Chapter 9 KING TOPOGRAPHY

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.

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 pre?minently 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.