How perverse is Yosemite Valley?

Yosemite Valley in California in the Sierra Nevada was the very first piece of land laid aside by the federal government for use as a public park; it was the first place in the U.S. where the term “national park” was applied. (Some other day we can discuss the more peculiar situation at Hot Springs National Park, which had some land reserved even earlier). It remains one of the most popular parks.  Yet later John Muir tried to use Yosemite as a common noun, terming large trunk valleys that had been glaciated “yosemites.” This never took off, but was he right in contending that the other valleys were pretty much the same deal?

The short answer seems to be no, and the difference between Yosemite and those other valleys seems to be a big part of the reason that Yosemite is unique.  Consider how this was described by Frederick Law Olmsted:

There are falls of water elsewhere finer, there are more stupendous rocks, more beetling cliffs, there are deeper and more awful chasms, there may be as beautiful streams, as lovely meadows, there are larger trees. It is in no scene or scenes the charm consists, but in the miles of scenery where cliffs of awful height and rocks of vast magnitude and of varied and exquisite coloring, are banked and fringed and draped and shadowed by the tender foliage of noble and lovely trees and hushes, reflected from the most placid pools, and associated with the most tranquil meadows, the most playful streams, and every variety of soft and peaceful pastoral beauty.

This union of the deepest sublimity with the deepest beauty of nature, not in one feature or another, not in one part or one scene or another, not any landscape that can be framed by itself, but all around and wherever the visitor goes, constitutes the Yo Semite the greatest glory of nature.

What exactly is the crux of this?

About the only way to have the vertical cliffs and the flat, meandering river right against one another is to make the valley deeper and then fill it back up (there is another way peculiar to sedimentary rocks, as Zion Canyon kind of shows, but it isn’t quite as successful as the river usually still has a pretty decent gradient).  Basically to get the flat gentle river you need a low grade, which is not terribly conducive to creating a big steep cliff right next door.

What happened in Yosemite was that glacial action made a very deep hole–seismic work in the 1930s put it at 600m deep, which has been largely confirmed by water wells drilled in the valley. The bedrock under the Ahwahnee Hotel and Yosemite Village Parking is about halfway between the elevation of those features and sea level.  Because bedrock blocks the Merced River from cutting down more at the mouth of the valley, this was a closed basin and when the glacier that cut it retreated, it filled with water.  To get the modern valley, you had to fill that deep lake with muds and silt and boulders.  And while some of that came from the weaker parts of the valley walls (e.g., Indian Canyon area), most of it probably came it from the Merced River and maybe Tenaya Creek.

A small mystery here is that there are small lakes upstream in both drainages that are not filled with sediment.  This means one of two things: either the sediment that filled Yosemite Valley came from below those lakes (Tenaya Lake and Merced Lake, if you are keeping score), or those lakes were scoured out by glaciers that postdated the one(s) that exhumed the deep bedrock bowl under Yosemite Valley. Almost certainly it was the latter that was true: presumably the glaciers in the Merced and Tenaya Canyons terminated for a considerable time in the vicinity of the valley’s edges, dumping loads of material derived from the mountains to the east and refilling the valley to a nice even grade. Only after largely filling the valley could the glaciers retreat above the lakes, leaving them relatively free of fill.

By filling a glacial “U shaped” valley, the sloping bottom part of the U was buried and instead Yosemite in cross section resembles a boxcar. Most other Sierran “yosemites”–Kern Canyon, Cedar Grove on the South Fork of the Kings, Simpson Meadow on the Middle Fork of the Kings, the San Joaquin canyon, Hetch Hetchy–tend to still show that rounded bottom of the U-shape acquired from glacial action.  That juxtaposition of cliff and placid pools that Olmsted lauded is really more unique than perhaps even Muir appreciated.  Choosing Yosemite as the first federal withdrawal of land for a park wasn’t quite the happenstance it might seem; it really is a fairly unique place.

Of course, there is a deeper story yet in why such erosion occurred in this one rather small drainage of the Sierra, but that is a tale for another day.

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