POSTGLACIAL HISTORY OF THE YOSEMITE VALLEY
PRODUCTION OF ROCK WASTE IN TEE YOSEMITE VALLEY
UNEQUAL DISTRIBUTION OF ROCK WASTE
Because of the sheerness of its walls the Yosemite Valley has been the scene of many rock falls. The glaciers, having transformed it from a V-shaped canyon to a U-shaped trough, left the Yosemite at the end of the ice age with oversteepened sidesthat is, with cliffs more precipitous than the ordinary processes of weathering and erosion would have produced. As a consequence, when these processes again prevailed upon the departure of the glaciers, the cliffs naturally tended to revert to less bold and more stable forms.
On the whole this dismantling has as yet made little progress, and most of the cliffs still retain their glacial profiles. The extent of the damage they have suffered is most readily estimated from the size of the piles of rock waste that lie at their bases. These piles, or taluses, are for the most part smallsurprisingly small, even, compared with the long slopes of rock waste that partly cloak the sides of the glaciated valleys in the Rocky Mountains, the Cascade Range, and the Alps. So insignificant do they seem under the imposing rock façades of the Yosemite Valley that many observers have commented on the apparent absence of débris in the valley. Whitney69 endeavored to explain the paucity of débris by supposing that the bulk of the rock fragments shed from the cliffs had "gone down to fill the abyss" that was formed, according to his hypothesis, when the bottom of the valley dropped out. In reality, however, there is hardly a spot at the base of the cliffs where rock waste is entirely wanting. Only in a few places are there piles less than 50 feet high. At the toe of El Capitan (pls. 3 and 17), which has the appearance of rising directly from the valley floor, there is fully 100 feet of débris. Under most of the great cliffs the débris attains heights of 250 to 500 feet: in a few recesses it reaches 2,000 feet.
What has been said thus far, however, applies strictly to the upper Yosemite chamber. The lower chamber is, by contrast, fairly lined with rock waste. The taluses slope out so far from each side as to leave but a narrow strip of valley bottom free. The great Rock Slides (pl. 3) across which the Big Oak Flat Road is graded are 2 miles broad and reach almost to the brink of the upland. This marked difference in the amount of rock waste in the upper and lower. chambers of the Yosemite Valley is explained in the first place by the fact that in the upper chamber rock waste has accumulated only since the last glaciationthat is, for a period of about 20,000 to 40,000 years. In the lower chamber, where the last Yosemite glacier did not penetrate, it has been accumulating ever since the earlier or El Portal glaciationthat is, for a period of about 200,000 to 300,000 years. But in addition there is the fact that the walls of the upper chamber are composed of prevailingly massive or sparsely jointed rocks, whereas the walls of the lower chamber are composed in large part of well-jointed rocks. Its north walk, which consists mostly of closely fractured gabbro, has suffered the most complete demolition. Only a few crags of it remain standing above the vast rock talus.
The spacing of the joint fractures in the rocks, which was so potent a factor in determining the rate of glacial excavation, has also effectually controlled the rate of the postglacial dismantling: Where the joints are most numerous and most closely spaced, naturally the dismantling processes have worked to best advantage; where the joints are few and far apart the dismantling has been correspondingly slow. Throughout the Yosemite region there is consequently a close relationship between the size of the débris piles and the joint structure of the walls above them. The upper Yosemite chamber, especially, offers the most striking variations. At the extreme point of the Cathedral Rocks, where the granite is particularly massive, there is scarcely 50 feet of débris, but in the recesses adjoining the Cathedral Spires, where shattered diorite and gabbro prevail, the débris reaches a height of 2,000 feet. East of Taft Point, where the cliffs are more sparingly fractured, the débris again dwindles to a mere 100 feet. The sheeted granodiorite in and about Sentinel Rock has given rise to taluses 600 to 1,500 feet in height, but the relatively massive granite which alternates with the granodiorite in the cliffs under Union Point has produced a talus only 50 feet high. On the north side of the valley, likewise, the débris piles vary from 100 feet at the base of the Three Brothers to 1,100 feet in the embayment to the east, where they afford a convenient slope for the zigzag trail to the Yosemite Falls. Under Columbia Rock, on the other hand, there is less than 50 feet of débris. In general each embayment or recess that marks a place of weakness in the walls of the valley contains a great cone of débris; each promontory that marks a point of strength has a minimum of débris at its base.
MODES OF PRODUCTION
The belief prevails among visitors to the Yosemite Valley that most of the rock débris has been thrown down from its walls by earthquakes. One reason is that some of the boulders are of astonishingly large size and lie far out in the valley. Another is that the walls are so massive that nothing less than an earthquake would seem to suffice to break them down. Muir, who was privileged to witness the rare spectacle of the downfall of a pinnaclethe Eagle Rock, which was on the south wall of the valley, not far from Moran pointat the time of the Owens Valley earthquake in 1872, was so deeply impressed by the sight of the great avalanche of bounding rock fragments that resulted from the crash that he was inclined to attribute most of the rock taluses to earthquake avalanches. As most of these taluses bear mature forests and appear to have received no noteworthy increments for long periods, he concluded that "more than nine-tenths" of all the rock waste in the Sierra Nevada had been shaken down by a single mighty quake, a short time after the end of the ice age.70
It is indeed probable that a considerable part of the rock waste in the Yosemite Valley is of earthquake origin, for tremors must have occurred from time to time during the postglacial interval as a result of minor movements on the fault fractures at the east base of the range. It is fairly certain, furthermore, that at least one strong shock was felt in the Yosemite Valley, for only 30 miles away, near the west shore of Mono Lake, the moraines at the mouth of Lundy Canyon are cut across by a fault scarp 50 feet high that was produced probably by a single, sudden earth movement. But whether as much as nine-tenths of all the rock waste in the Yosemite Valley is to be accredited to this one strong quake, or even to all the postglacial quakes together, seems doubtful, nevertheless, for rock waste is being shed from the cliffs in quantities by no means negligible at the present time, as the result of the normal weathering and disintegration of the rock walls and of the recurrent action of such destructive agencies as snow avalanches and torrential rains, and this intermittent shedding of débris doubtless has gone on ever since the glaciers vanished. To credit earthquakes with five-tenths of the total amount of rock waste in the valley, therefore, would probably be liberal.
Significant light was shed on this question some years ago, when an excavation for road metal was begun at the toe of the great rock talus under the Big Oak Flat Road. In the head wall of this excavation were visible no less than four distinct layers of rock débris, each several feet in thickness and separated from the next by a thin layer of dark earthy matter of vegetal origin, doubtless ancient soil. Roots and stumps of forest trees were embedded in these soils. It was thus made clear that this talus had been formed not by a single huge rock avalanche, but by successive avalanches that were separated by long intervals, during each of which a forest was able to establish itself upon the slope.
Whether each of these avalanches was thrown down by an earthquake, it is impossible to say. Some may have fallen without the intervention of a quake. For it often happens that a cliff that has stood for a long time essentially unchanged in outer appearance becomes so weakened internally by the solvent action of acid-bearing water that percolates through the joint cracks that its component parts at last lose coherence and crash down under the pull of gravity alone. An occurrence of this kind was observed near the base of the Three Brothers in 1923. A large sheet or spall of rock that had been in process of being loosened for centuries, perhaps, suddenly detached itself from the cliff face without being impelled by any noticeable earth tremor and, as it fell, crushed and obliterated with its débris a forest of pine trees that had grown up on the talus below. It is probable that spontaneous rock falls of this sort account for a large share of the vast talus over which the Big Oak Flat Road is laid and likewise of other taluses in the valley.
There is little need for alarm on the part of the visitor lest any spontaneous rock avalanches should occur while he is in the Yosemite Valley, for such avalanches are in the nature of things very infrequent in a valley whose walls are so prevailingly massive. Only in those few spots where the rock is shattered into small fragments and where ground water is fairly abundant, are rock falls likely to occur annually or at shorter intervals, for there frost and thaw, heat and cold, working in alternation, are effective agents. In those places, which are for the most part in recesses, the talus has a relatively fresh appearance and is either bare or only scantily covered by bushes.
Most of the minor rock falls in the Yosemite region take place in the winter or in the spring, either in conjunction with snow avalanches or in thawing weather. On the steep northwest slope of Clouds Rest snow avalanches are the principal agency and have worn smooth, funnel-shaped tracks. During the long dry summers rock falls are rare but not wholly absent. Tenaya Canyon has a particularly evil reputation on their account. The few mountaineers who have had the hardihood to traverse this canyon appear to have been as much impressed by the peril of being struck by falling rocks as by the unusual obstacles that are to be overcome in climbing.
One agency whose effectiveness in causing the mechanical disruption of the granitic rocks in the Yosemite region deserves to be considered here is the radiant heat of the sun. Several circumstances operate to intensify its action on those rocks. Because of the utter absence of vegetation and even of soil the rock over large areas is directly exposed to the rays of the sun. The dryness and purity of the air, coupled with its thinness at the higher altitudes, cause the rays to suffer relatively little loss of heat through absorption or reflection by particles of water vapor and dust. And the cloudless skies that prevail during the summer permit them to strike the rock unhindered day after day, often for weeks or months at a stretch. As a consequence the rock is subjected every year to prolonged periods of intense daily insolation alternating with rapid cooling through irradiation at night.
Now all granitic rocks are aggregates of several different mineralsmainly feldspar, quartz, mica, and hornblendethe crystals of which are tightly interlocked. When these crystals expand upon being heated they tend to wedge and pry one another apart. A few heatings have no effect, but thousands upon thousands of heatings are bound to loosen the crystals by degrees until at last they are detached in flakes or individually. In roaming through the Sierra Nevada one sometimes finds rock fragments that, although apparently solid, can not be picked up entire: when lifted they break up into loose grains that run through the fingers like coarse sand. "Rotten" rock fragments of this kind occur only in places where the heat of the sun is felt daily.
The summits of those domes that were not over-ridden by the glaciers of the ice age, such as Sentinel Dome and Half Dome, and the summits of those domes that were overridden by the earlier glaciers only, such as North Dome, Basket Dome, the Quarter Domes, and Moraine Dome, afford particularly good places to observe the disruptive effect of solar heat on granite. The granite there is characteristically flaky parallel to the surface, the tendency evidently being for the outer, more intensely heated layers to burst loose by expansion from the less intensely heated layers underneath. (The production of curving shells, 1 foot or several feet in thickness, by exfoliation involves other agencies besides solar heat and is not to be confounded with the flaking here discussed.) The flakes break up into individual rock grains, and ultimately these are washed away by the rain water. The great accumulations of granite sand that occur in the vicinity of domes and other bare masses of granite are produced largely in this way. Frost presumably plays a part in this mechanical disruption, but if so, only a very subordinate part, for typical frost cracks are absent in the domes and the other rock masses referred to.
It is a highly significant fact that the loose grains derived from disintegrating granite in the Yosemite region show scarcely any effects of chemical decomposition. The crystals of feldspar are but slightly cloudy at the edges, and the flakes of biotite and rods of hornblende show as a rule no alteration whatever. This is a very different state of things from that which prevails in the eastern part of the United States and in other regions of fairly high humidity, for there the granitic rocks are reduced by chemical decomposition into clayey soils. However, this crumbling of the granite into undecomposed grains takes place only on the domes, cliffs, and other conspicuously bare rock masses in the Yosemite region that are subjected to intense insolation. In the densely forested areas on the uplands, where the heat of the sun is partly excluded by the foliage of the trees and where the granite is covered by a layer of moisture-conserving, acid-producing humus the chemical processes reduce the granite in much the same way as in a humid region.
In closing this discussion of the different processes whereby rock waste is produced in the Yosemite region it will be appropriate to point out a few of the outstanding evidences of earthquake action. Most spectacular among these are the gigantic blocks commonly referred to as earthquake boulders. A few isolated blocks that are as large as cabins and seldom fail to attract the attention of the passer-by lie near the Le Conte Memorial Lodge. Other blocks of various sizes are interspersed among the houses in the old Yosemite Village, which was built at the toe of a chaotic pile of rock débris. Still others encumber the channel of the Merced River in the gorge below the valley, causing the stream to dash itself into foam in its wild descent; these are not boulders rolled down by the current but blocks that have fallen from the walls of the gorge. Most remarkable of all is the famous Arch Rock, between and under whose enormous overarching fragments the automobile road passes about 3 miles above El Portal.
Though earthquake shocks undeniably afford the most plausible explanation for the dislodgment of these huge blocks, nevertheless it must be admitted that there is no clear, uncontrovertible proof at hand. Any or all of these blocks could have been dislodged without the aid of an earthquake, simply as the result of the normal weathering and disintegration of the cliffs. For in many places in the Yosemite's walls, owing to irregularities in the joint structure, very large blocks are flanked or underlain by thin slabs or mere slivers of rock. As these small fragments are loosened and dislodged, ultimately the large blocks are left in unstable positions. Even if no earth tremors should intervene the time would eventually come when the blocks would tumble down for lack of adequate support. Nor is the great distance at which each of the blocks in question lies from the parent wall necessarily any indication of earthquake action. Any block falling from a great height is likely to bound far out, and the larger and heavier the block the greater its momentum and the farther out it will bound.
The probability that the blocks described are of earthquake origin is greatly increased by the fact that there are in the Yosemite region several masses of rock débris of enormous extent and wholly distinct from the ordinary sloping taluses by reason of their irregular, hummocky, sprawling forms, which can scarcely be accounted for save by the agency of earthquakes. One of these chaotic, far-flung masses obstructs the mouth of Tenaya Canyon and impounds Mirror Lake. It was derived from the wall back of the Washington Column and met lesser avalanches that fell, presumably at the same time, from the cliffs west of Half Dome. The water that issues from Mirror Lake in part percolates through the mass of débris, as is manifest at times of low water, when the stream bed is dry for a considerable distance.
Another far-flung mass of rock débris of the same character lies at the head of the Yosemite Valley, just south of Tenaya Canyon. It is spread out over a space of many acres and projects a quarter of a mile from the base of the head wall, necessitating a curve in the road south of Tenaya Creek. But the most remarkable body of earthquake débris is that which lies in front of El Capitannot the talus of blocks that slopes steeply from the cliff to the valley floor, but the much vaster hummocky mass, partly obscured by a growth of trees and brush (it can not be detected in pl. 3), that sprawls nearly half a mile out into the valley, as far as the automobile road, which makes a detour around its edge. There can be no doubt that it is the product in the main of one colossal avalanche that came down from the whole height of the cliff faceprobably the most spectacular rock avalanche that has fallen in the Yosemite Valley since the glacial epoch.
El Capitan is so often referred to as the very embodiment of the proverbial "rock of ages" that to many readers this may seem surprising news. Indeed, the sweeping concave lines of its great façade are usually regarded as characteristic products of glacial erosion that have suffered scarcely any change from postglacial weathering. Some observers, even, have believed that they could detect on its face the gleam of glacier polish. Yet the quantity of débris that fell in this stupendous earthquake avalanche is so great it covers nearly a quarter of a square mile of ground to an estimated depth of fully 100 feet-that its removal doubtless altered appreciably the contour and appearance of El Capitan.
All the great masses of débris just described must have fallen a long time ago, for they all support old and large trees. Perhaps they were all precipitated by that severe earthquake which originated near Mono Lake. Apparently only moderate quakes have occurred since then. The quake of 1872 probably was one of the strongest that has taken place during historic time. No tremors of any consequence have made themselves felt in the Yosemite Valley since. Indeed, the Sierra Nevada is to-day a region of marked stability, and this fact is strikingly attested by the presence on the walls of the Yosemite of precariously balanced pinnacles and rocks such as the Agassiz Column (pl. 47, B), which stands near the trail just below Union Point.
TORRENT FANS AND TORRENT CHANNELS
The débris piles everywhere are subject to the washing action of rain-water rills. As a result they are fringed by gently sloping aprons of granite sand that spread some distance out upon the floor of the valley. In those places, however, where the water is concentrated into torrents of considerable volume not merely sand grains are washed down but blocks of all sizes up to 10 and even 15 feet in diameter, and these coarse materials are spread out in what are properly termed fans. This is true especially at the mouths of deep-cut recesses from which the storm waters issue in short-lived torrents of well-nigh incredible swiftness and power. The larger of these torrent fans of coarse débris are indicated by a special symbol on the map of glacial and postglacial deposits. (See pl. 29.)
Most typically developed is the torrent fan at the mouth of the recess between the Three Brothers and the massif of El Capitan. Narrowest at its upper end, where the water issues from a rock channel, it deploys symmetrically downward in true fan shape, its surface furrowed by numerous diverging channels. Its upper part, which is composed of the coarsest material, is steepest and has a gradient of 5 feet in 10; its lower margin, which is composed of the finest material, is flattest and merges into the floor of the valley. The size of the fan and the quantity of material which it contains might seem out of proportion to the small extent of the recess and the circumscribed area that is tributary to it. The recess is only 1-1/4 miles long and receives no drainage from any hanging upland valley but heads abruptly against the rim, not far back of Eagle Peak. Its entire drainage area is considerably less than 1 square mile. However, the upper funnel-shaped part of the recess is inclosed almost wholly by steep slopes of bare, smooth granite from which the storm waters run off with amazing speed; and the principal drainage lines converge to one point and are so nearly of equal length and equal steepness that the waters reach the point of confluence almost simultaneously. The conditions, therefore, all operate to intensify torrential action. Finally, the recess contains, in addition to the rock waste derived from its own walls, a large body of morainal débris that was left in it by the ancient glaciers. This unconsolidated material gives way readily before the rushing waters and makes up a considerable part of the fan.
The most extraordinary features of the fan are the walls of blocks that flank the diverging channels on it. These are not natural levees of débris that was dropped at times of overflowthey are really walls, built of blocks superimposed one upon another. Most of them are only 3 to 5 feet in height, but some are twice as high. A few contain blocks 10 to 15 feet in diameter upon which rest smaller fragments that somehow have been thrown up there by the tumultuous waters.
The precise manner in which these channel walks are built by the torrents is not known from direct observation, but that the blocks are really tossed up and not merely dropped by overflowing water, is to be inferred from certain facts that were observed in 1919, a short time after unusually violent torrential floods had taken place in the recesses near the Cathedral Spires as the result of a severe cloud-burst. The trees standing near to the channel walls were found barked and bruised by leaping blocks to a height of 5 to 8 feet, and many new blocks had been added to the walls, yet there was no evidence of overflow on the ground behind the walls. At the foot of each fan, where the water lost its momentum and spread out unconfined, was a field of blocks aggregating thousands of tons in weight. All of this material had been brought down in less than half an hour, probably during one great rush of water at the climax of the flood.
Last Updated: 28-Nov-2006