Chapter 11
The Ancient Yosemite Glaciers:
How the Valley Was Formed
Part 2
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.
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