One Hundred Hikes in Yosemite

Evolution of the Yosemite Landscape — Sierra Nevada Uplift

In 1990, after four months of field work examining evidence of past glaciers in the Stanislaus River drainage, I realized that the giant glaciers there-as large as the ones in Yosemite Valley-had barely eroded the granitic landscape. Along the North Fork Stanislaus River, past glaciers had cut, at best, a narrow stream channel just a mere 30 feet below the base of a remnant of a 23-million-year-old rhyolite deposit. This observation conflicted with the widely held view that glaciers in the Sierra Nevada (and elsewhere around the world) had performed tremendous erosion. Furthermore, this deposit and other old, dated deposits just above the floors of other canyons suggested that they had been almost as deep as they are today. For example, just above Hetch Hetchy Reservoir about 1/4 mile west of Rancheria Falls, there are small volcanic remnants at 4500 feet elevation. These indicate that the ancient Tuolumne River canyon was already about 3000 feet deep. But because these remnants lie on slopes, they do not represent the ancient canyon's floor, only its lower slopes. The floor would have been lower, and the ancient canyon would have been almost as deep as the modern one. In short, the range's major canyons, including the Merced River's Yosemite Valley, have been deep for 20+ million years.

This conclusion posed a problem with regard to uplift. Over the last 20+ million years, geoscientists believed that the Sierra Nevada had been tilted westward, raising crest lands in our vicinity by about 11,000 feet. Because it was tilted as a rigid block, a point one fourth up from its base would have been raised one fourth this amount, or about 2750 feet; a point one-half up from its base would have been raised one half, or about 5500 feet; and so on. Today the canyon floors of the Tuolumne and Merced rivers at the halfway point are about 2000 feet elevation. But since some 5500 feet of uplift supposedly has occurred at that point, the canyon floors initially must have been about 3500 below sea level-an impossibility. (Geoscientists got around this by assuming that over this period the canyons had been deepened by thousands of feet.)

In 1990, when I was reaching my conclusions, Peter Molnar of M.I.T. wrote an article for the Irish Journal of Earth Sciences entitled"The Rise of Mountain Ranges and the Evolution of Humans: a Causal Relation?" It proposed that in the last few million years the world's mountain ranges rose exponentially faster, which caused human brains to evolve exponentially larger. The article was a spoof, written because he was fed up with so many geoscientists asserting that their favorite mountain range had risen in Quaternary time (about the last two million years). The Sierra Nevada is no exception. As I document in my "Riddles" book, there have been many attempts to prove major uplift in the last few million years, and not one of them stands up to scrutiny. In that book I also offer three field methods-not artificial constructs-to quantify this uplift, and each indicates no measurable uplift for tens of millions of years.

So far as I can determine, the Quaternary-uplift myth of the world's glaciated ranges originated in the 1850s or '60s. Early geologists logically had realized that glaciers in these ranges developed around the beginning of the Quaternary. Therefore, the ranges must have risen just before then, achieving sufficient height by the early Quaternary to generate mammoth glaciers. Because the ranges must have been low before this uplift, their canyons would have been shallow. Hence today's deep canyons must have been excavated first by invigorated mountain rivers and then perhaps even more by the ensuing glaciers.

Earlier I said that about 80 million years ago an oceanic plate greatly increased its velocity and shallowed its angle of diving, and that where extension is extreme the base of the upper crust will separate from the top of the lower crust along a detachment fault. As the oceanic plate beneath the Sierra Nevada greatly increased its velocity and shallowed its angle of diving, it would have imparted severe stresses on the crust. Through extension the lower crust would have thinned as it stretched laterally, and a detachment fault would have developed between the lower and upper crust. In perhaps only a few million years-this figure based on rates of detachment-fault movements elsewhere-the upper crust would have been removed from the Sierra Nevada, exposing mainly the upper parts of plutons but also some lower remnants of metamorphic terranes. Where did the range's upper crust go? I suggest that it was faulted west with respect to the lower crust, and then was transported northward as one or more terranes along a right-lateral fault system. We know that in the past other terranes originating south of California were transported on such faults to northern lands.

With the removal of the upper crust, the lower crust of the Sierran block would have risen, although not to its original height, which is why the pre-faulted range should have stood higher. The Sierran crustal block, supported by underlying mantle, is similar to a block of wood floating in water. This may float, say, about 1/2 inch above water, and if you saw off the part protruding above water, the block will rise, due to the removal of the weight of the upper 1/2 inch. However, it will rise less than 1/2 inch. An ice cube melting in water is another useful analog. Be aware that since the raised lower crust no longer had the weight of the upper crust upon it, it now experienced less pressures and lower temperatures and so became brittle, capable of fracture.

Conventional wisdom requires that Sierran uplift is greatest where the crust is thickest. Consequently, because elevations are greatest along the crest, the crust should be thickest beneath it and should thin westward. However, 1990s seismic studies show that the crust thickens westward, and in our vicinity it is thickest-about 25 miles-near the western boundary of Yosemite National Park. Therefore, by conventional wisdom, uplift should have been greatest there, producing the highest elevations near El Portal. In 1997 David Jones suggested that uplift actually was greater in the western part of the range than at the crest, about 10 miles versus 4. This conclusion is based on the mineralogy of granitic rocks exposed at the surface. Each mineral forms at certain pressures and temperatures, so by knowing the minerals now exposed at the surface, we can estimate the original depth at which they solidified. As I mentioned above, during the Nevadan orogeny the Sierra Nevada was part of a much longer range. Two parts of it were what are now southern California's Transverse Ranges and Peninsular Ranges, the latter extending south to the tip of Baja California. Mineralogical studies in them show that they had uplifts of similar timing and magnitudes to what we now envisage for the Sierra Nevada.


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