The first of these tiltings occurred in that far age which geologists call the Cretaceous. It was inconsiderable, but enough to hasten the speed of the streams and establish general outlines for all time. About the middle of the Tertiary Period volcanic eruptions changed all things. Nearly all the valleys except the Yosemite became filled with lava. Even the crest of the range was buried a thousand feet in one place. This was followed by a rise of the Sierra Crest a couple of thousand feet, and of course a much sharper tilting of the western slopes. The Merced and Tenaya Rivers must have rushed very fast indeed during the many thousand years that followed. The most conservative estimate of the duration of the Tertiary Period is four or five million years, and until its close volcanic eruptions continued to fill valleys with lava, and the Great Basin kept settling, and the crest of the Sierra went on rising; and with each lifting of the crest, the tilt of the rivers sharpened and the speed of the torrents hastened. The canyon deepened during this time from seven hundred to a thousand feet. The Yosemite was then a mountain valley whose sloping sides were crossed by cascades. Then, about the beginning of the Quaternary Period, came the biggest convulsion of all. The crest of the Sierra was hoisted, according to Matthes's calculations, as much as eight thousand feet higher in this one series of movements, and the whole Sierra block was again tilted, this time, of course, enormously. For thousands of centuries following, the torrents from Lyell's and McClure's melting snows must have descended at a speed which tore boulders from their anchorages, ground rocks into sand, and savagely scraped and scooped the river beds. Armed with sharp hard-cutting tools ripped from the granite cirques of Sierra's crest, these mad rivers must have scratched and hewn deep and fast. And because certain valleys, including the Yosemite, were never filled with lava like the rest, these grew ever deeper with the centuries. The great crust movement of the Quaternary Period was not the last, by any means, though it was the last of great size. There were many small ones later. Several even have occurred within historic times. On March 26, 1872, a sudden earth movement left an escarpment twenty-five feet high at the foot of the range in Owens Valley. The village of Lone Pine was levelled by the accompanying earthquake. John Muir, who was in the Yosemite Valley at the time, describes in eloquent phrase the accompanying earthquake which was felt there. A small movement, doubtless of similar origin, started the San Francisco fire in 1906. Conditions created by the great Quaternary tilting deepened the valley from eighteen hundred feet at its lower end to twenty-four hundred feet at its upper end. It established what must have been an unusually interesting and impressive landscape, which suggested the modern aspect, but required completion by the glaciers.