Chapter 11
The Ancient Yosemite Glaciers:
How the Valley Was Formed
All California has been glaciated, the low
plains and valleys as well
as the mountains. Traces of an ice-sheet, thousands of feet in
thickness,
beneath whose heavy folds the present landscapes have been molded, may
be found everywhere, though glaciers now exist only among the peaks of
the High Sierra. No other mountain chain on this or any other of the
continents that I have seen is so rich as the Sierra in bold, striking,
well-preserved glacial monuments. Indeed, every feature is more or
less tellingly glacial. Not a peak, ridge, dome, cañon,
yosemite,
lake-basin, stream or forest will you see that does not in some way
explain the past existence and modes of action of flowing, grinding,
sculpturing, soil-making, scenery-making ice. For, notwithstanding the
post-glacial agents--the air, rain, snow, frost, river, avalanche,
etc.--have been at work upon the greater portion of the Range for tens
of thousands of stormy years, each engraving its own characters more
and more deeply over those of the ice, the latter are so enduring and
so heavily emphasized, they still rise in sublime relief, clear and
legible, through every after-inscription. The landscapes of North
Greenland, Antarctica, and some of those of our own Alaska, are still
being fashioned beneath a slow-crawling mantle of ice, from a quarter
of a mile to probably more than a mile in thickness, presenting noble
illustrations of the ancient condition of California, when its sublime
scenery lay hidden in process of formation. On the Himalaya, the
mountains of Norway and Switzerland, the Caucasus, and on most of those
of Alaska, their ice-mantle has been melted down into separate glaciers
that flow river-like through the valleys, illustrating a similar past
condition in the Sierra, when every cañon and valley was the
channel
of an ice-stream, all of which may be easily traced back to their
fountains, where some sixty-five or seventy of their topmost residual
branches still linger beneath protecting mountain shadows.
The change from one to another of those
glacial conditions was slow as
we count time. When the great cycle of snow years, called the Glacial
Period, was nearly complete in California, the ice-mantle, wasting from
season to season faster than it was renewed, began to withdraw from the
lowlands and gradually became shallower everywhere. Then the highest
of the Sierra domes and dividing ridges, containing distinct glaciers
between them, began to appear above the icy sea. These first river-like
glaciers remained united in one continuous sheet toward the summit of
the Range for many centuries. But as the snow-fall diminished, and the
climate became milder, this upper part of the ice-sheet was also in
turn separated into smaller distinct glaciers, and these again into
still smaller ones, while at the same time all were growing shorter and
shallower, though fluctuations of the climate now and then occurred
that brought their receding ends to a standstill, or even enabled them
to advance for a few tens or hundreds of years.
Meanwhile, hardy, home-seeking plants and
animals, after long waiting,
flocked to their appointed places, pushing bravely on higher and
higher,
along every sun-warmed slope, closely following the retreating ice,
which, like shreds of summer clouds, at length vanished from the
new-born mountains, leaving them in all their main, telling features
nearly as we find them now.
Tracing the ways of glaciers, learning how
Nature sculptures
mountain-waves in making scenery-beauty that so mysteriously influences
every human being, is glorious work.
The most striking and attractive of the
glacial phenomena in the upper
Yosemite region are the polished glacier pavements, because they are so
beautiful, and their beauty is of so rare a kind, so unlike any portion
of the loose, deeply weathered lowlands where people make homes and
earn
their bread. They are simply flat or gently undulating areas of hard
resisting granite, which present the unchanged surface upon which with
enormous pressure the ancient glaciers flowed. They are found in most
perfect condition in the subalpine region, at an elevation of from
eight
thousand to nine thousand feet. Some are miles in extent, only slightly
interrupted by spots that have given way to the weather, while the best
preserved portions reflect the sunbeams like calm water or glass, and
shine as if polished afresh every day, notwithstanding they have been
exposed to corroding rains, dew, frost, and snow measureless thousands
of years.
The attention of wandering hunters and
prospectors, who see so many
mountain wonders, is seldom commanded by other glacial phenomena,
moraines however regular and artificial-looking, cañons however
deep
or strangely modeled, rocks however high; but when they come to these
shining pavements they stop and stare in wondering admiration, kneel
again and again to examine the brightest spots, and try hard to account
for their mysterious shining smoothness. They may have seen the winter
avalanches of snow descending in awful majesty through the woods,
scouring the rocks and sweeping away like weeds the trees that stood
in their way, but conclude that this cannot be the work of avalanches,
because the scratches and fine polished strife show that the agent,
whatever it was, moved along the sides of high rocks and ridges and up
over the tops of them as well as down their slopes. Neither can they
see
how water may possibly have been the agent, for they find the same
strange polish upon ridges and domes thousands of feet above the reach
of any conceivable flood. Of all the agents of whose work they know
anything, only the wind seems capable of moving across the face of the
country in the directions indicated by the scratches and grooves. The
Indian name of Lake Tenaya is "Pyweak"--the lake of shining rocks. One
of the Yosemite tribe, Indian Tom, came to me and asked if I could tell
him what had made the Tenaya rocks so smooth. Even dogs and horses,
when
first led up the mountains, study geology to this extent that they gaze
wonderingly at the strange brightness of the ground and smell it, and
place their feet cautiously upon it as if afraid of falling or sinking.
In the production of this admirable hard
finish, the glaciers in many
places flowed with a pressure of more than a thousand tons to the
square
yard, planing down granite, slate, and quartz alike, and bringing out
the veins and crystals of the rocks with beautiful distinctness. Over
large areas below the sources of the Tuolumne and Merced the granite is
porphyritic; feldspar crystals in inch or two in length in many places
form the greater part of the rock, and these, when planed off level
with
the general surface, give rise to a beautiful mosaic on which the happy
sunbeams plash and glow in passionate enthusiasm. Here lie the
brightest
of all the Sierra landscapes. The Range both to the north and south of
this region was, perhaps, glaciated about as heavily, but because the
rocks are less resisting, their polished surfaces have mostly given way
to the weather, leaving only small imperfect patches. The lower
remnants
of the old glacial surface occur at an elevation of from 3000 to 5000
feet above the sea level, and twenty to thirty miles below the axis of
the Range. The short, steeply inclined cañons of the eastern
flank also
contain enduring, brilliantly striated and polished rocks, but these
are
less magnificent than those of the broad western flank.
One of the best general views of the
brightest and best of the Yosemite
park landscapes that every Yosemite tourist should see, is to be had
from the top of Fairview Dome, a lofty conoidal rock near Cathedral
Peak
that long ago I named the Tuolumne Glacier Monument, one of the most
striking and best preserved of the domes. Its burnished crown is about
1500 feet above the Tuolumne Meadows and 10,000 above the sea. At first
sight it seems inaccessible, though a good climber will find it may
be scaled on the south side. About half-way up you will find it so
steep that there is danger of slipping, but feldspar crystals, two or
three inches long, of which the rock is full, having offered greater
resistance to atmospheric erosion than the mass of the rock in which
they are imbedded, have been brought into slight relief in some places,
roughening the surface here and there, and affording helping footholds.
The summit is burnished and scored like the
sides and base, the
scratches and strife indicating that the mighty Tuolumne Glacier swept
over it as if it were only a mere boulder in the bottom of its channel.
The pressure it withstood must have been enormous. Had it been less
solidly built it would have been carried away, ground into moraine
fragments, like the adjacent rock in which it lay imbedded; for, great
as it is, it is only a hard residual knot like the Yosemite domes,
brought into relief by the removal of less resisting rock about it;
an illustration of the survival of the strongest and most favorably
situated.
Hardly less wonderful is the resistance it
has offered to the trying
mountain weather since first its crown rose above the icy sea. The
whole
quantity of post-glacial wear and tear it has suffered has not degraded
it a hundredth of an inch, as may readily be shown by the polished
portions of the surface. A few erratic boulders, nicely poised on its
crown, tell an interesting story. They came from the summit-peaks
twelve
miles away, drifting like chips on the frozen sea, and were stranded
here when the top of the monument merged from the ice, while their
companions, whose positions chanced to be above the slopes of the sides
where they could not find rest, were carried farther on by falling back
on the shallowing ice current.
NEXT...
|
SPONSORED LINKS
RELATED 