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.