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 ma.s.s of water in the lake rhythmically swings from sh.o.r.e to sh.o.r.e. And, moreover, he shows that the water oscillates according to the two princ.i.p.al 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 sh.o.r.es 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 ma.s.s 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 sh.o.r.es 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 sh.o.r.es.
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 sh.o.r.e, 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 circ.u.mstance 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.[3]
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 3: 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 ma.s.ses 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 Aga.s.siz 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 pa.s.sage 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 ma.s.sive 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 ma.s.s. 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 quant.i.ties 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 ma.s.ses consist of breccias containing numerous d.y.k.es and necks of ma.s.sive 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 lat.i.tude 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 k.n.o.b, Mount Mildred and Twin Peak. The andesite ma.s.ses 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 ma.s.ses, fresh from the factory of G.o.d. There was not a tree, not a shrub, not a flower, not a blade of gra.s.s. 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.
[Ill.u.s.tration: Gilmore Lake, Pyramid Peak and the Crystal Range, in winter, from summit of Mount Tallac]
[Ill.u.s.tration: Desolation Valley, Looking Toward Mosquito Pa.s.s]
[Ill.u.s.tration: Heather Lake, near Glen Alpine]
[Ill.u.s.tration: Susie Lake, near Glen Alpine Springs]
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 alt.i.tudes, 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 b.u.t.tressed 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 ma.s.ses 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-ma.s.ses also have created a number of _cirques_, where remnants of the winter"s snow ma.s.ses are yet to be seen. These snow ma.s.ses 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 acc.u.mulated and acc.u.mulating 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 a.s.sumed 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 ma.s.ses, what the geologists term _neve_. This is a granular snow, intermediate between snow and ice. A little lower down this _neve_ is converted into true glacial ice-beds, which grow longer, broader, deeper and thicker as the _neve_ 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 ma.s.ses, pressing down upon the hundreds of thousands of tons of ice that lie below. Slowly the ma.s.s 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 ma.s.ses of the mountain summits. Some of these have weathered into toppling ma.s.ses, 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.