Central California - May/June 2009
Yosemite National Park - Yosemite Valley
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A shaded
relief map of Yosemite Valley |
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The origins
of Yosemite Valley
begin about 200 million years ago (MYA). At this time the granites of the
Sierra Nevada began as rising plumes
of magma deep underground (technically called
“diapirs”). Over
time these diapirs slowly cooled and hardened underground forming huge masses
of granite. Then about 100 MYA the rocks were uplifted, and the surface
material covering them eroded away. Beginning about 3 MYA a series of “Ice
Ages” caused large glaciers to cyclically advance and retreat. It was these
glaciers that carved the deep valleys of the Sierra. During some of these
cycles, the glacier covered all but the highest peaks (e.g. El Capitan and
Half Dome). At other times the glacier was only as high as Bridalveil falls
(this includes the most recent Ice Age). That works out to a difference of at
least 1000ft. The term
“river of ice” is a very good description of a glacier. Over time, the ice
flows downhill carrying anything that happens to fall onto its surface. This
includes the rocks eroding off the High Sierra peaks. Some of the material is
carried to the face of the glacier, where it then falls off and creates a
pile of debris called a “moraine”.
If the glacier front is stable (i.e. neither retreating or advancing) these
moraines can reach hundreds of feet in height. One such moraine formed across
the valley at roughly the base of El Capitan. When the glacier retreated this
moraine formed a dam, and consequently a lake behind it. The flatness of the
valley floor is due to soil deposited as lake sediments. During
inter-glacial times, the Merced
River and its ancestors drained the valley. As the river meandered in its
floodplain it undermined points along the valley walls. This action helped
create the near vertical cliffs in parts of the valley. |
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Looking east into Yosemite Valley from “Tunnel View”. The tall vertical cliff on the left is El Capitan. This granite
monolith rises 3000ft above the valley floor. The waterfall right of center is Bridalveil
Falls (more shots of these falls later). |
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A shot along the eastern extreme of the park’s
road network. |
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A shot of Half Dome taken from the
valley floor |
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A view of Half Done from Glacier Point |
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The Merced River (foreground) and Bridalveil
Falls (background, left of center). Note the “Hanging
Valley” that Bridalveil Falls emerge from. Glaciers, just like rivers,
can have tributaries. As the larger glacier cut Yosemite Valley (flowing left
to right in this shot) a smaller tributary glacier cut a shallower valley in
the rock wall. When the glaciers retreated the shallow valley appears to be
suspended above the main vally. |
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Looking west at the Merced River leaving
Yosemite Valley. Here the valley constricts, and the river becomes a
white-water torrent. Note the road at the bottom of the image. This
is El Portal Rd (CA140).
The road parallels the river for about 20mi. |
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A beautiful spot along El Portal Rd (CA120).
This shot was taken while standing on a huge boulder that juts out into the
river. While relaxing here, I could feel the eddies of cold air flowing off
the water’s surface. Since the vast majority of this water is snow melt
runoff, it is extremely cold! |
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Another shot of some rapids along the Merced
River. |
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Here we see part a rockslide that buried about
600ft of CA140 in 2006. Since
stopping is not allowed along this stretch of the road, this shot was taken without
the viewfinder while driving along at about 10mph. Known as the “Ferguson Slide”, a USGS report (PDF)
on the slide can be found here.
While the text is a little dry, the last 3 pages have interesting aerial
views/diagrams. |
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One of the two temporary bridges built for the
detour around the slide. This detour is along a one-lane one-way road.
Traffic lights at either end control access. This shot was taken at the west
end of the detour, while waiting for the 10~15min cycle of the traffic light. |
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Yosemite
Valley |
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All images copyright ©2009 by Al Hann All rights reserved |
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