Yosemite Natioanl Park: Geology
The story of Yosemite began about 500 million years ago when
the Sierra Nevada region lay beneath an ancient sea. Thick
layers of sediment lay on the sea bed, which eventually was
folded and twisted and thrust above sea level. Simultaneously
molten rock welled up from deep within the earth and cooled
slowly beneath the layers of sediment to form granite. Erosion
gradually wore away almost all the overlying rock and exposed
the granite. And even as uplifts continued to form the Sierra,
water and then glaciers went to work to carve the face of
Yosemite. Weathering and erosion continue to shape it today.
Tuolumne Meadows and the High Country - This section of Yosemite
has some of the most rugged sublime scenery in the Sierra.
In summer the meadows, lakes, and exposed granite slopes teem
with life. Because of the short growing season, the plants
and animals take maximum advantage of the warm days to grow,
reproduce, and store food for the long, cold winter ahead.
The Tioga Road (California 120), crosses this area. This scenic
highway, originally built as a mining road in 1882-83, was
realigned and modernized in 1961. The road passes through
an area of sparkling lakes, fragile meadows, domes, and lofty
peaks that only 10,000 years ago lay under glacial ice. Scenic
turnouts along the road afford superb views. At Tioga Pass
the road crosses the Sierra's crest at 9,945 feet, the highest
automobile pass in California.
Tuolumne Meadows (at 8,600 feet) is the largest sub-alpine
meadow in the Sierra. It is 55 miles from Yosemite Valley
via the Tioga Road. Long a focal point of summer activity,
it is also growing in popularity as a winter mountaineering
area. In the summer Tuolumne Meadows is a favorite starting
point for backpacking trips and day hikes. The meadows are
spectacular in early summer, abounding in wildflowers and
wildlife.
Giant Sequoia Groves - The Mariposa Grove, 35 miles south
of Yosemite Valley, is the largest of three Sequoia groves
in Yosemite. The Tuolumne and Merced Groves are near Crane
Flat. Despite human pressures, these towering trees, largest
of all living things, have endured for thousands of years.
Only in recent years, however, have we begun to understand
the Giant Sequoia environment. During the last 100 years protection
has sometimes been inadequate and sometimes excessive. For
example, in the late 1800s tunnels were cut through two trees
in the Mariposa Grove. Conversely, good intentions created
another problem, protection from fire has resulted in adverse
effects.
Sequoias are wonderfully adapted to fire. The wood and bark
are fire-resistant. Black scars on a number of large trees
that are still prospering indicate they have survived many
scorching fires. Sequoia reproduction also depends on fire.
The tiny seeds require minimal soil for germination, and seedlings
need sunlight. Historically, frequent natural fires opened
the forest, thinned out competing plant species, and left
rich mineral soil behind. But years of fire suppression have
allowed debris, such as fallen branches, to accumulate, stifling
reproduction and allowing shade-tolerant trees to encroach.
Prescribed fires, intended to simulate natural fires and improve
the health of the forest, are now set by the National Park
Service.
As you look at these trees, keep in mind that they have been
here since the beginning of history in the western world.
The Mariposa Grove's Grizzly Giant is 2,700 years old and
is thought to be the oldest of all Sequoias.
Yosemite Valley - "The Incomparable Valley", it
has been called, is probably the world's best known example
of a glacier-carved canyon. Its leaping waterfalls, towering
cliffs, rounded domes, and massive monoliths make it a preeminent
natural marvel. These attributes have inspired poets, painters,
photographers, and millions of visitors beginning with John
Muir for more than one hundred years. Nowhere in Yosemite
is the sense of scale so dramatic.
Yosemite Valley is characterized by sheer walls and a flat
floor. Its evolution began when alpine glaciers lumbered through
the canyon of the Merced River. The ice carved through weaker
sections of granite plucking and scouring rock but leaving
harder, more solid portions—such as El Capitan and Cathedral
Rocks—intact and greatly enlarging the canyon that the
Merced River had carved through successive uplifts of the
Sierra. Finally the glacier began to melt and the terminal
moraine left by the last glacial advance into the valley dammed
the melting water to form ancient Lake Yosemite, which sat
in the newly carved U-shaped valley. Sediment eventually filled
in the lake, forming the flat valley floor you see today.
This same process is now filling Mirror Lake at the base of
Half Dome.
In contrast to the valley's sheer walls, the Merced Canyon
along California 140 outside the park is a typical river-cut,
V-shaped canyon, for the glaciers did not extend this far.
Back from the rim of the valley itself, forested slopes show
some glacial polish. But for the most part these areas also
were not glaciated.
The valley is a mosaic of open meadows sprinkled with wildflowers
and flowering shrubs, oak woodlands, and mixed-conifer forests
of ponderosa pine, incense-cedar, and Douglas-fir. Wildlife
from monarch butterflies to mule deer and black boars flourishes
in these communities. Around the valley's perimeter, waterfalls,
which reach their maximum flow in May and June, crash to the
floor. Yosemite, Bridalveil, Vernal, Nevada, and Illilouette
are the most prominent of these falls, some of which have
little or no water from mid-August through early fall.
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