One Hundred Hikes in Yosemite

Evolution of the Yosemite Landscape — Origin of a Modern Sierra Nevada

Uplift of the lower crust in response to removal of the upper crust created a range that resembled today's, one of a similar size and of similar elevations. Uplift also created the Sierra's crest and therefore the valleys east of it. The foremost ones are, from north to south, the Truckee-Lake Tahoe basin (no lake until about 2 1/4 million years ago), Mono Basin, and Owens Valley. These three (and others) have been considered youthful because faulting and volcanism have occurred in each in the last few million years. However, 1990s mapping and dating of Owens Valley faults demonstrated that some of the faults are extremely old-Mesozoic in age-active when the ancient Sierra Nevada was growing through terrane accretion. Additional evidence of the valley's antiquity comes from measuring how fast its granitic bedrock is weathering, which in the Alabama Hills, below Mt. Whitney, is about 7 feet per million years. If the Owens Valley began to form only 3 to 4 million years ago (the commonly cited dates), then only 21 to 28 feet of weathering has occurred along the resistant ridges that today descend thousands of feet from the Sierra crest. These ridges have changed very little in this period, so back then the valley already was very deep.

The extensional forces that led to the birth of the modern Sierra Nevada and its east-side basins also fractured the range's recently raised bedrock. It was about then, some 80 million years ago, that the range's modern rivers originated on the newly created surface, cutting into what was once lower crust and having their courses dictated in part by recently formed fractures, or joints. These also dictated the courses of some tributaries, particularly north of the Tuolumne River, where linear canyons are the rule, not the exception.

The composition of bedrock can also influence the development of a landscape. For example, if Yosemite Valley had been carved in diorite instead of granodiorite and granite, it likely wouldn't be part of a national park today (and the Sierra Nevada wouldn't be"the Range of Light"). Diorite does not form massive cliffs, and where it forms summits, they tend to be low hills, such as Ackerson Mountain, south of Camp Mather, along the road to Hetch Hetchy. Diorite also does not form impressive domes such as Mt. Starr King, North Dome, and Half Dome.

Joints are more important than the composition of bedrock, and good evidence for this is in western Yosemite Valley and the adjacent upper Merced Gorge. In this area are the Rockslides, which are a veneer of talus derived through rockfall from minor, upper diorite cliffs to cover minor, lower ones. Diorite here has fractured into many small pieces and has weathered much more readily than does the more-massive granite. However, the situation is more complicated than that. In actuality only the western part of the cliffs above the Rockslides is diorite, the eastern part is granite, the same bedrock that makes up El Capitan-one of the world's most resistant monoliths-and also the part of the upper Merced Gorge west of the Rockslides. So there are three very different landforms created in this granite: the enormous vertical walls of El Capitan, the highly fractured, minor cliffs of the eastern part of the Rockslides (and those on the opposite walls), and the V-shaped upper Merced Gorge. When it comes to directing the evolution of the Yosemite landscape, the alignment and spacing of joints are more important than the type of bedrock.


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