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.
The general view from the summit consists of a sublime assemblage of
ice-born rocks and mountains, long wavering ridges, meadows, lakes, and
forest-covered moraines, hundreds of square miles of them. The lofty
summit-peaks rise grandly along the sky to the east, the gray pillared
slopes of the Hoffman Range toward the west, and a billowy sea of
shining rocks like the Monument, some of them almost as high and which
from their peculiar sculpture seem to be rolling westward in the middle
ground, something like breaking waves. Immediately beneath you are the
Big Tuolumne Meadows, smooth lawns with large breadths of woods on
either side, and watered by the young Tuolumne River, rushing cool and
clear from its many snow- and ice-fountains. Nearly all the upper part
of the basin of the Tuolumne Glacier is in sight, one of the greatest
and most influential of all the Sierra ice-rivers. Lavishly flooded by
many a noble affluent from the ice-laden flanks of Mounts Dana, Lyell,
McClure, Gibbs, Conness, it poured its majestic outflowing current full
against the end of the Hoffman Range, which divided and deflected it to
right and left, just as a river of water is divided against an island
in the middle of its channel. Two distinct glaciers were thus formed,
one of which flowed through the great Tuolumne Cañon and Hetch Hetchy
Valley, while the other swept upward in a deep current two miles wide
across the divide, five hundred feet high between the basins of the
Tuolumne and Merced, into the Tenaya Basin, and thence down through the
Tenaya Cañon and Yosemite.
The map-like distinctness and freshness of this glacial landscape
cannot fail to excite the attention of every beholder, no matter how
little of its scientific significance may be recognized. These bald,
westward-leaning rocks, with their rounded backs and shoulders toward
the glacier fountains of the summit-mountains, and their split, angular
fronts looking in the opposite direction, explain the tremendous
grinding force with which the ice-flood passed over them, and also the
direction of its flow. And the mountain peaks around the sides of the
upper general Tuolumne Basin, with their sharp unglaciated summits and
polished rounded sides, indicate the height to which the glaciers rose;
while the numerous moraines, curving and swaying in beautiful lines,
mark the boundaries of the main trunk and its tributaries as they
existed toward the close of the glacial winter. None of the commerical
highways of the land or sea, marked with buoys and lamps, fences, and
guide-boards, is so unmistakably indicated as are these broad, shining
trails of the vanished Tuolumne Glacier and its far-reaching
tributaries.
I should like now to offer some nearer views of a few characteristic
specimens of these wonderful old ice-streams, though it is not easy to
make a selection from so vast a system intimately inter-blended. The
main branches of the Merced Glacier are, perhaps, best suited to our
purpose, because their basins, full of telling inscriptions, are the
ones most attractive and accessible to the Yosemite visitors who like
to look beyond the valley walls. They number five, and may well be
called Yosemite glaciers, since they were the agents Nature used in
developing and fashioning the grand Valley. The names I have given them
are, beginning with the northern-most, Yosemite Creek, Hoffman, Tenaya,
South Lyell, and Illilouette Glaciers. These all converged in admirable
poise around from northeast to southeast, welded themselves together
into the main Yosemite Glacier, which, grinding gradually deeper, swept
down through the Valley, receiving small tributaries on its way from
the Indian, Sentinel, and Pohono Cañons; and at length flowed out of
the Valley, and on down the Range in a general westerly direction. At
the time that the tributaries mentioned above were well defined as to
their boundaries, the upper portion of the valley walls, and the
highest rocks about them, such as the Domes, the uppermost of the Three
Brothers and the Sentinel, rose above the surface of the ice. But
during the Valley’s earlier history, all its rocks, however lofty, were
buried beneath a continuous sheet, which swept on above and about them
like the wind, the upper portion of the current flowing steadily, while
the lower portion went mazing and swedging down in the crooked and
dome-blocked cañons toward the head of the Valley.
Every glacier of the Sierra fluctuated in width and depth and length,
and consequently in degree of individuality, down to the latest glacial
days. It must, therefore, be borne in mind that the following
description of the Yosemite glaciers applies only to their separate
condition, and to that phase of their separate condition that they
presented toward the close of the glacial period after most of their
work was finished, and all the more telling features of the Valley and
the adjacent region were brought into relief.
The comparatively level, many-fountained Yosemite Creek Glacier was
about fourteen miles in length by four or five in width, and from five
hundred to a thousand feet deep. Its principal tributaries, drawing
their sources from the northern spurs of the Hoffman Range, at first
pursued a westerly course; then, uniting with each other, and a series
of short affluents from the western rim of the basin, the trunk thus
formed swept around to the southward in a magnificent curve, and poured
its ice over the north wall of Yosemite in cascades about two miles
wide. This broad and comparatively shallow glacier formed a sort of
crawling, wrinkled ice-cloud, that gradually became more regular in
shape and river-like as it grew older. Encircling peaks began to
overshadow its highest fountains, rock islets rose here and there amid
its ebbing currents, and its picturesque banks, adorned with domes and
round-backed ridges, extended in massive grandeur down to the brink of
the Yosemite walls.
In the meantime the chief Hoffman tributaries, slowly receding to the
shelter of the shadows covering their fountains, continued to live and
work independently, spreading soil, deepening lake-basins and giving
finishing touches to the sculpture in general. At length these also
vanished, and the whole basin is now full of light. Forests flourish
luxuriantly upon its ample moraines, lakes and meadows shine and bloom
amid its polished domes, and a thousand gardens adorn the banks of its
streams.
It is to the great width and even slope of the Yosemite Creek Glacier
that we owe the unrivaled height and sheerness of the Yosemite Falls.
For had the positions of the ice-fountains and the structure of the
rocks been such as to cause down-thrusting concentration of the Glacier
as it approached the Valley, then, instead of a high vertical fall we
should have had a long slanting cascade, which after all would perhaps
have been as beautiful and interesting, if we only had a mind to see it
so.
The short, comparatively swift-flowing Hoffman Glacier, whose fountains
extend along the south slopes of the Hoffman Range, offered a striking
contrast to the one just described. The erosive energy of the latter
was diffused over a wide field of sunken, boulder-like domes and
ridges. The Hoffman Glacier, on the contrary moved right ahead on a
comparatively even surface, making descent of nearly five thousand feet
in five miles, steadily contracting and deepening its current, and
finally united with the Tenaya Glacier as one of its most influential
tributaries in the development and sculpture of the great Half Dome,
North Dome and the rocks adjacent to them about the head of the Valley.
The story of its death is not unlike that of its companion already
described, though the declivity of its channel, and its uniform
exposure to sun-heat prevented any considerable portion of its current
from becoming torpid, lingering only well up on the Mountain slopes to
finish their sculpture and encircle them with a zone of moraine soil
for forests and gardens. Nowhere in all this wonderful region will you
find more beautiful trees and shrubs and flowers covering the traces of
ice.
The rugged Tenaya Glacier wildly crevassed here and there above the
ridges it had to cross, instead of drawing its sources direct from the
summit of the Range, formed, as we have seen, one of the outlets of the
great Tuolumne Glacier, issuing from this noble fountain like a river
from a lake, two miles wide, about fourteen miles long, and from 1500
to 2000 feet deep.
In leaving the Tuolumne region it crossed over the divide, as mentioned
above, between the Tuolumne and Tenaya basins, making an ascent of five
hundred feet. Hence, after contracting its wide current and receiving a
strong affluent from the fountains about Cathedral Peak, it poured its
massive flood over the northeastern rim of its basin in splendid
cascades. Then, crushing heavily against the Clouds’ Rest Ridge, it
bore down upon the Yosemite domes with concentrated energy.
Toward the end of the ice period, while its Hoffman companion continued
to grind rock-meal for coming plants, the main trunk became torpid, and
vanished, exposing wide areas of rolling rock-waves and glistening
pavements, on whose channelless surface water ran wild and free. And
because the trunk vanished almost simultaneously throughout its whole
extent, no terminal moraines are found in its cañon channel; nor, since
its walls are, in most places, too steeply inclined to admit of the
deposition of moraine matter, do we find much of the two main laterals.
The lowest of its residual glaciers lingered beneath the shadow of the
Yosemite Half Dome; others along the base of Coliseum Peak above Lake
Tenaya and along the precipitous wall extending from the lake to the
Big Tuolumne Meadows. The latter, on account of the uniformity and
continuity of their protecting shadows, formed moraines of considerable
length and regularity that are liable to be mistaken for portions of
the left lateral of the Tuolumne tributary glacier.
Spend all the time you can spare or steal on the tracks of this grand
old glacier, charmed and enchanted by its magnificent cañon, lakes and
cascades and resplendent glacier pavements.
The Nevada Glacier was longer and more symmetrical than the last, and
the only one of the Merced system whose sources extended directly back
to the main summits on the axis of the Range. Its numerous fountains
were ranged side by side in three series, at an elevation of from
10,000 to 12,000 feet above the sea. The first, on the right side of
the basin, extended from the Matterhorn to Cathedral Peak; that on the
left through the Merced group, and these two parallel series were
united by a third that extended around the head of the basin in a
direction at right angles to the others.
The three ranges of high peaks and ridges that supplied the snow for
these fountains, together with the Clouds’ Rest Ridge, nearly inclose a
rectangular basin, that was filled with a massive sea of ice, leaving
an outlet toward the west through which flowed the main trunk glacier,
three-fourths of a mile to a mile and a half wide, fifteen miles long,
and from 1000 to 1500 feet deep, and entered Yosemite between the Half
Dome and Mount Starr King.
Could we have visited Yosemite Valley at this period of its history, we
should have found its ice cascades vastly more glorious than their tiny
water representatives of the present day. One of the grandest of these
was formed by that portion of the Nevada Glacier that poured over the
shoulder of the Half Dome.
This glacier, as a whole, resembled an oak, with a gnarled swelling
base and wide-spreading branches. Picturesque rocks of every
conceivable form adorned its banks, among which glided the numerous
tributaries, mottled with black and red and gray boulders, from the
fountain peaks, while ever and anon, as the deliberate centuries passed
away, dome after dome raised its burnished crown above the ice-flood to
enrich the slowly opening landscapes.
The principal moraines occur in short irregular sections along the
sides of the cañons, their fragmentary condition being due to
interruptions caused by portions of the sides of the cañon walls being
too steep for moraine matter to lie on, and to down-sweeping torrents
and avalanches. The left lateral of the trunk may be traced about five
miles from the mouth of the first main tributary to the Illilouette
Cañon. The corresponding section of the right lateral, extending from
Cathedral tributary to the Half Dome, is more complete because of the
more favorable character of the north side of the cañon. A short
side-glacier came in against it from the slopes of Clouds’ Rest; but
being fully exposed to the sun, it was melted long before the main
trunk, allowing the latter to deposit this portion of its moraine
undisturbed. Some conception of the size and appearance of this fine
moraine may be gained by following the Clouds’ Rest trail from
Yosemite, which crosses it obliquely and conducts past several sections
made by streams. Slate boulders may be seen that must have come from
the Lyell group, twelve miles distant. But the bulk of the moraine is
composed of porphyritic granite derived from Feldspar and Cathedral
Valleys.
On the sides of the moraines we find a series of terraces, indicating
fluctuations in the level of the glacier, caused by variations of
snow-fall, temperature, etc., showing that the climate of the glacial
period was diversified by cycles of milder or stormier seasons similar
to those of post-glacial time.
After the depth of the main trunk diminished to about five hundred
feet, the greater portion became torpid, as is shown by the moraines,
and lay dying in its crooked channel like a wounded snake, maintaining
for a time a feeble squirming motion in places of exceptional depth, or
where the bottom of the cañon was more steeply inclined. The numerous
fountain-wombs, however, continued fruitful long after the trunk had
vanished, giving rise to an imposing array of short residual glaciers,
extending around the rim of the general basin a distance of nearly
twenty-four miles. Most of these have but recently succumbed to the new
climate, dying in turn as determined by elevation, size, and exposure,
leaving only a few feeble survivors beneath the coolest shadows, which
are now slowly completing the sculpture of one of the noblest of the
Yosemite basins.
The comparatively shallow glacier that at this time filled the
Illilouette Basin, though once far from shallow, more resembled a lake
than a river of ice, being nearly half as wide as it was long. Its
greatest length was about ten miles, and its depth perhaps nowhere much
exceeded 1000 feet. Its chief fountains, ranged along the west side of
the Merced group, at an elevation of about 10,000 feet, gave birth to
fine tributaries that flowed in a westerly direction, and united in the
center of the basin. The broad trunk at first poured northwestward,
then curved to the northward, deflected by the lofty wall forming its
western bank, and finally united with the grand Yosemite trunk,
opposite Glacier Point.
All the phenomena relating to glacial action in this basin are
remarkably simple and orderly, on account of the sheltered positions
occupied by its ice-fountains, with reference to the disturbing effects
of larger glaciers from the axis of the main Range earlier in the
period. From the eastern base of the Starr King cone you may obtain a
fine view of the principal moraines sweeping grandly out into the
middle of the basin from the shoulders of the peaks, between which the
ice-fountains lay. The right lateral of the tributary, which took its
rise between Red and Merced Mountains, measures two hundred and fifty
feet in height at its upper extremity, and displays three well-defined
terraces, similar to those of the south Lyell Glacier. The comparative
smoothness of the upper-most terrace shows that it is considerably more
ancient than the others, many of the boulders of which it is composed
having crumbled. A few miles to the westward, this moraine has an
average slope of twenty-seven degrees, and an elevation above the
bottom of the channel of six hundred and sixty feet. Near the middle of
the main basin, just where the regularly formed medial and lateral
moraines flatten out and disappear, there is a remarkably smooth field
of gravel, planted with arctostaphylos, that looks at the distance of a
mile like a delightful meadow. Stream sections show the gravel deposit
to be composed of the same material as the moraines, but finer, and
more water-worn from the action of converging torrents issuing from the
tributary glaciers after the trunk was melted. The southern boundary of
the basin is a strikingly perfect wall, gray on the top, and white down
the sides and at the base with snow, in which many a crystal brook
takes rise. The northern boundary is made up of smooth undulating
masses of gray granite, that lift here and there into beautiful domes
of which the Starr King cluster is the finest, while on the east tower
of the majestic fountain-peaks with wide cañons and neve amphitheaters
between them, whose variegated rocks show out gloriously against the
sky.
The ice-plows of this charming basin, ranged side by side in orderly
gangs, furrowed the rocks with admirable uniformity, producing
irrigating channels for a brood of wild streams, and abundance of rich
soil adapted to every requirement of garden and grove. No other section
of the Yosemite uplands is in so perfect a state of glacial
cultivation. Its domes and peaks, and swelling rock-waves, however
majestic in themselves, and yet submissively subordinate to the garden
center. The other basins we have been describing are combinations of
sculptured rocks, embellished with gardens and groves; the Illilouette
is one grand garden and forest, embellished with rocks, each of the
five beautiful in its own way, and all as harmoniously related as are
the five petals of a flower. After uniting in the Yosemite Valley, and
expending the down-thrusting energy derived from their combined weight
and the declivity of their channels, the grand trunk flowed on through
and out of the Valley. In effecting its exit a considerable ascent was
made, traces of which may still be seen on the abraded rocks at the
lower end of the Valley, while the direction pursued after leaving the
Valley is surely indicated by the immense lateral moraines extending
from the ends of the walls at an elevation of from 1500 to 1800 feet.
The right lateral moraine was disturbed by a large tributary glacier
that occupied the basin of Cascade Creek, causing considerable
complication in its structure. The left is simple in form for several
miles of its length, or to the point where a tributary came in from the
southeast. But both are greatly obscured by the forests and underbrush
growing upon them, and by the denuding action of rains and melting
snows, etc. It is, therefore, the less to be wondered at that these
moraines, made up of material derived from the distant
fountain-mountains, and from the Valley itself, were not sooner
recognized.
The ancient glacier systems of the Tuolumne, San Joaquin, Kern, and
Kings River Basins were developed on a still grander scale and are so
replete with interest that the most sketchy outline descriptions of
each, with the works they have accomplished would fill many a volume.
Therefore I can do but little more than invite everybody who is free to
go and see for himself.
The action of flowing ice, whether in the form of river-like glaciers
or broad mantles, especially the part it played in sculpturing the
earth, is as yet but little understood. Water rivers work openly where
people dwell, and so does the rain, and the sea, thundering on all the
shores of the world; and the universal ocean of air, though invisible,
speaks aloud in a thousand voices, and explains its modes of working
and its power. But glaciers, back in their white solitudes, work apart
from men, exerting their tremendous energies in silence and darkness.
Outspread, spirit-like, they brood above the predestined landscapes,
work on unwearied through immeasurable ages, until, in the fullness of
time, the mountains and valleys are brought forth, channels furrowed
for rivers, basins made for lakes and meadows, and arms of the sea,
soils spread for forests and fields; then they shrink and vanish like
summer clouds.