CHAPTER VI

JOHN LE CONTE'S PHYSICAL STUDIES OF LAKE TAHOE

In certain numbers (November and December 1883 and January 1884) of the Overland Monthly, Professor John Le Conte, of the State University, Berkeley, California, presented the results of his physical studies of Lake Tahoe in three elaborate chapters. From these the following quotations of general interest are taken:

Hundreds of Alpine lakes of various sizes, with their clear, deep, cold, emerald or azure waters, are embosomed among the crags of the Sierra Nevada Mountains. The most extensive, as well as the most celebrated, of these bodies of fresh water is Lake Tahoe.

This Lake, ... occupies an elevated valley at a point where the Sierra Nevada divides into two ranges. It is, as it were, ingulfed between two lofty and nearly parallel ridges, one lying to the east and the other to the west. As the crest of the principal range of the Sierra runs near the western margin of this Lake, this valley is thrown on the eastern slope of this great mountain system.

The boundary line between the States of California and Nevada makes an angle of about 131 degrees in this Lake, near its southern extremity, precisely at the intersection of the 39th parallel of north latitude with the 120th meridian west from Greenwich. Inasmuch as, north of this angle, this boundary line follows the 120th meridian, which traverses the Lake longitudinally from two to four miles from its eastern shore-line, it follows that more than two-thirds of its area falls within the jurisdiction of California, the remaining third being within the boundary of Nevada. It is only within a comparatively recent period that the geographical coordinates of this Lake have been accurately determined.

Its greatest dimension deviates but slightly from a medium line. Its maximum length is about 21.6 miles, and its greatest width is about 12 miles. In consequence of the irregularity of its outline, it is difficult to estimate its exact area; but it cannot deviate much from 192 to 195 square miles.

The railroad surveys indicate that the elevation of the surface of its waters above the level of the ocean is about 6247 feet.

Its drainage basin, including in this its own area, is estimated to be about five hundred square miles. Probably more than a hundred affluents of various capacities, deriving their waters from the amphitheater of snow-clad mountains which rise on all sides from 3000 to 4000 feet above its surface, contribute their quota to supply this Lake. The largest of these affluents is the Upper Truckee River, which falls into its southern extremity.

The only outlet to the Lake is the Truckee River, which carries the surplus waters from a point on its northwestern shore out through a magnificent mountain gorge, thence northeast, through the arid plains of Nevada, into Pyramid Lake. This river in its tortuous course runs a distance of over one hundred miles, and for about seventy miles (from Truckee to Wadsworth) the Central Pacific Railroad follows its windings. According to the railroad surveys, this river makes the following descent:

[Footnote 1: The elevation of Pyramid Lake above the sea-level has never, as far as we know, been accurately determined. Henry Gannet, in his Lists of Elevation (4th ed., Washington, 1877, p. 143), gives its altitude above the sea as 4890 feet; and credits this number to the Pacific Railroad Reports. But as this exact number appears in Frémont's Report of Exploring Expedition to Oregon and North California in the Years 1843-44. (Doc. No. 166, p. 217), it is probable that the first rude and necessarily imperfect estimate has been copied by subsequent authorities. This number is evidently more than 800 feet too great; for the railroad station at Wadsworth (about eighteen or twenty miles from the lake), where the line of the railroad leaves the banks of the Truckee River, is only 4077 feet above the sea-level. So that these numbers would make Pyramid Lake 813 feet above the level of its affluent at Wadsworth; which, of course, is impossible. Under this state of facts, I have assumed the elevation of this lake to be 3890 feet.]

During the summer of 1873, the writer embraced the opportunity afforded by a six weeks' sojourn on the shores of the Lake to undertake some physical studies in relation to this largest of the "gems of the Sierra." Furnished with a good sounding-line and a self-registering thermometer, he was enabled to secure some interesting and trustworthy physical results.

(1.) Depth. It is well known that considerable diversity of opinion has prevailed in relation to the actual depth of Lake Tahoe. Sensational newsmongers have unhesitatingly asserted that, in some portions, it is absolutely fathomless. It is needless to say that actual soundings served to dispel or to rectify this popular impression. The soundings indicated that there is a deep subaqueous channel traversing the whole Lake in its greatest dimension, or south and north. Beginning at the southern end, near the Lake House, and advancing along the long axis of the Lake directly north towards the Hot Springs at the northern end—a distance of about eighteen miles—we have the following depths:

These figures show that this lake exceeds in depth the deepest of the Swiss lakes (the Lake of Geneva), which has a maximum depth of 334 meters. On the Italian side of the Alps, however, Lakes Maggiore and Como are said to have depths respectively of 796.43 and 586.73 meters. These two lakes are so little elevated above the sea that their bottoms are depressed 587 and 374 meters below the level of the Mediterranean.

(2.) Relation of Temperature to Depth. By means of a self-registering thermometer (Six's) secured to the sounding-line, a great number of observations were made on the temperature of the water of the Lake at various depths and in different portions of the same. These experiments were executed between the 11th and 18th of August, 1873. The same general results were obtained in all parts of the Lake. The following table contains the abstract of the average results, after correcting the thermometric indications by comparison with a standard thermometer:

It will be seen from the foregoing numbers that the temperature of the water decreases with increasing depth to about 700 or 800 feet (213 or 244 meters), and below this depth it remains sensibly the same down to 1506 feet (459 meters). This constant temperature which prevails at all depths below say 250 meters is about 4 degrees Cent. (39.2 Fah.). This is precisely what might have been expected; for it is a well established physical property of fresh water, that it attains its maximum density at the above-indicated temperature. In other words, a mass of fresh water at the temperature of 4 deg. Cent. has a greater weight under a given volume (that is, a cubic unit of it is heavier at this temperature) than it is at any temperature either higher or lower. Hence, when the ice-cold water of the snow-fed streams of spring and summer reaches the Lake, it naturally tends to sink as soon as its temperature rises to 4 deg. Cent.; and, conversely, when winter sets in, as soon as the summer-heated surface water is cooled to 4 deg., it tends to sink. Any further rise of temperature of the surface water during the warm season, or fall of temperature during the cold season, alike produces expansion, and thus causes it to float on the heavier water below; so that water at 4 deg. Cent., perpetually remains at the bottom, while the varying temperature of the seasons and the penetration of the solar heat only influence a surface stratum of about 250 meters in thickness. It is evident that the continual outflow of water from its shallow outlet cannot disturb the mass of liquid occupying the deeper portions of the Lake. It thus results that the temperature of the surface stratum of such bodies of fresh water for a certain depth fluctuates with the climate and with the seasons; but at the bottom of deep lakes it undergoes little or no change throughout the year, and approaches to that which corresponds to the maximum density of fresh water.

(3.) Why the Water does not freeze in Winter. Residents on the shore of Lake Tahoe testify that, with the exception of shallow and detached portions, the water of the Lake never freezes in the coldest winters. During the winter months, the temperature of atmosphere about this Lake must fall as low, probably, as 0 degrees Fah. (-17.78 deg. Cent.). According to the observations of Dr. George M. Bourne, the minimum temperature recorded during the winter of 1873-74 was 6 deg. Fah. (-14.44 deg. Cent.). As it is evident that during the winter season the temperature of the air must frequently remain for days, and perhaps weeks, far below the freezing point of water, the fact that the water of the Lake does not congeal has been regarded as an anomalous phenomenon. Some persons imagine that this may be due to the existence of subaqueous hot springs in the bed of the Lake—an opinion which may seem to be fortified by the fact that hot springs do occur at the northern extremity of the Lake. But there is no evidence that the temperature of any considerable body of water in the Lake is sensibly increased by such springs. Even in the immediate vicinity of the hot springs (which have in summer a maximum temperature of 55 deg. C. or 131 F.), the supply of warm water is so limited that it exercises no appreciable influence on the temperature of that portion of the Lake. This is further corroborated by the fact that no local fogs hang over this or any other portion of the Lake during the winter which would most certainly be the case if any considerable body of hot water found its way into the Lake.

The true explanation of the phenomenon may, doubtless, be found in the high specific heat of water, the great depth of the Lake, and in the agitation of its waters by the strong winds of winter. In relation to the influence of depth, it is sufficient to remark that, before the conditions preceding congelation can obtain, the whole mass of water—embracing a stratum of 250 meters in thickness—must be cooled down to 4 deg. Cent.; for this must occur before the vertical circulation is arrested and the colder water floats on the surface. In consequence of the great specific heat of water, to cool such a mass of the liquid through an average temperature of 8 deg. Cent, requires a long time, and the cold weather is over before it is accomplished. In the shallower portions, the surface of the water may reach the temperature of congelation, but the agitations due to the action of strong winds soon breaks up the thin pellicle of ice, which is quickly melted by the heat generated by the mechanical action of the waves. Nevertheless, in shallow and detached portions of the Lake, which are sheltered from the action of winds and waves—as in Emerald Bay—ice several inches in thickness is sometimes formed.

(4.) Why Bodies of the Drowned do not Rise. A number of persons have been drowned in Lake Tahoe—some fourteen between 1860 and 1874—and it is the uniform testimony of the residents, that in no case, where the accident occurred in deep water, were the bodies ever recovered. This striking fact has caused wonder-seekers to propound the most extraordinary theories to account for it. Thus one of them says, "The water of the Lake is purity itself, but on account of the highly rarified state of the air it is not very buoyant, and swimmers find some little fatigue; or, in other words, they are compelled to keep swimming all the time they are in the water; and objects which float easily in other water sink here like lead." Again he says, "Not a thing ever floats on the surface of this Lake, save and except the boats which ply upon it."

It is scarcely necessary to remark that it is impossible that the diminution of atmospheric pressure, due to an elevation of 6250 feet (1905 meters) above the sea-level, could sensibly affect the density of the water. In fact, the coefficient of compressibility of this liquid is so small that the withdrawal of the above indicated amount of pressure (about one-fifth of an atmosphere) would not lower its density more than one hundred-thousandth part! The truth is, that the specific gravity is not lower than that of any other fresh water of equal purity and corresponding temperature. It is not less buoyant nor more difficult to swim in than any other fresh water; and consequently the fact that the bodies of the drowned do not rise to the surface cannot be accounted for by ascribing marvelous properties to its waters.

The distribution of temperature with depth affords a natural and satisfactory explanation of the phenomenon, and renders entirely superfluous any assumption of extraordinary lightness in the water. The true reason why the bodies of the drowned do not rise to the surface is evidently owing to the fact that when they sink into water which is only 4 deg. Cent. (7.2 deg. Fah.) above the freezing temperature, the gases usually generated by decomposition are not produced in the intestines; in other words, at this low temperature the bodies do not become inflated, and therefore do not rise to the surface. The same phenomenon would doubtless occur in any other body of fresh water under similar physical conditions. [1]

[Footnote 1: It should be noted that since 1874 there have been remarkably few deaths from drowning in Lake Tahoe, and that the major cases of those referred to by Dr. LeConte were of workmen and others who were generally under the influence of intoxicants.]

(5.) Transparency of the Water. All visitors to this beautiful Lake are struck with the extraordinary transparency of the water. At a depth of 15 to 20 meters (49.21 to 65.62 feet), every object on the bottom—on a calm sunny day—is seen with the greatest distinctness. On the 6th of September, 1873, the writer executed a series of experiments with the view of testing the transparency of the water. A number of other experiments were made August 28 and 29, under less favorable conditions. By securing a white object of considerable size—a horizontally adjusted dinner-plate about 9.5 inches in diameter—to the sounding-line, it was ascertained that (at noon) it was plainly visible at a vertical depth of 33 meters, or 108.27 English feet. It must be recollected that the light reaching the eye from such submerged objects must have traversed a thickness of water equal to at least twice the measured depth; in the above case, it must have been at least 66 meters, or 216.54 feet. Furthermore, when it is considered that the amount of light regularly reflected from such a surface as that of a dinner-plate, under large angles of incidence in relation to the surface, is known to be a very small fraction of the incident beam (probably not exceeding three or four per cent.), it is evident that solar light must penetrate to vastly greater depths in these pellucid waters.

Moreover, it is quite certain that if the experiments in relation to the depths corresponding to the limit of visibility of the submerged white disk had been executed in winter instead of summer, much larger numbers would have been obtained. For it is now well ascertained, by means of the researches of Dr. F.A. Forel of Lausanne, that the waters of Alpine lakes are decidedly more transparent in winter than in summer. Indeed, it is reasonable that when the affluents of such lakes are locked in the icy fetters of winter, much less suspended matter is carried into them than in summer, when all the sub-glacial streams are in active operation.

Professor Le Conte goes into this subject (as he later does into the subject of the color of Lake Tahoe) somewhat exhaustively in a purely scientific manner and in too great length for the purposes of this chapter, hence the scientific or curious reader is referred to the original articles for further information and discussion.

Color of the Waters of Lake Tahoe. One of the most striking features of this charming mountain Lake is the beautiful hues presented by its pellucid waters. On a calm, clear, sunny day, wherever the depth is not less than from fifty to sixty meters, to an observer floating above its surface, the water assumes various shades of blue; from a brilliant Cyan blue (greenish-blue) to the most magnificent ultramarine blue or deep indigo blue. The shades of blue increasing in darkness in the order of the colors of the solar spectrum, are as follows: Cyan-blue (greenish blue), Prussian-blue, Cobalt-blue, genuine ultramarine-blue, and artificial ultramarine-blue (violet blue). While traversing one portion of the Lake in a steamer, a lady endowed with a remarkable natural appreciation and discrimination of shades of color declared that the exact tint of the water at this point was "Marie-Louise blue."

The waters of this Lake exhibit the most brilliant blueness in the deep portions, which are remote from the fouling influences of the sediment-bearing affluents, and the washings of the shores. On a bright and calm day, when viewed in the distance, it had the ultramarine hue; but when looked fair down upon, it was of almost inky blackness—a solid dark blue qualified by a trace of purple or violet. Under these favorable conditions, the appearance presented was not unlike that of the liquid in a vast natural dyeing-vat.

A clouded state of the sky, as was to be expected, produced the well-known effects due to the diminished intensity of light; the shades of blue became darker, and, in extreme cases, almost black-blue. According to our observations, the obscurations of the sky by the interposition of clouds produced no other modifications of tints than those due to a diminution of luminosity.

In places where the depth is comparatively small and the bottom is visibly white, the water assumes various shades of green; from a delicate apple-green to the most exquisite emerald-green. Near the southern and western shores of the Lake, the white, sandy bottom brings out the green tints very strikingly. In the charming cul-de-sac called "Emerald Bay," it is remarkably conspicuous and exquisitely beautiful. In places where the stratum of water covering white portions of the bottom is only a few meters in thickness, the green hue is not perceptible, unless viewed from such a distance that the rays of light emitted obliquely from the white surface have traversed a considerable thickness of the liquid before reaching the eye of the observer.

The experiments with the submerged white dinner-plate, in testing the transparency of the water, incidentally manifested, to some extent, the influence of depth on the color of the water. The white disk presented a bluish-green tint at the depth of from nine to twelve meters; at about fifteen meters it assumed a greenish-blue hue, and the blue element increased in distinctness with augmenting depth, until the disk became invisible or undistinguishable in the surrounding mass of blue waters. The water intervening between the white disk and the observer did not present the brilliant and vivid green tint which characterized that which is seen in the shallow portions of the Lake, where the bottom is white. But this is not surprising, when we consider the small amount of diffused light which can reach the eye from so limited a surface of diffusion.

In studying the chromatic tints of these waters, a hollow pasteboard cylinder, five or six centimeters in diameter, and sixty or seventy centimeters in length, was sometimes employed for the purpose of excluding the surface reflection and the disturbances due to the small ripples on the water. When quietly floating in a small row-boat, one end of this exploring tube was plunged under the water, and the eye of the observer at the other extremity received the rays of light emanating from the deeper portions of the liquid. The light thus reaching the eye presented essentially the same variety of tints in the various portions of the Lake as those which have been previously indicated.

Hence it appears that under various condition—such as depth, purity, state of sky and color of bottom—the waters of this Lake manifest nearly all the chromatic tints presented in the solar spectrum between greenish-yellow and the darkest ultramarine-blue, bordering upon black-blue.

It is well known that the waters of oceans and seas exhibit similar gradations of chromatic hues in certain regions. Navigators have been struck with the variety and richness of tints presented, in certain portions, by the waters of the Mediterranean Sea, the Atlantic and Pacific Oceans, and especially those of the Caribbean Sea. In some regions of the oceans and seas, the green hues, and particularly those tinged with yellow, are observed in comparatively deep waters, or, at least, where the depths are sufficiently great to prevent the bottom from being visible. But this phenomenon seems to require the presence of a considerable amount of suspended matter in the water. In no portion of Lake Tahoe did I observe any of the green tints, except where the light-colored bottom was visible. This was, probably, owing to the circumstance that no considerable quantity of suspended matter existed in any of the waters observed.

Rhythmical Variations of Level in Lakes: or "Seiches."—As might be expected, the waters of Lake Tahoe are subject to fluctuations of level, depending upon the variable supplies furnished by its numerous affluents. In mid-winter, when these streams are bound in icy fetters, the level falls; while in the months of May and June, when the snows of the amphitheater of mountain-slopes are melting most rapidly, the level of the Lake rises, and a maximum amount of water escapes through its outlet. According to the observations of Capt. John McKinney, made at his residence on the western shore of this Lake, the average seasonal fluctuation of level is about 0.61 of a meter; but in extreme seasons it sometimes amounts to 1.37 meters. The Lake of Geneva, in like manner, is liable to fluctuations of level amounting to from 1.95 to 2.60 meters, from the melting of the Alpine snows.

But besides these variations of level due to the variable quantities of water discharged into them by their affluents, many lakes of moderate dimensions are liable to rhythmical oscillations of level of short duration, which are, obviously, but produced by fluctuations in the supply of water. It is to this kind of species of variation of level that our attention will be directed in the sequel.

This interesting phenomenon was first recognized in the Lake of Geneva; but was subsequently found to be common to all the Swiss lakes, as well as to those of Scotland. It is, therefore, a general phenomenon, which may be observed in all lakes of moderate dimensions. The inhabitants of the shores of the Lake of Geneva have long designated this rhythmical oscillation of the level of the water by the term of Seiche; and this designation has been adopted by scientific writers.

These Seiches were first signalized in the Lake of Geneva in 1730, by Fatio de Duillier, who ascribed them to the checking of the flow of the waters of the Rhone on the shoal near Geneva by the force of the wind at mid-day. Addison and Jallabert, in 1742, supposed them to be caused by sudden increments in the discharge of the affluents, due to the augmentation in the amount of snow melted after mid-day; or to the sudden increase in the flow of the Arve, checking the outflow of water by the Rhone. Bertrand supposed that electrified clouds might locally attract and elevate the waters of the lake, and thus produce oscillations of level. H.B. de Saussure, in 1799, attributed the phenomenon to rapid local variations of atmospheric pressure on different parts of the lake. J.P.E. Vaucher, in 1802 and 1804, adopted de Saussure's explanation, and confirmed it by many excellent observations. He, moreover, established that Seiches, more or less considerable, occur in all the Swiss lakes; and that they take place at all seasons of the year, and at all times of the day; but, in general, more frequently in spring and autumn. As regards the cause of the phenomenon, Vaucher shows how rapid local alterations of atmospheric pressure would produce oscillations in the level of the lake, and compares them to the vibrations of a liquid in a recurved tube or siphon. Finally, Arago maintained that Seiches may arise from various causes, and traced the analogy between them and certain remarkable oscillations of the sea, including those arising from earthquakes.

But physical science is indebted to Professor F.A. Forel, of Lausanne, for the most complete and exhaustive investigation in relation to the phenomena of Seiches. This accomplished physicist began his researches in 1869, and has continued them up to the present time. He has been able to demonstrate that these rhythmical oscillations occur in nearly all the Swiss Lakes (he studied the phenomena in nine of them), and that they follow in all cases the same general laws. Those of the Lake of Geneva have received the most elaborate and prolonged investigation. In March, 1876, Forel established a self-registering tide-gauge (limni-metre enregistreur) on the northern shore of this lake, at Morges; and, with the coöperation of P. Plantamour, another one was installed in June, 1877, at Secheron, near the city of Geneva, at the southern extremity. Since these dates, these two instruments have, respectively, been registering oscillations of the level of the water of the Lake of Geneva; and they are so sensitive as to indicate the waves generated by a steamer navigating the lake at a distance of ten or fifteen kilometers.

From a most searching investigation of all the phenomena presented by the Seiches in the Swiss Lakes, Forel deduces the conclusion that they are really movements of steady uninodal oscillations (balanced undulations), in which the whole mass of water in the lake rhythmically swings from shore to shore. And, moreover, he shows that the water oscillates according to the two principal dimensions of the lake; thus, giving rise to longitudinal Seiches and transverse Seiches. They occur in series of tautochronous oscillations of decreasing amplitude; the first wave produced by the action of a given cause having a maximum amplitude.

Causes. The disturbances of hydrostatic equilibrium which generate Seiches may be produced by a variety of causes. Among these, the following may be cited: (a) Sudden local variations of atmospheric pressure on different parts of the lake. (b) A descending wind, striking the surface of the lake over a limited area, (c) Thunder-storms, hail-storms, and water-spouts; and especially when the accompanying winds act vertically. (d) The fall of a large avalanche, or of a land-slide into the lake. (e) And lastly, earthquakes.

Observations show that the most frequent and evident of these causes are variations of atmospheric pressure and local storms. With regard to earthquake shocks as a cause of such fluctuations of level, it is a singular and significant fact that since Forel has established the delicate self-registering apparatus on the shores of the Lake of Geneva, no less than twelve earthquake shocks have been experienced in this portion of Switzerland, and they have had no sensible influence on these sensitive instruments. In fact, a little consideration in relation to the character of such shocks renders it highly improbable that such brief tremors of the earth's crust could have been any agency in the generation of rhythmical oscillations of the whole mass of water in the lake. Indeed, it is very questionable whether any earthquake waves are ever produced in the ocean, except when the sea-bottom undergoes a permanent vertical displacement.

Lake Tahoe. From inquiries made of the inhabitants of the shores of Lake Tahoe, I was not able to discover that any rhythmical oscillations of the level of its waters have ever been noticed. Some residents declared that they had observed sudden fluctuations of level, which, from their suddenness, they were disposed to ascribe to disturbances of the bottom of the Lake due to volcanic agencies, although they were unable to coordinate such oscillations with any earthquake manifestations on the adjacent shores.

It is evident, however, that until arrangements are consummated for recording systematic observations on the variations of the level of this Lake, we cannot expect that its Seiches will be detected. Of course, self-registering gauges would give the most satisfactory results; but any graduated gauge, systematically observed, would soon furnish evidence of the phenomenon. For the longitudinal Seiches, "Hot Springs," at the northern extremity of the Lake, or "Lake House," at the southern end, would be eligible stations for gauges; and for the transverse Seiches, Glenbrook, on the eastern shore, or Capt. McKinney's on the western margin, would afford good stations. As far as I am aware, true Seiches have never been observed in any of the American lakes. This fact is the more remarkable from the circumstance that long-continued and careful observations have been made on the fluctuations of level of several of the large Canadian lakes, with the view of testing the possible existence of lunar tides. Perhaps these lakes may be too large to manifest the uninodal rhythmical oscillations which have been so successfully studied by Forel in the smaller lakes of Switzerland.[2]

Be this as it may, there can be no doubt that Lake Tahoe is a body of water in all respects adapted for the manifestation of this species of oscillation; and that, like the Swiss lakes, it is subject to Seiches. Indeed, the far greater simplicity in the configuration of the basin of Lake Tahoe than that of the Lake of Geneva must render the phenomena much less complicated in the former than in the latter.

Professor LeConte then gives his computations as to the probable duration of the oscillations on Lake Tahoe, should they occur there.

[Footnote 2: It is proper to add that Fluctuations of level in the North American lakes have been noticed by various observers, from the time of the Jesuit Fathers of the period of Marquette, in 1673, down to the present epoch. Among those who have discussed this problem may be mentioned in chronological order: Fra Marquette in 1673, Baron La Hontan 1689, Charlevoix 1721, Carver 1766, Weld 1796, Major S.A. Storrow 1817, Capt. Henry Whiting 1819, H.R. Schoolcraft 1820, Gen. Dearborn 1826-29.]

CHAPTER VII

HOW LAKE TAHOE WAS FORMED

Lindgren, the geologist, affirms that after the Sierra Nevada range was thrust up, high into the heavens, vast and long continued erosion "planed down this range to a surface of comparatively gentle topography." He claims that it must originally have been of great height. Traces of this eroded range (Cretaceous) "still remain in a number of flat-topped hills and ridges that rise above the later tertiary surface. There is reason to believe that this planed-down mountain range had a symmetrical structure, for somewhat to the east of the present divide is a well-marked old crest line extending from the Grizzly Peak Mountains on the north, in Plumas County, at least as far south as Pyramid Peak, in Eldorado County. At sometime in the later part of the Cretaceous period the first breaks took place, changing the structure of the range from symmetrical to monoclinal and outlining the present form of the Sierra Nevada."

This great disturbance he thinks, "was of a two-fold character, consisting of the lifting up of a large area including at least a part of the present Great Basin [Nevada and Utah] and a simultaneous breaking and settling of the higher portions of the arch. Along the eastern margin a system of fractures was thus outlined which toward the close of the Tertiary was to be still further emphasized. The main break probably extended from a point south of Mono Lake to Antelope Valley and from Markleeville northward toward Sierra Valley. A large part of the crust block to the west of this dislocation also sank down. This sunken area is now indicated by Lake Tahoe and by its northward continuation, Sierra Valley, separated from each other only by masses of Tertiary lavas.... It is worthy of note that within the area of the range no volcanic eruptions accompanied this subsidence."

He continues: "As a consequence of this uplift the erosive power of the streams was rejuvenated, the Cretaceous surface of gentle outline was dissected, and the rivers began to cut back behind the old divide, carrying their heads nearly to the present crest line that separates the slope of the Sierra from the depression of Lake Tahoe."

These rivers are the great gold bearing streams that caused the mining excitement of 1849. They all head near the Tahoe region, and include the Yuba, Feather, American, Mokelumne, Calaveras, Cataract, and Tuolumne.

Here, then, were two crest lines—the old Cretaceous line of which the Crystal Range immediately overlooking Desolation Valley on the west, with Pyramid and Agassiz Peaks as its salient points,—and the new Tertiary crest line, reaching somewhat irregularly from Honey Lake in the north to Mono Lake in the south. At the north of Lake Tahoe, "southwest of Reno, a large andesitic volcano poured forth lavas which extend between the Truckee River Canyon and the Washoe Valley. In the region extending northward from Lake Tahoe to Sierra Valley enormous andesitic eruptions took place, and the products of these volcanoes are now piled up as high mountains, among which Mount Pluto nearly attains 9000 feet."

These are the volcanic lavas which united the two crests forming the eastern and western borders of the Tahoe basin or depression, and through which the Truckee River had in some way to find passage ere it could discharge its waters into Pyramid Lake, resting in the bosom of the Great Basin.

Here, then, we have the crude Tahoe basin ready for the reception of water. This came from the snow and rainfall on its large and mountainous drainage area, a hundred greater and lesser streams directly and indirectly discharging their flow into its tremendous gulf.

Its later topography has been materially modified by glacial action, and this is fully discussed by Professor Joseph Le Conte in the following chapter.

It should not be forgotten, however, that while Mt. Pluto was being formed, other vast volcanic outpourings were taking place. Well back to the west of the Tahoe region great volcanoes poured out rhyolite, a massive rock of light gray to pink color and of fine grain, which shows small crystals of quartz and sanidine in a streaky and glossy ground mass. On the summits nearer to Tahoe the volcanic outflows were of andesite, a rough and porous rock of dark gray to dark brown color. Lindgren says: "By far the greater part of the andesite occurs in the form of a tuffaceous breccia in numerous superimposed flows. These breccias must have issued from fissures near the summit of the range and were, either before their eruption or at the time of issue, mixed with enormous quantities of water, forming mud flows sufficiently fluid to spread down the slope for distances of fifty or sixty miles. The derivation of the water and the exact mode of eruption are difficult to determine.... Towards the summits the breccias gradually lose their stratified character and become more firmly cemented. Over large areas in the Truckee quadrangle the andesite masses consist of breccias containing numerous dykes and necks of massive andesite....

"The andesite volcanoes were mainly located along the crest of the Sierra, in fact, almost continuously from Thompson Peak, west of Honey Lake, down to latitude 38° degrees 10'. Farther south the eruptions diminished greatly in intensity.... Along the first summit of the range west of Tahoe the greatest number of vents are found. Beginning at Webber Lake on the north, they include Mount Lola, Castle Peak, Mount Lincoln, Tinker Knob, Mount Mildred and Twin Peak. The andesite masses here in places attain a thickness of 2000 feet. An interval followed in the northern part of the Pyramid Peak quadrangle where no important volcanoes were located, but they appear again in full force in Alpine County. Round Top, attaining an elevation of 10,430 feet, and the adjacent peaks, were the sources of the enormous flows which covered a large part of Eldorado County. Still another volcanic complex with many eruptive vents is that situated in the western part of Alpine County, near Markleeville, which culminates in Highland Peak and Raymond Peak, the former almost reaching 11,000 feet. The total thickness of the volcanic flows in this locality is as much as 4000 feet."

It is to these breccias we owe the volcanic appearances in the Truckee River Canyon, a few miles before reaching the Lake. There are several layers of the andesites breccias at the head of Bear Creek Canyon, above Deer Park Springs.

"None of the craters," says Lindgren, "of these volcanoes are preserved, and at the time of their greatest activity they may have reached a height of several thousand feet above the present summits."

CHAPTER VIII

THE GLACIAL HISTORY OF LAKE TAHOE

We have already seen in the preceding chapter how the great basin, in which Lake Tahoe rests, was turned out in the rough from Nature's workshop. It must now be smoothed down, its angularities removed, its sharpest features eliminated, and soft and fertile banks prepared upon which trees, shrubs, plants and flowers might spring forth to give beauty to an otherwise naked and barren scene.

It is almost impossible for one to picture the Tahoe basin at this time. There may have been water in it, or there may not. All the great mountain peaks, most of them, perhaps, much higher by several thousands of feet than at present, were rude, rough, jagged masses, fresh from the factory of God. There was not a tree, not a shrub, not a flower, not a blade of grass. No bird sang its cheering song, or delighted the eye with its gorgeous plumage; not even a frog croaked, a cicada rattled, or a serpent hissed. All was barren desolation, fearful silence and ghastly newness.

What were the forces that produced so marvelous a change?

Snowflakes,—"flowers of the air",—as John Muir so poetically calls them. They accomplished the work. Falling alone they could have done nothing, but coming down in vast numbers, day after day, they piled up and became a power. Snow forms glaciers, and glaciers are mighty forces that create things.

Let us, if possible, stand and watch the Master Workman doing the work that is to make this region our source of present day joy. We will make the ascent and stand on the summit of Pyramid Peak. This is now 10,020 feet above sea level, rising almost sheer above Desolation Valley immediately at our feet.

The first thing that arrests the visitor's attention is the peculiar shape of the peak upon which he stands, and of the whole of the Crystal Range. Both east and west it is a great precipice, with a razor-like edge, which seems to have been especially designed for the purpose of arresting the clouds and snow blown over the mountain, ranges of the High Sierras, and preventing their contents falling upon the waste and thirsty, almost desert-areas of western Nevada, which lie a few miles further east.

Whence do the rains and snow-storms come?

One hundred and fifty miles, a trifle more or less, to the westward is the vast bosom of the Pacific Ocean. Its warm current is constantly kissed by the fervid sun and its water allured, in the shape of mist and fog, to ascend into the heavens above. Here it is gently wafted by the steady ocean breezes over the land to the east. In the summer the wind currents now and again swing the clouds thus formed northward, and Oregon and Washington receive rain from the operation of the sun upon the Pacific Ocean of the south. In June and July, however, the Tahoe region sees occasional rains which clear the atmosphere, freshen the flowers and trees, and give an added charm to everything. But in the fall and winter the winds send the clouds more directly eastward, and in crossing the Sierran summits the mist and fog become colder and colder, until, when the clouds are arrested by the stern barriers of the Crystal Range, and necessity compels them to discharge their burden, they scatter snow so profusely that one who sees this region only in the summer has no conception of its winter appearance. The snow does not fall as in ordinary storms, but, in these altitudes, the very heavens seem to press down, ladened with snow, and it falls in sheets to a depth of five, ten, twenty, thirty and even more feet, on the level.

Look now, however, at the western edge of the Crystal Range. It has no "slopes." It is composed of a series of absolute precipices, on the edge of one of which we stand. These precipices, and the razor edge, are fortified and buttressed by arms which reach out westward and form rude crescents, called by the French geologists cirques, for here the snow lodges, and is packed to great density and solidity with all the force, fervor and fury of the mountain winds.

But the snow does not fall alone on the western cirques. It discharges with such prodigality, and the wind demands its release with such precipitancy, that it lodges in equally vast masses on the eastern slopes of the Crystal Range. For, while the eastern side of this range is steep enough to be termed in general parlance "precipitous," it has a decided slope when compared with the sheer drop of the western side. Here the configuration and arrangement of the rock-masses also have created a number of cirques, where remnants of the winter's snow masses are yet to be seen. These snow masses are baby glaciers, or snow being slowly manufactured into glaciers, or, as some authorities think, the remnants of the vast glaciers that once covered this whole region with their heavy and slowly-moving icy cap.

On the Tallac Range the snow fell heavily toward Desolation Valley, but also on the steep and precipitous slopes that faced the north. So also with the Angora Range. Its western exposure, however, is of a fairly gentle slope, so that the snow was blown over to the eastern side, where there are several precipitous cirques of stupendous size for the preservation of the accumulated and accumulating snow.

Now let us, in imagination, ascend in a balloon over this region and hover there, seeking to reconstruct, by mental images, the appearance it must have assumed and the action that took place in the ages long ago.

Snow, thirty, fifty, one hundred or more feet deep lay, on the level, and on the mountain slopes or in precipitous cirques twice, thrice, or ten times those depths. Snow thus packed together soon changes its character. From the light airy flake, it becomes, in masses, what the geologists term névé. This is a granular snow, intermediate between snow and ice. A little lower down this névé is converted into true glacial ice-beds, which grow longer, broader, deeper and thicker as the névé presses down from above.

Lay minds conceive of these great ice-beds of transformed snow as inert, immovable bodies. They think the snow lies upon the surface of the rocks or earth. The scientific observer knows better. By the very inertia of its own vast and almost inconceivable weight the glacier is compelled to move. Imagine the millions of millions of tons of ice of these sloping masses, pressing down upon the hundreds of thousands of tons of ice that lie below. Slowly the mass begins to move. But all parts of it do not move with equal velocity. The center travels quicker than the margins, and the velocity of the surface is greater than that of the bottom. Naturally the velocity increases with the slope, and when the ice begins to soften in the summer time its rate of motion is increased.

But not only does the ice move. There have been other forces set in motion as well as that of the ice. The fierce attacks of the storms, the insidious forces of frost, of expansion and contraction, of lightning, etc., have shattered and loosened vast masses of the mountain summits. Some of these have weathered into toppling masses, which required only a heavy wind or slight contractions to send them from their uncertain bases onto the snow or ice beneath. And the other causes mentioned all had their influences in breaking up the peaks and ridges and depositing great jagged bowlders of rock in the slowly-moving glaciers.

Little by little these masses of rock worked their way down lower into the ice-bed. Sometime they must reach the bottom, yet, though they rest upon granite, and granite would cleave to granite, the irresistible pressure from above forces the ice and rock masses forward. Thus the sharp-edged blocks of granite become the blades in the tools that are to help cut out the contours of a world's surface. In other words the mass of glacial ice is the grooving or smoothing plane, and the granite blocks, aided by the ice, become the many and diverse blades in this vast and irresistible tool. Some cut deep and square, others with flutings and bevelings, or curves, but each helps in the great work of planing off, in some way, the rocky masses over which they move. Hence it will be seen that the grooving and marking, the fluting and beveling, the planing and smoothing processes of the ice are materially aided and abetted by the very hardness and weight of the granite and other rocks it carries with it.

Now let Joseph LeConte take up the theme and give us of the rich treasure-store of his knowledge and observation. In the American Journal of Science and Arts, Third Series, for 1875, he discussed the very field we are now interested in, and his fascinating and illuminating explanations render the subject perfectly clear. Said he:

Last summer I had again an opportunity of examining the pathways of some of the ancient glaciers of the Sierra. One of the grandest of these is what I call the Lake Valley Glacier.[3] Taking its rise in snow fountains among the high peaks in the neighborhood of Silver Mountain, this great glacier flowed northward down Lake Valley, and, gathering tributaries from the summit ridges on either side of the valley, but especially from the higher western summits, it filled the basin of Lake Tahoe, forming a great "mer de glace," 50 miles long, 15 miles wide, and at least 2000 feet deep, and finally escaped northeastward to the plains. The outlets of this great "mer de glace" are yet imperfectly known. A part of the ice certainly escaped by Truckee Canyon (the present outlet of the Lake); a part probably went over the northeastern margin of the basin. My studies during the summer were confined to some of the larger tributaries of this great glacier.

[Footnote 3: This is the name given by Dr. LeConte to the Basin in which Lake Tahoe rests and including the meadow lands above Tallac.]

Truckee Canyon and Donner Lake Glaciers. I have said that one of the outlets of the great "mer de glace" was by the Truckee River Canyon. The stage road to Lake Tahoe runs in this canyon for fifteen miles. In most parts of the canyon the rocks are volcanic and crumbling, and therefore ill adapted to retain glacial marks; yet in some places where the rock is harder these marks are unmistakable. On my way to and from Lake Tahoe, I observed that the Truckee Canyon glacier was joined at the town of Truckee by a short but powerful tributary, which, taking its rise in an immense rocky amphitheater surrounding the head of Donner Lake, flowed eastward. Donner Lake, which occupies the lower portion of this amphitheater, was evidently formed by the down-flowing of the ice from the steep slopes of the upper portion near the summit. The stage road from Truckee to the summit runs along the base of a moraine close by the margin of the lake on one side, while on the other side, along the apparently almost perpendicular rocky face of the amphitheater, 1000 feet above the surface of the lake, the Central Pacific Railroad winds its fearful way to the same place. In the upper portion of this amphitheater large patches of snow still remain unmelted during the summer.

My examination of these two glaciers, however, was very cursory. I hasten on, therefore, to others which I traced more carefully.

Lake Tahoe lies countersunk on the very top of the Sierra. This great range is here divided into two summit ridges, between which lies a trough 50 miles long, 20 miles wide, and 3000 to 3500 feet deep. This trough is Lake Valley. Its lower half is filled with the waters of Lake Tahoe. The area of this Lake is about 250 square miles, its depth 1640 feet, and its altitude 6200 feet. It is certain that during the fullness of glacial times this trough was a great "mer de glace," receiving tributaries from all directions except the north. But as the Glacial Period waned—as the great "mer de glace" dwindled and melted away, and the lake basin became occupied by water instead, the tributaries still remained as separate glaciers flowing into the Lake. The tracks of these lingering small glaciers are far more easily traced and their records more easily read, than those of the greater but more ancient glacier of which they were once but the tributaries.

Of the two summit ridges mentioned above the western is the higher. It bears the most snow now, and in glacial times gave origin to the grandest glaciers. Again: the peaks on both these summits rise higher and higher as we go toward the upper or southern end of the Lake. Hence the largest glaciers ran into the Lake at its southwestern end. And, since the mountain slopes here are toward the northeast and therefore the shadiest and coolest, here also the glaciers have had the greatest vitality and lived the longest, and have, therefore, left the plainest records. Doubtless, careful examination would discover the pathways of glaciers running into the Lake from the eastern summit also; but I failed to detect any very clear traces of such, either on the eastern or on the northern portion of the western side of the Lake; while between the southwestern end and Sugar Pine Point, a distance of only eight or ten miles, I saw distinctly the pathways of five or six. North of Sugar Pine Point there are also several. They are all marked by moraine ridges running down from the summits and projecting as points into the Lake. The pathways of three of these glaciers I studied somewhat carefully, and after a few preliminary remarks, will describe in some detail.

Mountains are the culminating points of the scenic grandeur and beauty of the earth. They are so, because they are also the culminating points of all geological agencies—igneous agencies in mountain formation, aqueous agencies in mountain sculpture. Now, I have already said that the mountain peaks which stand above the Lake on every side are highest at the southwestern end, where they rise to the altitude of 3000 feet above the lake surface, or between 9000 and 10,000 feet above the sea. Here, therefore, ran in the greatest glaciers; here we find the profoundest glacial sculpturings; and here also are clustered all the finest beauties of this the most beautiful of mountain lakes. I need only name Mount Tallac, Fallen Leaf Lake, Cascade Lake, and Emerald Bay, all within three or four miles of each other and of the Tallac House. These three exquisite little lakes (for Emerald Bay is also almost a lake), nestled closely against the loftiest peaks of the western summit ridge, are all perfect examples of glacial lakes.

South of Lake Tahoe, Lake Valley extends for fifteen miles as a plain, gently rising southward. At its lower end it is but a few feet above the lake surface, covered with glacial drift modified by water, and diversified, especially on its western side, by débris ridges, the moraines of glaciers which continued to flow into the valley or into the Lake long after the main glacier, of which they were once tributaries, had dried up. On approaching the south end of the Lake by steamer, I had observed these long ridges, divined their meaning, and determined on a closer acquaintance. While staying at the Tallac House I repeatedly visited them and explored the canyons down which their materials were brought. I proceed to describe them.

Fallen Leaf Lake Glacier. Fallen Leaf Lake lies on the plain of Lake Valley, about one and a half miles from Lake Tahoe, its surface but a few feet above the level of the latter Lake[4]; but its bottom far, probably several hundred feet, below that level. It is about three to three and one-half miles long and one and one-fourth miles wide. From its upper end runs a canyon bordered on either side by the highest peaks in this region. The rocky walls of this canyon terminate on the east side at the head of the lake, but on the west side, a little farther down. The lake is bordered on each side by an admirably marked débris ridge (moraine) three hundred feet high, four miles long, and one and one-half to two miles apart. These moraines may be traced back to the termination of the rocky ridges which bound the canyon. On one side the moraine lies wholly on the plain; on the other side its upper part lies against the slope of Mount Tallac. Near the lower end of the lake a somewhat obscure branch ridge comes off from each main ridge, and curving around it forms an imperfect terminal moraine through which the outlet of the lake breaks its way.

[Footnote 4: Professor Price informs me there is a difference of eighty feet between the level of Lake Tahoe and Fallen Leaf Lake.]

On ascending the canyon the glaciation is very conspicuous, and becomes more and more beautiful at every step. From Glen Alpine Springs upward it is the most perfect I have ever seen. In some places the white rocky bottom of the canyon, for many miles in extent, is smooth and polished and gently undulating, like the surface of a glassy but billowy sea. The glaciation is distinct also up the sides of the canyon 1000 feet above its floor.

There can be no doubt, therefore, that a glacier once came down this canyon filling it 1000 feet deep, scooped out Fallen Leaf Lake just where it struck the plain and changed its angle of slope, and pushed its snout four miles out on the level plain, nearly to the present shores of Lake Tahoe, dropping its débris on either side and thus forming a bed for itself. In its subsequent retreat it seems to have rested its snout some time at the lower end of Fallen Leaf Lake, and accumulated there an imperfect terminal moraine.

Cascade Lake Glacier. Cascade Lake, like Fallen Leaf Lake, is about one and one-half miles from Lake Tahoe, but, unlike Fallen Leaf Lake, its discharge creek has considerable fall, and the lake surface is, therefore, probably 100 feet above the level of the greater lake. On either side of this creek, from the very border of Lake Tahoe, runs a moraine ridge up to the lake, and thence along each side of the lake up to the rocky points which terminate the true mountain canyon above the head of the lake. I have never anywhere seen more perfectly defined moraines. I climbed over the larger western moraine and found that it is partly merged into the eastern moraine of Emerald Bay to form a medial at least 300 feet high, and of great breadth. From the surface of the little lake the curving branches of the main moraine, meeting below the lake to form a terminal moraine, are very distinct. At the head of the lake there is a perpendicular cliff over which the river precipitates itself, forming a very pretty cascade of 100 feet or more. On ascending the canyon above the head of the lake, for several miles, I found, everywhere, over the lip of the precipice, over the whole floor of the canyon, and up the sides 1000 feet or more, the most perfect glaciation.

There cannot, therefore, be the slightest doubt that this also is the pathway of a glacier which once ran into Lake Tahoe. After coming down its steep rocky bed, this glacier precipitated itself over the cliff, scooped out the lake at its foot, and then ran on until it bathed its snout in the waters of Lake Tahoe, and probably formed icebergs there. In its subsequent retreat it seems to have dropped more débris in its path and formed a more perfect terminal moraine than did Fallen Leaf Glacier.

Emerald Bay Glacier. All that I have said of Fallen Leaf Lake and Cascade Lake apply, almost word for word, to Emerald Bay. This beautiful bay, almost a lake, has also been formed by a glacier. It also is bounded on either side by moraines, which run down to and even project into Lake Tahoe, and may be traced up to the rocky points which form the mouth of the canyon at the head of the bay. Its eastern moraine, as already stated, is partly merged into the western moraine of Cascade Lake, to form a huge medial moraine. Its western moraine lies partly against a rocky ridge which runs down to Lake Tahoe to form Rubicon Point. At the head of the bay, as at the head of Cascade Lake, there is a cliff about 100 feet high, over which the river precipitates itself and forms a beautiful cascade. Over the lip of this cliff, and in the bed of the canyon above, and up the sides of the cliff-like walls, 1000 feet or more, the most perfect glaciation is found. The only difference between this glacier and the two preceding is, that it ran more deeply into the main lake and the deposits dropped in its retreat did not rise high enough to cut off its little rock basin from that lake, but exists now only as a shallow bar at the mouth of the bay. This bar consists of true moraine matter, i.e., intermingled bowlders and sand, which may be examined through the exquisitely transparent water almost as perfectly as if no water were present.

All that I have described separately and in detail, and much more, may be taken in at one view from the top of Mount Tallac. From this peak nearly the whole course of these three glaciers, their fountain amphitheaters, their canyon beds, and their lakes enclosed between their moraine arms, may be seen at once. The view from this peak is certainly one of the finest that I have ever seen. Less grand and diversified in mountain forms than many from peaks above the Yosemite, it has added beauty of extensive water surface, and the added interest of several glacial pathways in a limited space. The observer sits on the very edge of the fountain amphitheaters still holding large masses of snow; immediately below, almost at his feet, lie glistening, gem-like, in dark rocky setting, the three exquisite little lakes; on either side of these, embracing and protecting them, stretch out the moraine arms, reaching toward and directing the eye to the great Lake, which lies, map-like, with all its sinuous outlines perfectly distinct, even to its extreme northern end, twenty-five to thirty miles away. As the eye sweeps again up the canyon-beds, little lakes, glacier scooped rock basins, filled with ice-cold water, flash in the sunlight on every side. Twelve or fifteen of these may be seen.

From appropriate positions on the surface of Lake Tahoe, also, all the moraine ridges are beautifully seen at once, but the glacial lakes and the canyon-beds, of course, cannot be seen.

There are several questions of a general nature suggested by my examination of these three glacial pathways, which I have thought best to consider separately.

a. Evidences of the existence of the Great Lake Valley Glacier. On the south shore of Lake Tahoe, and especially at the northern or lower end of Fallen Leaf Lake, I found many pebbles and some large bowlders of a beautiful striped agate-like slate. The stripes consisted of alternate bands of black and translucent white, the latter weathering into milk-white, or yellowish, or reddish. It was perfectly evident that these fragments were brought down from the canyon above Fallen Leaf Lake. On ascending this canyon I easily found the parent rock of these pebbles and bowlders. the It is a powerful outcropping ledge of beautifully striped siliceous slate, full of fissures and joints, and easily broken into blocks of all sizes, crossing the canyon about a half mile above the lake. This rock is so peculiar and so easily identified that its fragments become an admirable index of the extent of the glacial transportation. I have, myself, traced these pebbles only a little way along the western shores of the great Lake, as my observations were principally confined to this part; but I learn from my brother, Professor John LeConte, and from Mr. John Muir, both of whom have examined the pebbles I have brought home, that precisely similar fragments are found in great abundance all along the western shore from Sugar Pine Point northward, and especially on the extreme northwestern shore nearly thirty miles from their source. I have visited the eastern shore of the Lake somewhat more extensively than the western, and nowhere did I see similar pebbles. Mr. Muir, who has walked around the Lake, tells me that they do not occur on the eastern shore. We have, then, in the distribution of these pebbles, demonstrative evidence of the fact that Fallen Leaf Lake glacier was once a tributary of a much greater glacier which filled Lake Tahoe.

The only other agency to which we could attribute this transportation is that of shore ice and icebergs, which probably did once exist on Lake Tahoe; but the limitation of the pebbles to the western, and especially the northwestern shores, is in exact accordance with the laws of glacial transportation, but contrary to those of floating ice transportation—for lake ice is carried only by winds, and would, therefore, deposit equally on all shores.

Again: I think I find additional evidence of a Lake Tahoe "mer de glace" in the contrasted character of the northern and southern shores of this Lake.

All the little glacial lakes described above are deep at the upper end and shallow at the lower end. Further, all of them have a sand beach and a sand flat at the upper end, and great bowlders thickly scattered in the shallow water, and along the shore at the lower end. These facts are easily explained, if we remember that while the glacial scooping was principally at the upper end, the glacial droppings were principally at the lower end. And further: that while the glacial deposit was principally at the lower end, the river deposit, since the glacial epoch, has been wholly at the upper end.

Now the great Lake, also, has a similar structure. It also has a beautiful sand and gravel beach all along its upper shore, and a sand flat extending above it; while at its lower, or northern end, thickly strewed in the shallow water, and along the shore line, and some distance above the shore line, are found in great abundance bowlders of enormous size. May we not conclude that similar effects have been produced by similar causes—that these huge bowlders were dropped by the great glacier at its lower end? Similar bowlders are also found along the northern portion of the eastern shore, because the principal flow of the ice-current was from the southwest, and in the fulness of glacial times the principal exit was over the northeastern lip of the basin.

b. Origin of Lake Tahoe. That Lake Tahoe was once wholly occupied by ice, I think, is certain; but that it was scooped out by the Lake Valley glacier is perhaps more doubtful. All other Sierra lakes which I have seen certainly owe their origin to glacial agency. Neither do I think we should be staggered by the size or enormous depth of this Lake. Yet, from its position, it may be a plication-hollow, or a trough produced by the formation of two parallel mountain ridges, and afterward modified by glacial agency, instead of a pure glacial-scooped rock-basin. In other words, Lake Valley, with its two summit ridges, may be regarded as a phenomenon belonging to the order of mountain-formation and not to the order of mountain sculpture. I believe an examination of the rocks of the two summit ridges would probably settle this. In the absence of more light than I now have, I will not hazard an opinion.[5]

c. Passage of slate into granite. From the commencement of the rocky canyon at the head of Fallen Leaf Lake, and up for about two miles, the canyon walls and bed are composed of slate. The slate, however, becomes more and more metamorphic as we go up, until it passes into what much resembles trap. In some places it looks like diorite and in others like porphyry. I saw no evidence, however, of any outburst. This latter rock passes somewhat more rapidly into granite at Glen Alpine Springs. From this point the canyon bed and lower walls are granite, but the highest peaks are still a dark, splintery, metamorphic slate. The glacial erosion has here cut through the slate and bitten deep into the underlying granite. The passage from slate through porphyritic diorite into granite may, I think, be best explained by the increasing degree of metamorphism, and at the same time a change of the original sediments at this point; granite being the last term of metamorphism of pure clays, or clayey sandstones, while bedded diorites are similarly formed from ferruginous and calcareous slates. Just at the junction of the harder and tougher granite with the softer and more jointed slates, occur, as might be expected, cascades in the river. It is probable that the cascades at the head of Cascade Lake and Emerald Bay mark, also, the junction of the granite with the slate—only the junction here is covered with débris. Just at the same junction, in Fallen Leaf Lake Canyon (Glen Alpine Basin), burst out the waters of Glen Alpine Springs, highly charged with bicarbonates of iron and soda.

[Footnote 5: This question practically has been settled by Mr. Lindgren, and his conclusions are given in an earlier chapter.]

d. Glacial Deltas. I have stated that the moraines of Cascade Lake and Emerald Bay glaciers run down to the margin of Lake Tahoe. An examination of this portion of the Lake shore shows that they run far into the Lake—that the Lake has been filled in, two or three miles, by glacial débris. On the eastern margin of Lake Tahoe, the water, close along the shore, is comparatively shallow, the shore rocky, and along the shore-line, above and below the water, are scattered great bowlders, probably dropped by the main glacier. But on the west margin of the Lake the shoreline is composed wholly of moraine matter, the water very deep close to shore, and the bottom composed of precisely similar moraine matter. In rowing along the shore, I found that the exquisite ultramarine blue of the deep water extends to within 100 to 150 feet of the shore-line. At this distance, the bottom could barely be seen. Judging from the experiments of my brother, Professor John Le Conte, according to which a white object could be seen at a depth of 115 feet, I suppose the depth along the line of junction of the ultramarine blue and the emerald green water is at least 100 feet. The slope of the bottom is, therefore, nearly, or quite, 45 degrees. It seems, in fact, a direct continuation beneath the water of the moraine slope. The materials, also, which may be examined with ease through the wonderfully transparent water, are exactly the same as that composing the moraine, viz: earth, pebbles, and bowlders of all sizes, some of them of enormous dimensions. It seems almost certain that the margin of the great Lake Valley glacier, and of the Lake itself when this glacier had melted and the tributaries first began to run into the Lake, was the series of rocky points at the head of the three little lakes, about three or four miles back from the present margin of the main Lake; and that all lakeward from these points has been filled in and made land by the action of the three glaciers described. At that time Rubicon Point was a rocky promontory, projecting far into the Lake, beyond which was another wide bay, which has been similarly filled in by débris brought down by glaciers north of this point. The long moraines of these glaciers are plainly visible from the Lake surface; but I have not examined them. Thus, all the land, for three or four miles back from the Lake-margin, both north and south of Rubicon Point, is composed of confluent glacial deltas, and on these deltas the moraine ridges are the natural levees of these ice-streams.

e. Parallel Moraines. The moraines described above are peculiar and almost unique. Nowhere, except about Lake Tahoe and near Lake Mono, have I seen moraines in the form of parallel ridges lying on a level plain and terminating abruptly without any signs of transverse connection (terminal moraine) at the lower end. Nor have I been able to find any description of similar moraines in other countries. They are not terminal moraines, for the glacial pathway is open below. They are not lateral moraines, for these are borne on the glacier itself, or else stranded on the deep canyon sides. Neither do I think moraines of this kind would be formed by a glacier emerging from a steep narrow canyon and running out on a level plain; for in such cases, as soon as the confinement of the bounding walls is removed, the ice stream spreads out into an ice lake. It does so as naturally and necessarily as does water under similar circumstances. The deposit would be nearly transverse to the direction of the motion, and, therefore, more or less crescentic. There must be something peculiar in the conditions under which these parallel ridges were formed. I believe the conditions were as described below.

We have already given reason to think that the original margin of the Lake, in glacial times, was three or four miles back from the present margin, along the series of rocky points against which the ridges abut; and that all the flat plain thence to the present margin is made land. If so, then it is evident that at that time the three glaciers described ran far out into the Lake, until reaching deep water, where they formed icebergs. Under these conditions, it is plain that the pressure on this, the subaqueous portion of the glacial bed, would be small, and become less and less until it becomes nothing at the point where the icebergs float away. The pressure on the bed being small, not enough to overcome the cohesion of ice, there would be no spreading. A glacier running down a steep narrow canyon and out into the deep water, and forming icebergs at its point, would maintain its slender, tongue-like form, and drop its débris on each side, forming parallel ridges, and would not form a terminal moraine because the materials not dropped previously would be carried off by icebergs. In the subsequent retreat of such a glacier, imperfect terminal moraines might be formed higher up, where the water is not deep enough to form icebergs. It is probable, too, that since the melting of the great "mer de glace" and the formation of the Lake, the level of the water has gone down considerably, by the deepening of the Truckee Canyon outlet by means of erosion. Thus not only did the glaciers retreat from the Lake, but also the Lake from the glaciers.

As already stated, similar parallel moraine ridges are formed by the glaciers which ran down the steep eastern slope of the Sierras, and out on the level plains of Mono. By far the most remarkable are those formed by Bloody most Canyon Glacier, described by me in a former paper. These moraines are six or seven miles long, 300 to 400 feet high, and the parallel crests not more than a mile asunder. There, also, as at Lake Tahoe, we find them terminating abruptly in the plain without any sign of terminal moraine. But higher up there are small, imperfect, transverse moraines, made during the subsequent retreat, behind which water has collected, forming lakes and marshes. But observe: these moraines are also in the vicinity of a great lake; and we have abundant evidence, in very distinct terraces described by Whitney[6] and observed by myself, that in glacial times the water stood at least six hundred feet above the present level. In fact, there can be no doubt that at that time the waters of Mono Lake (or a much greater body of water of which Mono is the remnant) washed against the bold rocky points from which the débris ridges start. The glaciers in this vicinity, therefore, must have run out into the water six or seven miles, and doubtless formed icebergs at their point, and, therefore, formed there no terminal moraine.

That the glaciers described about Lake Tahoe and Lake Mono ran out far into the water and formed icebergs I think is quite certain, and that parallel moraines open below are characteristic signs of such conditions I also think nearly certain.