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."