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.
[Illustration: Gilmore Lake, Pyramid Peak and the Crystal Range,
in winter, from summit of Mount Tallac]
[Illustration: Desolation Valley, Looking Toward Mosquito Pass]
[Illustration: Heather Lake, near Glen Alpine]
[Illustration: 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 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_.[1] 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 1: This is the name given by Dr. LeConte to the Basin in
which Lake Tahoe rests and including the meadow lands above Tallac.]
[Illustration: Pyramid Peak and Lake of the Woods, near Lake Tahoe,
Calif.]
[Illustration: Snow Bank, Desolation Valley, near Lake Tahoe]
[Illustration: Grass Lake, near Glen Alpine Springs]
_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[2];
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 2: 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.[3]
[Footnote 3: This question practically has been settled by Mr.
Lindgren, and his conclusions are given in an earlier chapter.]
_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.
_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[4] 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.
[Footnote 4: _Geological Survey of California_, Vol. I, 451.]
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.
_f. Glacial Erosion_. My observations on glacial pathways
in the High Sierra, and especially about Lake Tahoe, have
greatly modified my views as to the nature of glacial erosion.
Writers on this subject seem to regard glacial erosion as
mostly, if not wholly, a _grinding_ and _scoring_;
the débris of this erosion as rock-meal; the great bowlders,
which are found in such immense quantities in the terminal
deposit, as derived wholly from the crumbling cliffs above the
glacial surface; the _rounded_ bowlders, which are often
the most numerous, as derived in precisely the same way, only
they have been engulfed by crevasses, or between the sides of
the glacier and the bounding wall, and thus carried between
the moving ice and its rocky bed, as between
the upper and nether millstone. In a word, all bowlders,
whether angular or rounded, are supposed to owe their
_origin_ or _separation_ and _shaping_ to
glacial agency.
Now, if such be the true view of glacial erosion, evidently
its effect in mountain sculpture must be small indeed.
_Roches moutonnées_ are recognized by all as the most
universal and characteristic sign of a glacial bed. Sometimes
these beds are only imperfect _moutonnées_, i.e., they
are composed of _broken angular surface with only the points
and edges planed off_. Now, _moutonnées_ surfaces
always, and especially angular surfaces with only points and
edges beveled, show that the erosion by grinding has been only
very superficial. They show that if the usual view of glacial
erosion be correct, the great canyons, so far from being
_formed_, were only very _slightly modified_
by glacial agency. But I am quite satisfied from my
own observations, that this is not the only _nor the
principal_ mode of glacial erosion. I am convinced that
a glacier, by its enormous pressure and resistless onward
movement, is _constantly breaking off large blocks_ from
its bed and bounding walls. Its erosion is not only a grinding
and scoring, but also a _crushing and breaking_. It
makes by its erosion not only rock-meal, but also large
_rock-chips_. Thus, a glacier is constantly breaking off
blocks and making angular surfaces, and then grinding off the
angles both of the fragments and the bed, and thus forming
rounded bowlders and _moutonnées_ surfaces. Its erosion
is a constant process of alternate _rough hewing and
planing_. If the rock be full of fissures, and the glacier
deep and heavy, the rough hewing so predominates that the
plane has only time to touch the corners a little before the
rock is again broken and new angles formed. This is the case
high up on the _canyon walls_, at the head of Cascade
Lake and Emerald Bay, but also in the _canyon beds wherever
the slate is approached_. If, on the other hand, the rock
is very hard and solid, and the glacier be not very deep and
heavy, the planing will predominate over the rough hewing, and
a smooth, gentle billowy surface is the result. This is the
case in the hard granite forming the beds of all the canyons
high up, but especially high up the canyon of Fallen Leaf Lake
(Glen Alpine Basin), where the canyon spreads out and extensive
but comparatively thin snow sheets have been at work. In some
cases _on the cliffs_, subsequent disintegration of a
glacier-polished surface may have given the appearance of
angular surfaces with beveled corners; but, in other cases,
in the _bed of the canyon_, and on elevated level places,
where large loosened blocks could not be removed by water nor
by gravity, I observed the same appearances, under conditions
which forbid this explanation. Mr. Muir, also, in his
_Studies in the Sierra_, gives many examples of undoubted
rock-breaking by ancient glaciers.
_Angular_ blocks are mostly, therefore, the ruins of
crumbling cliffs, borne on the surface of the glacier and
deposited at its foot. Many _rounded_ bowlders also have
a similar origin, having found their way to the bed of the
glacier through crevasses, or along the sides of the glacier.
But _most of the rounded bowlders_ in the terminal
deposit of _great glaciers_ are fragments _torn off by
the glacier itself_. The proportion of rounded bowlders--of
upper or air-formed--to nether or glacier-formed fragments,
depends on the depth and extent of the ice-current. In the
case of the universal ice-sheet (ice-flood) there are, of
course, no upper formed or angular blocks at all--there is
nothing borne on the surface. The moraine, therefore, consists
wholly of nether-formed and nether-borne severely triturated
materials (_moraine profunde_). The bowlders are, of
course, all rounded. This is one extreme. In the case of the
thin moving ice-fields, the _glacierets_ which still
linger among the highest peaks and shadiest hollows of the
Sierra, on the other hand, the moraines are composed _wholly
of angular blocks_. This is the character of the terminal
moraine of Mount Lyell glacier. These glacierets are too thin
and feeble and torpid to break off fragments--they can
only _bear_ away what falls on them. This is the
other extreme. But in the case of ordinary
glaciers--ice-streams--the bowlders of the terminal deposit
are mixed; the angular or upper-formed predominating in the
small existing glaciers of temperate climates, but the rounded
or nether-formed greatly predominating in the grand old
glaciers of which we have been
speaking. In the terminal deposits of these, especially in the
materials pushed into the Lake, it is somewhat difficult
to find a bowlder which has not been subjected to severe
attrition.