f 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