Showing posts with label Sierra Vista Scenic Byway. Show all posts
Showing posts with label Sierra Vista Scenic Byway. Show all posts

Wednesday, February 9, 2011

The Other California: What do you do with a used forest?

Humans have cast a long shadow over Sierra National Forest (oh, sorry about literal metaphors).


The Sierra Nevada is a world-class mountain range. Scenic, in turns rugged and visitor-friendly, spectacular, destination of millions of travelers from around the planet, the mountains host one of the earth's most precious parks in Yosemite, and the only slightly lesser-known (but no less incredible) Sequoia/Kings Canyon National Park. Regular readers of this blog will certainly know of the explorations we have covered in those parks, but my Other California series is about those significant places that aren't quite so famous. They are the places that don't always show up on the postcards...

There is a huge swath of mountain territory between the south boundary of Yosemite, and the northern reaches of Kings Canyon. This is the drainage of the San Joaquin River, and to most travelers it is terra incognita, a blank spot on the map.

Structurally the Sierra Nevada is a 400-mile-long block of mostly granitic rock that is tilted towards the west, with a long gentle western slope and a very steep eastern flank. Most of the major rivers start at the crest and flow west through deep canyons to the Central Valley. Due to a geological glitch of sorts, the San Joaquin has its headwaters more or less east of what by all rights should be the Sierra Crest. The high rugged ridge of Mts. Ritter and Banner and the jagged Minarets extends south from Yosemite, and reaches elevations exceeding 13,000 feet. This would seem to be the crest, but the river flows in a deep canyon around the south end of the Minarets, and the actual Sierra Crest off to the east is a somewhat uninteresting ridge of volcanic rock that barely exceeds 10,000 feet in elevation. Mammoth Mountain (that of the ski area) is the high point on that part of the crest, and from the west it doesn't look like much. Even the John Muir Trail skips this part of the crest and winds along the lakes and ridges below Ritter, Banner and the Minarets. The headwaters of the San Joaquin is a spectacular area, and Devils Postpile National Monument draws many visitors. But that's not the region I want to talk about today.

The blank spot I speak of is downstream, west of the Minarets, and between the national parks. The land is administered by the Department of Agriculture as Sierra National Forest, and there aren't generally a vast number of tourists to be seen there. That's not to say that the land is not known. It is known quite well by some. It is a landscape that has been fully utilized under the doctrine of multiple-use, the operating principle used by the Forest Service to justify any money-making enterprise that anyone can think of. The vast conifer forests have been logged heavily, huge dams have been constructed, herds of cattle graze the meadows, and mining took place, especially in the metamorphic rocks near the Minarets. In land-use terms, this is a used-car lot of a natural environment.
So what do you do with a used forest? What do you do when the biggest trees have been cut down, and the mills shut down? The mines have been abandoned for years even though they were probably the reason the Minarets were left out of Yosemite's boundaries in the first place (although a desire to build another paved trans-Sierra highway might have been another motivation). It starts with the understanding that time is a healer. Trees are growing back, and logging has left a network of roads through the region, some paved, others not, but somebody got creative and linked a series of utility roads into a rather marvelous route that explores some very scenic and interesting country. The public relations people labeled it the Sierra Vista Scenic Byway. We explored the road last fall, and I can give it a thumbs-up as a great place to explore.
Much of the region is a densely forested plateau pockmarked by green meadows and the occasional granitic dome. The San Juaquin River has carved a deep gorge through the bedrock, which is mostly an intrusion of granitic rock called the Mt. Givens granodiorite. It is one of the largest single intrusions of the Sierra Nevada batholith, with a length of about 50 miles and a width of 10 to 20 miles. It was emplaced about 90 million years ago (Bateman, P., 1992, Plutonism in the Central Part of the Sierra Nevada Batholith, California; USGS Professional Paper 1483). Glaciers carved the rugged peaks of the Ritter Range, and extended across much of the area betwen the parks.
The whole loop is more than 70 miles, and is paved road for the most part. Several sections were once paved, but they sort of gave up trying about 40 years ago, so there are a few bumps here and there, but most cars will have no problems. The road is closed during the winter and spring until the snow melts. The Forest Service has a handy map and guide online here, and America's Byways has a guide here. A number of interpretive displays have been constructed, and there are a few chemical/pit toilets at the picnic stops, but don't look for much in the way of amenities or gas stations. Supplies can be had down the hill at North Fork, Bass Lake, and Oakhurst.
Mammoth Pool Reservoir (above) impounds the waters of the San Joaquin River. A side road provides access. A second side road near the southern end of the loop road visits the geographic center of California. The Eagle Peaks (below) are exfoliating slopes of granitic rock seen at the Mile High Overlook, one of the best view points on the road. The Overlook provides the spectacular views of the Minarets seen in the pictures above as well (photos courtesy of Susan Hayes; first photo is mine, standing in Jackass Meadow, appropriately enough). Globe Rock, a huge balanced boulder, was discussed in a previous post.
Check it out! In a few months, that is. I didn't get a chance to visit the Nelder Sequoia Grove on our last trip out, so we might run into each other this summer...

Thursday, October 14, 2010

Balancing Boulders and Erratic Rocks!

Sunday's mystery photo produced a pretty good response of hypotheses, both of the scientific sort, but also of the fanciful sort (and sometimes the boundary gets blurred!). The rock in question is known as Globe Rock, and it can be seen along the Sierra Vista Scenic Byway, an 85-mile long loop road in the high forest country between Yosemite and Kings Canyon National Parks. The area is drained by the San Joaquin River. The elevation is about 7,000 feet.
As noted on the geologic map by N. King Huber of the Shuteye Peak quadrangle (USGS Geological Quadrangle GQ-728; get all of the central Sierra Nevada geologic maps here), the rock underlying the Globe is the Mt. Givens granodiorite, a medium to coarse-grained hornblende-biotite granodiorite and granite that is locally porphyritic with tabular K-spar phenocrysts with a U-Pb age of about 90 million years (my casual non-geologists readers are forgiven for not following this, but my students had better understand it; there's a midterm Tuesday on this stuff!). The rock is labeled on the geologic map, near the top and to the left.
A lot of the answers to my challenge on Sunday noted the possibility that this was a glacial erratic sitting on a pedestal. This is the most likely answer based on my cursory examination, although the possibility must be considered that it may also be an unusually weathered exfoliation slab (in other words, the rock formed in place, not transported from elsewhere). There are glacial deposits shown on Huber's map only a mile or so away, and the larger pre-Tahoe glaciers may have covered much of the region. In either case, the pillar would have developed because the large boulder protected the underlying rock from weathering, while the surrounding rock was eroded away. In this setting, the amount of erosion suggests that the boulder has been there for a very long time. The odd spherical shape of the boulder itself would also be an indicator of age; corners and edges tend to be weathered away first.

Glacial erratics are very often composed of a different kind of rock, and the black coloring on the boulder certainly causes the rock to appear to have a different composition, but a close look at the base of the boulder (above) shows the rocks to be actually quite similar, and they are both certainly granitic. The dark color is provided by dark oxides and lichens growing on the surface. Even if the rocks are the same, it quite probably could still be an erratic, as the Mt. Givens granodiorite is widely exposed in the region.
Rocks on pedestals are quite interesting for a number of reasons, not the least of which is their contribution to the scenery. If they can be tied to a particular glacial episode, they can be a clue to the rate of erosion in a region. If they are truly precarious, they provide some data about the likelihood of severe shaking during earthquakes. If they haven't toppled in the thousands of years they have existed in a particular spot, it would suggest a limited degree of ground movement (although I have to wonder...this rock is close to major faults in the eastern Sierra Nevada).

The other rock I photographed this weekend illustrates what probably amounts to a younger pedestal boulder, on the flanks of Sentinel Dome in Yosemite National Park. It is less rounded, and has not developed as high a pedestal. This could be a function of age, although differences in composition and local soil conditions could also be a factor. This site is described as being outside the limits of the Sierra glaciations, so it is either a rockfall 'erratic', or it is part of a slab that formed in place.

Balanced rocks are found in many different circumstances and many different places. One of the commenters provided a link to a balanced boulder in Kansas, and I've included a shot of the famous Balanced Rock at Arches National Park in Utah.

What other balanced rocks do you know about? Got any pictures?