upper half of the range scarcely altered by the tear and wear of thousands of years. Those of the lower half are far more ancient, and their material has evidently been shifted and reformed until their original characteristics are almost entirely lost. These fresh glacier-formed soils are subject to modifications of various kinds. After the coarse, unbolted moraine soils derived from granite, slate, and lava have been well watered and snow-pressed, they are admirably adapted for the ordinary food and anchorage of coniferous trees, but further manipulation is required to fit them for special grove and garden purposes. The first and most general action to which they are subjected is that of slow atmospheric decomposition, which mellows and smooths them for the reception of blooming robes of under- shrubs and grasses, and up to a certain point augments their capacity for the support of pines and firs. Streams of rain and melting snow rank next in importance as modifiers of glacial soils. Powerful torrents waste and change the most compact beds with great rapidity, but the work done by small rain-cur- Studies in the Sierra 75 rents and low-voiced brooks is very much less than is vaguely supposed. The brook which drains the south flank of the Clouds\u27 Rest ridge, above Yosemite Valley, in making its way southward to join the Nevada Creek, is deflected to the west by the right lateral moraine of the ancient Nevada glacier, and compelled to creep and feel its. way along the outside of the moraine as far as to where it is caught between the moraine and an escarpment which advances from the Clouds\u27 Rest crest. When halted here, it spread into a pool, and rose until it was able to effect its escape over the lowest portion of the barrier. Now this stream, which in ordinary stages is about five feet wide and a foot deep, seems to have flowed unfailingly in one channel throughout all the long post-glacial centuries, but the only erosion the moraine has suffered is the removal of sand, mud, and some of the smaller boulders, while the large stones, jammed into a kind of wall, are merely polished by the friction of the stream, and bid fair to last tens of thousands of years. The permanence of soils depends more upon their position and mechanical structure than upon their composition. Coarse porous moraine matter permits rains and melting snows to percolate unimpeded, while muddy and impermeable beds are washed and wasted on the surface. Snow avalanches more resemble glaciers in their methods of soil formation and distribution than any other of the post-glacial agents. The century avalanche sweeps down all the trees that chance to stand in its path, together with soils of every kind, mixing all together without reference to the size of their component fragments. Most of the uprooted trees are deposited in lateral windrows, heads downward, piled upon each other, and tucked snugly in alongside the clearing; while a few are carried down into the valley on the snout of the avalanche, and deposited with stones, leaves, and burs, in a kind of terminal moraine. The soil accumulations of annual avalanches are still more moraine-like in form, and frequently attain a depth of from forty to fifty feet. They are composed of mud, sand, coarse granules, and rough angular blocks, avalanched from the mountain side, and sometimes water-washed pebbles also, derived from the channels of streams. 76 Sierra Club Bulletin Thus, the largest of the Clouds\u27 Rest avalanches, in rushing down their magnificent pathway of nearly a mile in vertical depth, on their arrival at the Tenaya Creek (Fig. 3) dash across its channel and up the opposite bank to a height of more than a hundred feet, pushing all the pebbles and boulders of the stream up with them. Spring freshets bring down a fresh supply of pebbles and boulders from year to year, which the avalanches patiently add to their moraine, until in a few thousand Fig. 3. years these washed pebbles form a considerable proportion of the mass. Trees over a hundred years old occur upon the upper portions of some of these avalanche-beds, showing that no avalanche of sufficient power to disturb them had occurred since they began to grow. The lower portions of these beds are, on the contrary, in a raw formative condition, and about as plantless as the shining boulder-beds in the bottoms of rivers. Again, stone avalanches have their share in depositing soil. The observer among beetling Yosemitic cliffs occasionally sees a single boulder eight or ten feet in diameter whizzing down the sky like a comet with a tail of dust two thousand feet long. When these huge soil-grains strike among other boulders at the end of their course, they make a sound deeper and heavier than thunder; the ground trembles, and stone-spray is whirled and spattered like water-spray at the foot of a fall. The crushed and pounded soil-beds to which avalanches of this kind give rise seem excellently well adapted to the growth of forest trees, but few of them are sufficiently matured to be Studies in the Sierra 77 available, and the trees that venture upon them are in constant danger of their lives. These unplanted beds occur most commonly at the base of cliffs intersected by feldspathic veins, the decomposition of which causes the downfall of additional material from year to year. On the contrary, the rougher and far more important soil-beds resulting from earthquake avalanches are formed almost instantaneously, without being subsequently augmented in any appreciable degree for centuries. The trees, therefore, and various shrubs and flowers which find them tolerable or congenial dwelling-places soon take possession of them, and soothe their rugged features with a mantle of waving verdure. At first thought no one would suppose that in a tumultuous pellmell down-crash of rifted rocks any specialization could be accomplished in their deposition. Both the suddenness and the violence of the action would seem to preclude the possibility of the formation of any deposit more orderly than a battered rubbish-heap. Every atom, however, whether of the slow glacier or swift avalanche, is inspired and directed by law. The larger blocks, because they are heavier in proportion to the amount of surface they present to the impeding air, bound out farther; and, because obstructions of surface irregularities have less effect upon larger blocks, they also roll farther on the bottom of the valley. The small granules and sand-grains slip and roll close to the cliff, and come F to rest on the top of the talus, while the main mass of the talus is perfectly graduated between these extremes. Besides this graduation accomplished in a vertical and forward direction, beautiful sections are frequently made in a horizontal and lateral direction, as illustrated in Figure 4. A B is a kind of natural trough or spout near the base of the cliff, directed obliquely downward, into 78 Sierra Club Bulletin which a portion of the avalanche-stream, F, falls, and is spouted to the left of its original course. Because the larger boulders composing the spouted portion of the current move faster, their momentum carries them farther toward H, giving rise to the talus E, while- the finer material is deposited at D. Again, the blocks sufficiently large to bound out beyond the deflecting spout form the rough talus C, while the smallest fragments of allΓÇönamely, the fine dust derived from chafingΓÇöfloat out far beyond, and settle in thin films silently as dew. In portions of canon walls where diagonal cleavage is developed, inclines such as A B (Fig. 5) are common. If two boulders in falling from the heights above should strike glan- cingly at A, the greater mass or more favorable form of boulder B might cause it to bound sufficiently far to reach the second incline, which would carry it toward D; while the smaller boulder, C, falling short, might fall under the guidance of a third incline, and be shed off toward E, the two boulders finally coming to rest a hundred yards or more apart. By these means the most delicate decompositions of stone- torrents are effected, the various resulting soils being delivered at different shoots and spouts, like the bran, shorts, and fine flour of a grist-mill. The ages of the oldest trees growing upon these soils furnish data by which some approximation to the time of their forma-\u27 tion may be made. FlG-5. The first* post-glacial earthquake sufficiently severe to produce large avalanches occurred at least three centuries ago, and no other of equal power has occurred since. By this earthquake alone, thousands of acres of noble soil-beds were suddenly and simultaneously deposited throughout all the deep canons of the range. Though Studies in the Sierra 79 thus hurled into existence at a single effort, they are the most changeless and indestructible soil formations in the Sierra. Excepting those which were launched directly into the channels of rivers, scarcely one of their wedged and locked boulders has been moved since the day of their creation. In striking contrast with these terrible demonstrations of mechanical energy, made in the deposition of earthquake soils, is the silent and motionless transformation of solid granite into loose