The splendid scenic endowment of the Yosemite Valley, its stupendous
architecture and vast sculpturing, its natural landscape engineering,
are largely triumphs of the ice age. Many theories have been
advanced to account for the origin and the extraordinary features of
this valley, especial prominence being given to subsidence, uplift,
explosion, with earthquake modifications and influences of violent
cataclysmic nature. Stream erosion has been strongly urged. All these
theories attribute minor influences to one or more other factors.
[Illustration: HALF DOME, YOSEMITE]
The theory now generally accepted gives ice the leading part in the
scooping of the valley and the creation of its wondrous forms. There
is much evidence to support this conclusion. The ice theory is
championed by John Muir, by Clarence King, and by F. E. Matthes.
Matthes and Muir probably have made the most careful and exhaustive
studies of the geological history of the valley.
This famous depression is of varying width. Examination of its walls
shows that in the wider places it is composed of fissured rock that
was more readily carried away by the ice than the adjoining unfissured
rock-sections. These resisting unfissured places jut into the valley.
Erosion by ice probably was preceded and somewhat guided by stream
erosion. But this ice sculpture, the rock-forms and features wrought,
must have been determined in a marked measure by the rock-structure.
That is to say, the dense quality of the rock, the number and the
position of the cleavage joints, or their absence in the rock, were
factors that helped determine the rock-forms of Yosemite. Other
factors since the ice age have altered or modified this glacial
topography.
It is certain that a vast ice-stream poured over the walls and forced
through this valley. This is shown in the rock-groovings and perched
boulders high on the walls, and also by the massive moraine which dams
the outlet of the valley. It appears certain that this must have been
left when the ice vanished; and apparently it formed a lake that
filled the entire valley nearly to the height of the dam. The lake
finally filled with sediment and sand, its surface corresponding
approximately with the present surface of the valley. The valley floor
is noticeably smooth, and its margins along the bottoms of the walls
are comparatively free from rock-débris.
The landscape of the entire Yosemite National Park is preëminently
glacial. Ice-polished mountains and hundreds of sculptured figures of
vast size are a part of the matchless exhibit of the ice age in this
wonderland. Polished domes predominate. Much of the rock-surface was
dense granite comparatively free from cleavage lines, soft materials,
or stratification. The forms made by the ice in these have endured.
Since the ice age the softer and more fissured rocks have been far
more changed by the various erosive forces than the more resistant
rock of the domes and other sculptured forms.
Little Yosemite Valley is essentially similar to the Greater Yosemite
in features and also in the manner of creation. Its walls are from
fifteen hundred to two thousand feet high, its length is about three
miles, its width one half-mile. Its floor, like that of the Greater
Yosemite, was for a time a lake. In origin and history, the
Hetch-Hetchy Valley, too, is almost identical with the Yosemite.
Nature often changes the scene, often puts on a new landscape. The
forces of erosion are steadily at work; most of them work slowly, but
sometimes a change is wrought suddenly.
When the Sierra was first upheaved it was more or less tilted,
terraced, and fissured. The surface was uneven. The present topography
is the product of a long and complicated series of events. It has been
wrought out by many erosive forces. It probably has been acted upon by
two or more ice ages, but the last age shaped the splendid topography
of the Yosemite that is attracting the world to the scene.
The eroding power of ice is determined by its thickness, that is to
say, by its weight. The small, shallow glaciers wear much more slowly
than the deep ice-streams that bear heavily upon the surface passed
over. The ancient glaciers of the region took on vast proportions. An
enormous and deep ice-field accumulated from the snows of Mounts Dana,
Lyell, Gibbs, McClure, Conness, and other peaks. Flowing westward, it
came in contact with Mount Hoffman, against which it divided. The
right section flowed down into the Tuolumne; the left, a branch about
two miles wide, swept upward, climbing about five hundred feet over
the pass and descending upon the Lake Tenaya region.
Apparently, five glacier streams united in the Yosemite Valley. They
not only filled it but deeply overflowed the highest points on its
walls. Passing out of the lower end of the valley, the united glacier
was forced to climb upward several hundred feet.
About twenty-five small glaciers still remain in the Yosemite National
Park. There are about two hundred and fifty glacier lakes, mostly
small. Others have filled with sediment and are hidden and forgotten.
Lake Tenaya, the Lake-of-the-Shining-Rocks, has a surrounding of dense
rock-masses that still show the rounded form and the high polish given
by the ice.