The question of Yosemite's making has busied geologists from Professor Whitney of the University of California, who first studied the problem, down to F.E. Matthes, of the United States Geological Survey, whose recent exhaustive studies have furnished the final solution. Professor Whitney maintained that glaciers never had entered the valley; he did not even consider water erosion. At one time he held that the valley was simply a cleft or rent in the earth's crust. At another time he imagined it formed by the sudden dropping back of a large block in the course of the convulsions that resulted in the uplift of the Sierra Nevada. Galen Clark, following him, carried on his idea of an origin by force. Instead of the walls being cleft apart, however, he imagined the explosion of close-set domes of molten rock the riving power, but conceived that ice and water erosion finished the job. With Clarence King the theory of glacial origin began its long career. John Muir carried this theory to its extreme. Since the period of Muir's speculations, the tremendous facts concerning the part played by erosion in the modification of the earth's surface strata have been developed. Beginning with W.H. Turner, a group of Yosemite students under the modern influence worked upon the theory of the stream-cut valley modified by glaciers. The United States Geological Survey then entered the field, and Matthes's minute investigations followed; the manuscript of his monograph has helped me reconstruct the dramatic past. The fact is that the Yosemite Valley was cut from the solid granite nearly to its present depth by the Merced River; before the glaciers arrived, the river-cut valley was twenty-four hundred feet deep opposite El Capitan, and three thousand feet deep opposite Eagle Peak. The valley was then V-shaped, and the present waterfalls were cascades; those which are now the Yosemite Falls were eighteen hundred feet deep, and those of Sentinel Creek were two thousand feet deep. All this in pre-glacial times. Later on the glaciers of several successive epochs greatly widened the valley, and measurably deepened it, making it U-shaped. The cascades then became waterfalls. But none will see the Yosemite Valley and its cavernous tributary canyons without sympathizing a little with the early geologists. It is difficult to imagine a gash so tremendous cut into solid granite by anything short of force. One can think of it gouged by massive glaciers, but to imagine it cut by water is at first inconceivable.