After the visitor has recovered from his first shock of astonishment—for
it is no less—at the beauty of the Valley, inevitably he wonders how
nature made it. How did it happen that walls so enormous rise so nearly
perpendicular from the level floor of the Valley?
When the Sierra Nevada was formed by the gradual tipping of a great
block of the earth’s crust 400 miles long and 80 miles wide, streams
draining this block were pitched very definitely toward the west and
with torrential force cut deep canyons. The period of tipping and stream
erosion covered so many thousands of centuries that the Merced River was
able to wear away the sedimentary rocks several thousand feet in
thickness, which covered the granite and then in the Yosemite Valley
region to cut some 2,000 feet into this very hard granite. Meantime the
north and south flowing side streams of the Merced, such as Yosemite
Creek, not benefited by the tipping of the Sierra block, could not cut
as fast as their parent stream and so were left high up as hanging
valleys.
During the Ice Age great glaciers formed at the crest of the range and
flowed down these streams, cutting deep canyons and especially widening
them. At the maximum period the ice came within 700 feet of the top of
Half Dome. It overrode Glacier Point and extended perhaps a mile below
El Portal. Glaciers deepened Yosemite Valley 500 feet at the lower end
and 1,500 feet opposite Glacier Point; then widened it 1,000 feet at the
lower end and 3,600 feet in the upper half. The V-shaped canyon which
had resulted from stream erosion was now changed to a U-shaped trough;
the Yosemite Cataract was changed to Yosemite Fall. As the last glacier
melted back from the Valley a lake was formed, the filling in of which
by sediments has produced the practically level floor now found from El
Capitan to Half Dome.
Visitors to the park should join an auto caravan to study evidences
first hand and hear the story of the geology of Yosemite discussed by
the ranger-naturalists.