EARLY HISTORY OF THE YOSEMITE VALLEY
THE RISE OF THE SIERRA NEVADA
The history of the successive uplifts that gave the Sierra block its present height and tilted attitude is as yet imperfectly known. Only the major events stand out clearly. The reason will be apparent when the nature of the evidence is learned; it is mostly indirect or circumstantial, and some of it must be sought in places where the layman would hardly look for indications of earth movements that happened in remote ages. For these reasons it will be appropriate, before beginning the story of the rise of the Sierra Nevada, to give some idea of the sources from which it is pieced together.
SOURCES OF INFORMATION
First there is the testimony of the successive beds of rock waste that lie spread out at the western base of the range. They consist mainly of gravel, sand, and clay, brought down by the Sierra rivers and deposited in the shallow sea which throughout most of Tertiary time occupied the basin of the Great Valley of California. The varying degrees of coarseness or fineness of these materials afford an index of the velocity and transporting power of the rivers and therefore also of the degree of steepness of the river beds at different times. The layers of coarse gravel obviously record epochs of uplift, when the rivers dashed swiftly down steepened courses; the layers of finer gravel and sand record epochs of prevailing stability of the land, during which the streams flowed with gradually slackening speed, as they cut their beds down to lower and lower gradients; and the layers of silt and clay record epochs when the rivers had resumed tranquil, sluggish habits of flow on faint slopes, as a result either of long-continued down cutting or of gradual subsidence of the land. Moreover, the manner in which these different layers overlap one another and the rock body of the range and the occurrence of remnants of layers high above the present foothills tell clearly of former oscillations of the Sierra region with respect to sea level. The geologic epochs when these different oscillations took place are indicated by the fossil shells and other remains contained in the deposits.
Similar testimony is afforded by the layers of rock waste that have accumulated in the basins east of the Sierra Nevada and in the Mohave Desert, to the south.
Much has been learned also from the "fossil river beds" that occur in the north half of the rangeriver beds that were entombed by flows of lava and that were partly brought to light again later when newly formed rivers, accelerated by fresh uplifts, trenched deep canyons. Some of these ancient river beds are now situated on or near the divides between the canyons, a thousand or even several thousand feet above the present streams. From their sand and gravel considerable gold has been extracted, but, what to the geologist is more significant, petrified bones of animals, fragments of wood, and even delicately preserved imprints of leaves have occasionally been dug out. From these fossils some conception may be formed of the age of the stream beds.
Of particular interest are the fossil remains that have been found at and near Table Mountain (the Table Mountain of Tuolumne County), which stands about 40 miles northwest of the Yosemite Valley and which has become famous through the writings of Bret Harte. This long, flat-topped ridge, or really chain of ridges, consists of the remnants of a flow of dark basaltic lava that followed the course of an ancient valley all the way from the summit of the Sierra Nevada to the foothills. It represents the last notable volcanic flow that occurred in the north-central part of the range. Its prominence in the landscape to-day is due to the fact that in the long time that has elapsed since the lava was poured out the hills that flanked the path of the flow have been eroded away over distances of many miles, whereas the basalt, owing to its exceptional resistance to chemical decomposition as well as to mechanical disruption, has remained standing but little reduced in height. The fossil remains, which consist of casts of leaves and a tooth of an extinct species of horse, date back, according to determinations by Dr. Ralph W. Chaney and Dr. Chester Stock, to the later part of the Miocene epoch and thus tend to show that the upland on which Table Mountain stands is the remnant of a landscape of late Miocene time. The Yosemite upland, which is the correlative of the upland near Table Mountain, is therefore presumably also a remnant of the late Miocene landscape.
Other data concerning the successive uplifts have been derived from a critical study of the canyons in the south half of the Sierra Nevada, which were obstructed only by minor volcanic outpourings and therefore reveal clearly the effects of successive renewals of stream activity. The first studies of this kind were made in the Kern River region by Prof. Andrew C. Lawson32 in 1903; but the more recent investigations in those parts of the range which are drained by the Tuolumne, Merced, and San Joaquin Rivers have furnished most of the data for the sequence of Sierra uplifts here recorded. Other kinds of evidence might be mentioned, but to present them all would lead too far afield.
HISTORY OF UPLIFTS
The precise date in geologic history when the Sierra block first became outlined by fault fractures is as yet uncertain. Some geologists have placed it tentatively as far back as the end of the Cretaceous period, but it seems more likely, in the light of recent studies of the eastern front of the range, as well as of the fault scarps of several ranges in the Great Basin, that the faulting began later, presumably in the second half of the Tertiary period. If that interpretation is accepted, the earlier uplifts in the Sierra region are to be conceived as having been mostly in the nature of gradual upwarpings not attended by any breaking of the earth's crust.
The first upwarping that is definitely indicated in the sedimentary record at the western base of the Sierra Nevada took place early in the Eocene epochthat is, nearly 60,000,000 years ago.33 It increased the height of the land only moderately, not enough to make it truly mountainous. Indeed, throughout the first half of the Tertiary period the Sierra region apparently remained a land of moderate altitude, little or no higher than the country adjoining it on the east. It was, however, by no means a lowland or plain throughout, for it was traversed by a number of parallel northwestward-trending ridgesslowly vanishing remnants of the Cretaceous mountain system already described. Westward the region sloped down to the sea, but the coast line did not remain fixed in position; it advanced at times many miles to the west of the present foot of the range, and at other times receded to the east of it, as a result of alternating heaving and sinking movements. Dense growths of rain-loving vegetation of types similar to those which now prevail in the southern Atlantic and Gulf Stateslaurel, maple, poplar, elm, beech, and magnoliaflourished in the mild, humid climate.
Toward the end of the Eocene epochabout 40,000,000 years agothere commenced a period of intermittent volcanic outbreaks. A row of volcanoes at the eastern border of the Sierra region sent forth streams of white and pink rhyolitic lava and mud that flowed down the valleys, burying the river channels. The volcanic materials were gullied and in part washed away by the streams, but fresh eruptions undid this work. For a long period volcanism and stream erosion continued to act in alternation. Meanwhile, by intermittent, probably gradual uplifts the Sierra region was broadly raised and tilted to the southwest. At the same time the country lying to the east of it was warped and flexed; low ranges came into existence, and between them were formed wide basins in which the waters collected in shallow lakes.
The disturbances died out at last and were followed by a long interval of relative quiet, during which most of the rhyolite and much other rock waste was stripped from the Sierra region and deposited in the sea on its western border and in the basins to the east of it. Then, presumably in the second half of the Miocene epochroughly, about 12,000,000 years agovolcanic activity and earth movements began anew on a vast scale. This time the eruptions yielded mostly andesitic lava of brown, reddish, and grayish colors. Down the valleys these materials flowed, sheet upon sheet, obliterating the stream beds and compelling the waters to seek new paths. In the north half of the range the outpourings were especially frequent and voluminous; they piled up to thicknesses of a thousand feet or more, overwhelming all the features of the country save the higher peaks and crests. In the southern parts of the range the volcanic flows were less extensive and less thick; they filled only the bottoms of certain valleys and caused no notable displacements in the drainage system. Only the drainage basin of the Merced, in the central part of the range, remained free from volcanic outpourings.
The crustal movements of this epoch increased the height of the Sierra region by several thousand feet and gave it the aspect of a mountain range, or rather a belt of mountains, that dominated all the country roundabout. Mount Lyell probably attained an altitude of about 7,000 feet. Strong faulting took place along some parts of the eastern border, and the great depression in which Lake Tahoe is situated was formed by subsidence: the lava which dams the lake itself was not poured out, apparently, until after the depression was formed. The ranges and valleys of the Great Basin region also were accentuated, in part by warping, in part by faulting.
Next followed another lengthy interval of repose, or relative repose, that lasted through the entire Pliocene epoch. Only feeble eruptions took place from time to time, and meanwhile the waters in the lava-covered parts of the range reorganized themselves into new rivers and cut new canyons, some of which attained depths of more than 1,000 feet. Vegetation reestablished itself on the disintegrating volcanic materials, but this time in the form of forests adapted to a cooler and less equable climate. Sequoias, ancestors of the big trees of the present time, flourished throughout the region.
And then, at the beginning of the Quaternary periodabout 1,000,000 years agocommenced those great upheavals and tilting movements that gave the Sierra Nevada its present great altitude. The summit peaks were raised to almost double their previous height, Mount Lyell being lifted to more than 13,000 feet above sea level. At the same time fracturing and faulting took place on an enormous scale. Owens Valley and the other desert regions adjoining the range on the east and south dropped back or else suffered but slight uplifts as compared with the mountain block, and so the Sierra Nevada came to stand out in its present imposing form, with gentle westward slope, sharply defined crest, and abrupt eastward-facing escarpment. Strangely, the volcanic accompaniments of this great upheaval and in breaking of the earth's crust were not extensive. Though molten material forced its way up repeatedly through fractures in or near the zone of faulting and also through cracks in the Sierra block, the resulting volcanic cones and lava flows were insignificant compared with those of Tertiary time.
After the grand climax of mountain-building movements that ushered in the Quaternary period, the earth stresses abated in intensity. Upheaval and downfaulting took place less frequently, and as a result the Sierra Nevada has suffered no marked further changes in height or in general form. Minor movements, however, have continued to occur at intervals into historically recent times.
The magnitude of the more recent movements on the fault fractures is indicated by small escarpments that cut across the débris slopes at the east base of the range. Most of these scarps are only 10 to 50 feet in height. Near Genoa, Nev., for instance, is a fault scarp varying from 10 to 44 feet in height that extends for several miles along the east base of the Carson Range, that offshoot from the main Sierra block which forms the east wall of the Tahoe Basin.34 The dislocation there probably involved a sinking of Carson Valley, for that valley is lowest next to the base of the range and abuts almost directly upon it. Another escarpment of recent origin cuts across the glacial moraines that project from the base of the range at the mouth of Lundy Canyon, near the west border of Mono Lake. The vertical displacement is at least 50 feet, and its recency is attested by the fact that Mill Creek, which issues from Lundy Canyon, still cascades abruptly at the place where the fault crosses its bed, the stream not yet having had time to smooth out the step completely. Here too, apparently, the valley block was depressed relative to the mountain block and tilted toward it, for Mono Lake laps the immediate base of the range, and its ancient shore lines, which date back to the glacial epoch, decline gently toward the southwest.
At least one notable dislocation has taken place at the east base of the Sierra Nevada within historic time. It gave rise to the famous Owens Valley earthquake of March 26, 1872, which destroyed the village of Lone Pine and caused the death of many of its inhabitants. The fault scarps produced by this earth movement are still conspicuous and are easily traced for distances of several miles, although they cut for the most part only through unconsolidated débris at the foot of the range. They vary from 8 to 25 feet in height. A line of lesser scarps, facing westward, was produced in the floor of the valley, roughly parallel to the main scarps and about half a mile distant from them. The intermediate strip of valley floor was slightly depressed and locally warped, so that a number of shallow ponds came into existence. The tremors produced by this earth movement were so strong that even in the Yosemite Valley, more than a hundred miles away, great avalanches of rock fragments fell from the cliffs and one noted pinnacle was demolished, as has been graphically told by John Muir, who was an eyewitness of the scene.35
Since this great earthquake no further strong tremors have been felt in the Sierra Nevada nor in the lowlands immediately to the east of it. Evidently earth movements are now infrequent in these areascertainly infrequent compared with those in the mobile coastal beltand it may be concluded, therefore, that the Sierra block and its neighbors are in fairly stable adjustment.
REARRANGEMENT OF DRAINAGE SYSTEM AND ORIGIN OF THE MERCED RIVER
A direct consequence of the upwarping and tilting of the Sierra region in early Tertiary time was the rearrangement of its drainage system. Before the uplifts began, the master streams, with few exceptions, flowed in northwesterly or southeasterly directions, the trend of the valleys being determined in general by the parallel ridges of the ancestral mountains of Cretaceous time. But when the land acquired a definite slant to the southwest, this old drainage system was replaced by a new system whose master streams flowed prevailingly in southwesterly directions. The change doubtless took place very gradually and must have required considerable time, for it was effected mainly by the progressive headward growth of certain streams that already flowed in southwesterly directions and by their capture of other streams that were less favored by the tilting. The mountain ridges by that time, it may be presumed, were worn down and breached in places and thus afforded gaps through which transverse drainage could be established. Even so, they interfered not a little with the development of the southwesterly rivers, as is evident from the strongly winding courses of those streams.
The later tilting movements steepened the inclination of the land but did not appreciably alter its direction, and thus the system of southwestward-flowing master streams was perpetuated. It does not follow, however, that all the great rivers which now flow down the western slope of the Sierra Nevada are identical with the rivers which came into existence at that early epoch, for lava flows have repeatedly filled and buried the valleys, and the new streams that were formed along the margins or on the irregular surface of the volcanic materials established their courses regardless of the positions of their predecessors.
Such changes in drainage were most prevalent in the northern half of the Sierra region. Even the Tuolumne River was affected by them. Remnants of old gravel beds of that river, situated at different levels, partly capped by lava, clearly show that its course has been obliterated at least twice. The Merced, on the other hand, appears not to have been obstructed or displaced by lava flows at any time in its history. A careful survey of its drainage basin reveals no traces of volcanism, save two diminutive craters south and southeast of Merced Pass, in the upper basin of the South Fork. In this almost complete exemption from volcanic outpourings the Merced drainage basin is unique in the Sierra Nevada, and naturally, because of the absence of such complicating circumstances, it has had a relatively simple erosional history. In any event there is little doubt that, save for local shiftings here and there, the Merced has followed approximately the same general course throughout its existence.
The history of some of its tributaries has probably been less simple. Most of these streams flowed originally in northwesterly or southeasterly directions, between the parallel ridges of the Cretaceous mountain system. As long as these ridges, composed of resistant rocks, were prominent and continuous, doubtless the streams remained fixed in position; but when the ridges were finally worn low and in places cut through, a number of these streams were probably captured by the headward-growing Merced and annexed to its expanding drainage system. Here and there the direction of flow may have been reversed, a northwestward-flowing stream being supplanted by a southeastward-flowing stream, or vice versa, but the axes of the valleys, being controlled by the trend of the strata, remained unchanged.
On the lower Sierra slope, which is carved wholly from upturned strata, the original northwesterly or southeasterly trends of the valleys have, indeed, persisted down to the present time. There the resistant strata still form the ridges, the intermediate belts of weak strata are followed by the streams, and consequently almost all of the Merced's tributaries flow northwestward or southeastward, regardless of the general southwestward slope of the range. There, also, the Merced deviates most widely from a direct southwesterly course and swings in great loops to the northwest and the southeast.
In the Yosemite region, where the folded sedimentary rocks are now wholly stripped away and the granite is exposed, the drainage pattern is naturally more varied, and tributary streams join the Merced at different angles. But even there a number of streams have northwesterly or southeasterly courses that are probably an inheritance from the now vanished mountain system of the Cretaceous period. Among these streams are Ribbon Creek, Yosemite Creek, Indian Creek, Illilouette Creek, Sentinel Creek, Bridalveil Creek, and Meadow Brook. Their peculiar arrangement at right angles to the course of the Merced has had a far-reaching influence on the ultimate modeling of the Yosemite region, as will become clear further on.
In the High Sierra, above the Yosemite region, the northwestward trend of the mountain creststhe Clark Range, the Cathedral Range, Kuna Crest, and the main divideis also probably inherited from the Cretaceous mountain system, although these crests are composed largely of granitic rocks and are covered with sedimentary rocks only in places. The northwesterly courses of the Merced and Tuolumne Rivers there betray the same guiding influence. The upper valleys of those streams are therefore probably of great antiquity.
Last Updated: 28-Nov-2006