Popular Science Monthly/Volume 52/March 1898/The Great Sierra Nevada Fault Scarp
|THE GREAT SIERRA NEVADA FAULT SCARP.
By HAROLD W. FAIRBANKS, Ph. D.
THAT portion of California lying east of the crest of the Sierra Nevada Mountains has had a remarkable geological history. There are many phenomena to be witnessed in that region which possess much interest aside from their purely scientific aspect, and deserve to be better known than they are at present. The enormous scale on which faulting has taken place, resulting in the precipitous eastern wall of the Sierra Nevadas, and the variety and extent of the comparatively recent volcanic outbursts along the lines of fracture, bring the magnitude of geological processes vividly before our eyes. There are probably no better examples of topographic features due to the elevation and depression of great blocks of the earth's crust, through the formation of faults, to be found in any other portion of the world. From a scenic standpoint, also, the region is unique. The lofty, jagged crest of the Sierras rises over fourteen thousand feet, culminating in Mount Whitney, the highest peak in the United States south of Alaska, while in marked contrast to the ice and snow of these mountain heights are the hot and scorching wastes of Death Valley, which lies but a short distance to the east, depressed over three hundred feet below the level of the sea. Snow-clad mountain and desert sand, mighty earth blocks, volcanic craters, lava flows, and alkaline lakes—where can be found greater attractions for the student of Nature?
The Sierra Nevada Mountains, having a length of over three hundred miles and a width of seventy-five miles, consist essentially throughout much of their extent of one great block of the earth's crust, having been elevated along a north-and-south line through a series of movements which have been frequently repeated during a long period of geological time. The block as a whole was not elevated, but tilted from the east, so that on that side there is an abrupt wall many thousands of feet in height, while on the west the slope is long and gradual. Owing to this fact it is only when seen from some point at its eastern base that the magnitude of this range of mountains is appreciated. Although the valleys at the eastern foot of the mountains vary from three thousand to six thousand feet in altitude, the wall of rock forming the scarp rises so abruptly and with such colossal proportions that it is absolutely overpowering in its grandeur. To the east and running parallel with the Sierra Nevadas are other giant ranges formed in much the same manner, being only slightly lower but separated from each other by desert valleys.
The mighty eastern scarp in all its distinctness might be considered as beginning on the south in the vicinity of Walker's Pass. With a gentle curve it sweeps toward the north, rising higher and
Mount Whitney, from Lone Pine Cañon.
higher for a hundred miles, until culminating in the jagged peaks northwest of Owen's Lake, where Mount Whitney reaches an altitude of fifteen thousand six hundred feet above the level of the sea. From this point there is only a slight descent for a hundred miles more, beyond which toward Lake Tahoe its extreme height and ruggedness are lost, the single fault line being replaced by several whose displacements are less. The scenic effect is grandest from Owen's Valley, where the mountain wall bounding it on the west, even and regular in its general outline save for the deep transverse gorges, rises eight thousand to ten thousand feet above the valley. Viewed from the Inyo Range opposite, the evenness of the crest is remarkable. The great peaks are not isolated, as is Mount Shasta, so that their individual grandeur is lost in the general effect. Owen's Valley has a length of about a hundred miles and a width of six to twelve miles, with an even sand floor much of the distance. On the edges the floor gradually slopes upward through the débris fans covered with sagebrush to the bordering mountains. The scenery of this valley is not alone due to the Sierra Nevadas, for on its eastern side, running parallel with the former mountains, is another range known at its southern end as the Inyo Range, and toward the north as the White Mountains. These, if less elevated and rugged, nevertheless possess remarkable proportions, rising five thousand to eight thousand feet above the valley and terminating in White Mountain peak, with an altitude of more than thirteen thousand feet. It will thus be seen that Owen's Valley occupies a troughlike depression between two parallel earth ridges or fault blocks. It also divides the arid from the non-arid regions. The crests of the two ranges are but eighteen miles apart, with this depression nearly two miles deep between them. The western wall is white with snow much of the year, and from its rugged cañons issue numerous streams utilized for irrigating the valley; the eastern range, on the contrary, is comparatively barren, snow lies on it but a short time, and running streams reaching the valley are rare. Owen's Lake lies toward the south at the lower end of the valley. Its greatest diameter is about eighteen miles, but it has no outlet and is quite shallow. It is probable that at one time its waters emptied southward through the continuation of this fault valley into the great wastes of the Salt Wells Desert. The water of the lake is impregnated with sodium
Sierra Nevada from the Lone Pine Hills.
chloride, sodium carbonates, and sulphates. The soda is present in such large amount that it is obtained in commercial quantities. When the water is evaporated in shallow ponds or tanks the carbonates of soda crystallize out first, and are thus easily separated from the other ingredients. The lake is fed chiefly by Owen's River, but at the present time it is slowly sinking. Whether this is due to growing aridity of the climate or because so much water is taken from the streams for irrigation is not known.
As we go north from the upper end of Owen's Valley a vast table-land of lava and volcanic ash is encountered, extending to Mono Lake. On the extreme head of Owen's River the volcanic material rises to the summit of the Sierras, almost obliterating the fault scarp for a number of miles.
Mono Lake is nearly circular, with a diameter of twelve miles. It has an elevation of a little more than six thousand feet, being about two thousand feet higher than Owen's Lake, and like the latter extends up to the very base of the fault cliffs forming the eastern wall of the mountains. The water is intensely alkaline, quite closely resembling in composition that of Owen's Lake. It is situated in a depression in the sandy desert, being surrounded by sand and volcanic rocks on all sides except the west, where rise the snowy peaks of the Sierra Nevadas to an elevation of more than thirteen thousand feet. Although the mountains at this point are still very imposing, their grandeur does not compare with those farther south in Owen's Valley. As we continue northward toward Lake Tahoe it appears that there are two or more fault lines to be made out, the lake itself, according to geologists, occupying a depression caused by the sinking of one of these blocks.
The geological history of much of the region along this series of mighty fault fissures which have resulted in the formation of the Sierra Nevada Mountains is as yet not thoroughly known. The glacial history of Mono Lake and the adoining portion of the Sierras has been studied by Russell, and is better known than that farther south. It has been thought by Mr. Lindgren, of the United States Geological Survey, that, following the period of volcanic activity near the close of the Jurassic which finally culminated in the formation of enormous fused granite magmas, the portion of the crust embracing the Sierra Nevadas and the Great Basin became arched upward. As the result of this, a strain was set up which finally gave rise to north-and-south fault lines, and the region east of that now occupied by the Sierra Nevadas began to settle. This is supposed to have been inaugurated during the Cretaceous, and has continued at various times and in different amounts down to the present time. Toward the close of the Tertiary the displacement of the Sierra Nevada fault block was markedly increased, giving the range approximately its present outlines. There are many indications that an equilibrium has not yet been reached, as shown by the slip which took place at the time of the earthquake of 1872. West of Mono Lake the total displacement can not be less than six thousand feet, while farther south in Owen's Valley it must have reached eight thousand to ten thousand feet, if not more.
Following the first faulting in the Cretaceous outlining the region occupied by the Sierra Nevada Mountains, the erosive agencies began to work actively, for the area was undoubtedly high and rugged. In the course of time the mountains were worn down in places to an approximately base-leveled condition, and erosion almost ceased. As a result of the renewed elevation in the latter part of the
Glacial Rounded Bowlder, Head of Owen's River.
Tertiary, rapid erosion was again inaugurated and the present deep canons were excavated. While these canons on both slopes are exceedingly rugged, those on the east are very short, owing to the fact that the crest of the range almost overlooks the fault scarp. Those crossing this scarp have cut down nearly or quite to the level of the upper part of the débris fan which has grown up at their mouths. They are three thousand to five thousand feet deep, terminating upward in cirquelike amphitheaters of precipitous rock. Through the more rugged portion of the range the mountain peaks have in many instances flattened, plateaulike summits which present a marked contrast with the steep walled canons. This kind of topography is to be observed from a point south of Mount Whitney for more than a hundred and fifty miles northward past Mono Lake, and is undoubtedly the remnant of an old base-leveled condition reached during the early Tertiary and preceding the last important uplift. Mount Whitney even has an almost level summit, breaking away into vertical cliffs, which on its eastern side are over three thousand feet high, and form a portion of the cirque at the head of Lone Pine Creek. The plateaulike character is plainly discernible about Mono Lake, where it has been described by Russell.
At the mouth of each of the canons debouching upon the desert there is a débris cone or fan of remarkable proportions. These fans have been formed by the radial distribution of the débris over the valley bottom below the mouths of the gorges whence the streams issue. The material of which they are formed consists of gravel and bowlders, and has been spread out over many square miles of the valleys. They are among the most striking features of the desert. The long, even slopes, sometimes reaching six to eight miles into the desert, terminate abruptly against the rocky walls of the mountains. To one not familiar with the desert they do not seem of such immense size, but if a climb is attempted this illusion is soon dispelled, for the slope is long and rough, covered with bowlders and intersected with dry water courses. Lone Pine Creek, heading under the great precipices of Mount Whitney, has many of these huge bowlders strewn along its course. By some their position might be attributed to glacial action, but in reality this is not the cause. Six miles below the mouth of the canon there are some ten to twelve feet in diameter, while at various points one to three miles below occur others reaching a diameter of twenty to twenty-five feet. The swiftly flowing streams, at times augmented to torrents by sudden cloud-bursts upon the mountains, are enabled to do an almost incredible amount of work in transporting material.
At the close of the Miocene began the volcanic disturbances which modified so much of the old topography of the northern portion of the Sierra Nevadas. These flows were chiefly andesite with some rhyolite, and issued from fissures along the lines of faulting. The solid flows, breccias, conglomerates, and ash built up great ridges and mountains at many points. Between Lake Tahoe and Mono Lake much of the older surface was buried. From the latter lake southward for seventy-five miles there is a vast barren tableland, while between the North Fork of Owen's River and the San Joaquin the volcanic rocks reach up to and even form the crest of the Sierras for a number of miles. The volcanic eruptions continued through the Pliocene, and at its close occurred another elevation of the Sierras, making more marked still the great fault on its eastern side and differentiating it more completely from the region to the east. From the Mohave Desert northward, the whole length of the Sierras, these lines of weakness have given rise to an almost continuous line of volcanic craters and flows.
The Glacial period in the Sierra Nevadas was finally ushered in, being doubtless due to the increased elevation in this enormous fault block. Whether this period was contemporaneous with glaciation
Glacial Lake, Head of Owen's River.
in the East is not known. There are many things, however, which lead us to believe that it was more recent here. Glaciers still exist in the shadows of the higher peaks, and the facts that their scourings are so fresh and the moraines so slightly modified are strong evidences in favor of the view that the ice period terminated very recently. The glacial phenomena about Mono Lake have been studied by Le Conte and Russell, but farther south they remain still almost unknown. Owen's River heads in the Sierras about forty miles south of Mono Lake. It is a stream of considerable size, draining a large basin, and issues from its rocky gorge at an elevation of eight thousand feet, thence flowing for many miles across the elevated volcanic table-land. Because of the elevation the immense glacier, which once gathered its strength in the basins of the fifteen beautiful lakes that now exist there, flowed far out upon the volcanic mesa at the foot of the scarp, which here has very nearly obliterated the latter feature. The moraines are apparent for a number of miles, but just how far they extend has not yet been determined. This ice stream was probably the largest of any which issued from the mountains, a fact due in great part to the elevation of the mouth of the canon. Lakes fill all the depression in the bed of this ancient glacier, illustrating how much the present scenic features of the Sierras are due to ice action. As we go south from the head of Owen's River into the valley of the same name it is apparent that only at its northern end did the glaciers reach as low as the
Glacial Moraines, Head of Owen's River.
mouths of the cañons and flow out upon the débris cone. Opposite the highest and most rugged portion of the Sierras in the lower part of Owen's Valley, it is evident that the temperature was too high to permit the glaciers to reach an elevation as low as the valley floor. The size and symmetry of many of the moraines reaching into the valleys in the region of Mono Lake strike the attention at once. The Green Creek glacier in particular left morainal walls of great size and regularity. At the mouth of the canon the flat valley is about half a mile wide, with steep and even walls of bowlders and gravel rising three hundred to four hundred feet. Many of the lakes are due to the presence of the terminal moraines, while others fill rock basins.
Russell has described the terraces or gigantic stone steps by which many of the canons rise, and the deep-walled cirques at their heads. About Mount Whitney these features are illustrated on a grand scale. The vast rocky amphitheaters at the heads of the canons are bounded by precipitous walls one thousand to two thousand feet high, while their bottoms are generally occupied by lakes in basins of solid rock.
Lake Mono is at present gradually rising, thus differing from Owen's Lake. The history of these sheets of water has been a checkered one. Periods of high water have alternated with those of desiccation. The last high-water stage seems to have been contemporaneous with the glaciation. Well-defined terraces appear about Mono Lake, indicating the various stages of the water. In this region the opportunities for the study of the glacial phenomena are excellent, for only a small part of it has as yet been investigated.
What is perhaps of more interest than the glaciers is the recent volcanic action so remarkably exemplified about Mono Lake. Following the andesitic flows which took place near the close of the Miocene and in the early Pliocene, there were numerous eruptions of basalt, although some of the basalts of this region may be considerably older. South of Owen's Lake the basalt flows are numerous and extensive, volcanic action in that region being most strongly marked in the Coso Mountains. The most recent eruptions of all have, however, taken place in the vicinity of Mono Lake. Here the surface of the flows is often so fresh and free from soil that it seems as if they had but just cooled. Prom the lake southward for twenty miles there is a line of volcanic cones known as the Mono craters, which are connected with the most recent eruptions and possess great interest. The cones have been built up on extensive obsidian flows, the whole forming a considerable mountainous elevation. The glassy eruptions came first, then the cones were built up of lapilli and scoriaceous material, while the volcanic ash which covers the country for miles around was probably the last to be thrown out. These recent glassy lavas were very viscid and cooled in thick masses instead of thin sheets. This is finely shown in the southernmost of the eruptions, where the glistening, glassy mass which broke into fragments as it cooled rises in almost precipitous crags fifty to seventy-five feet. The bottom of the valley between two different coulees is occupied by a small stream which runs over an older lava. From the surface of the latter, which is covered with soil, spring immense pine trees which must have sprouted since the glass cooled, for no vegetation could have withstood the heat. A climb over the sharply angular fragments which form the wonderfully rugged surface of the obsidian flow is an interesting experience. Great jagged points project up here and there from the broken surface, while the total lack of soil and the bright, clean surface of the brecciated glass makes it appear as though it were but yesterday when the seething mass welled up, overspread the surface, and cooled. Several circular depressions in the valley adjacent to the lava attract the attention. They appear like craters, but it is clear
Débris Slope at Foot of Sierra Nevada Fault Scarp near Owen's Lake.
that no lava ever flowed from them, and they were probably formed by some explosive action from beneath. The greatest body of ash, lapilli, and obsidian flows lies immediately south of the lake and constitutes the Mono craters. They present a weird appearance, rising in their nakedness from the sandy sagebrush plains with the dark background of pines to the south. The Mono craters can not be more than a few hundred years old. They certainly date from post-glacial times.
The two islands in Mono Lake are possessed of great interest, and are well worthy of a careful examination. They are largely volcanic, and present some exceptional geological features. The larger, known as Pa-o-ha, is about two miles long, and consists in part of stratified lake deposits raised up by some convulsion, while the smaller, known as Negit, or Black Island, is wholly of volcanic origin. The lake beds on the larger island are faulted and generally much disturbed, but are probably not older than the Quaternary. The eastern and northern parts consist of a black basaltic lava, very fine-grained or even glassy. Much of it is younger than the sedimentary beds. The lava extends beneath the water on the northern side of the island in such a manner as to indicate that it flowed out during a stage of low water. At the southeast side hot springs issue from the lava just beneath the surface of the water. The bare rocks above are seamed and decomposed, and from holes and crevices there issue jets of steam which are visible for long distances in the cold weather. At the eastern end there are several peculiar craterlike depressions in the lava. One or two of them lie partly beneath the surface of the lake, and are being destroyed by the action of the waves. Several are, however, very perfect and symmetrical, about fifty feet deep, and as they extend beneath the lake level their bottoms are filled with water. No melted rock ever appears to have come from them, and there are several reasons for concluding that their origin is due to the same causes as those on the head of the North Fork of Owen's River already mentioned. This was the explosive action of gases beneath, completely shattering the massive lava and blowing out the material once occupying the depressions. It does not appear that any of this material could have been fused, for the fragments scattered around over the surrounding surface are all angular. Volcanic ash is strewn about and may have followed the explosions. A similar craterlike depression occurs in the sedimentary beds on the southern side of the island, and it can only be accounted for by a similar cause, or possibly by the falling out of the bottom.
Negit Island presents from a distance a dark, forbidding aspect. It is long and low, with a broad, truncated cone at its western end. A close examination shows this cone to be a mass of different colored lavas and scoria fissured and thrown into all kinds of shapes by earthquake movements. It is so shattered that it seems ready to crumble away and sink in the lake. The whole island has the appearance of having been elevated from beneath the water in very recent times, and shattered to its very center. On the eastern side the massive flows have evidently been lifted upward in such a manner as to leave great open fissures four to six feet wide and descending to unknown depths. From the level of the lake they are filled with water. These islands possess a most unique character, and, taken in connection with the Mono craters with their flows of acid lavas, illustrate volcanic phenomena more strikingly than almost anything else in the United States. The fact that the volcanic action has been so recent adds greatly to its interest. The oldest lavas in the region seem to have been rhyolites and andesites, then followed basic andesites and basalts, and last of all the acid and glassy rhyolites of the Mono craters.
On the north side of the lake the tufas, built up by springs issuing from beneath the water, form a most interesting study. Great masses of calcareous material have been formed about the orifices
Obsidian Flow South of Mono Lake.
of these mineral springs, assuming odd and striking shapes. In the desiccated lakes of the desert in southern Inyo County these calcareous tufa deposits stand out in all their proportions.
It is probable that the forces producing the recent volcanic action are but slumbering, and that in the future, as in the past, movements will continue to take place along this great fault line bounding the Sierra Nevadas on the east. When the strain becomes too great, earthquakes will be felt and possibly followed by eruptions of lava.
The most severe earthquake of which we have any record on the Pacific coast of the United States was due to movement along this, fault. On March 26, 1872, nearly the whole of California and Nevada was violently shaken. The loss of life was, however, confined to Owen's Valley. At Lone Pine, near the foot of the valley, out of a population of about three hundred people, twenty-three were killed and sixty injured. Goodyear has described in detail the effect of this earthquake. After the event an examination showed numerous fault lines, extending as a general thing parallel to the base of the Sierras. Local areas sank, and in addition to the vertical movement there was a horizontal one amounting in some instances to from twelve to eighteen feet. Owing to the slight rainfall, the fault scarps left by this earthquake may still be seen. They indicate either a depression of the valley or an elevation of the Sierras to the extent of several feet. Russell mentions a fault cliff near Mono Lake of fifty feet which he thinks may date from this disturbance. It is clear that an equilibrium has not yet been reached, and there is no telling when the shocks may be repeated. These things forcibly remind us that geological processes are going on to-day as in the past. The common phenomena around us teach the same thing, but we become so used to them that they are not noticed, and it is only when our attention is called to some great example, something out of the ordinary, that we realize the transitoriness of even the great mountains.
I have thus tried to trace in a general way the history of the fault fissures and the great mountains and deep valleys produced by them along eastern California from their inception in the Cretaceous down to the present. Many of the geological phenomena connected with this subject are without doubt displayed on a grander and more imposing scale in this region than anywhere else in the United States.