Central California - May/June 2009

Yosemite National Park - Yosemite Valley


A shaded relief map of Yosemite Valley


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.



Yosemite Valley From Tunnel Viewpoint

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).

A shot along the eastern extreme of the park’s road network.

A shot of Half Dome taken from the valley floor

A view of Half Done from Glacier Point

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.

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.

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!

Another shot of some rapids along the Merced River.


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.


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.


San Francisco

Yosemite Valley

Yosemite Waterfalls

Hetch Hetchy Reservoir

Tioga Pass Road 1

Tioga Pass Road 2

Mammoth Lakes – Fire…

Mammoth Lakes –  ... and Ice

South on US395

Moonrise at the Golden Gate


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All images copyright ©2009 by Al Hann

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