One Hundred Hikes in Yosemite

Evolution of the Yosemite Landscape — Oldest Rocks
  |  Gorp.com

For decades a signed Highway 140"oldest rocks" geologic exhibit stood along the east base of Ferguson Ridge about 10 road miles west of El Portal. However, 1980s research indicates that these rocks are a mere 245 million years old, plus or minus a few million years. The oldest rocks in the Park's vicinity are more than twice as old, having been deposited as sandy sediments around the start of the Paleozoic era (about 540 million years ago). These quartz-rich sandstones later were metamorphosed to quartzites, which today straddle the Park's lightly visited northern-boundary lands. You can see them between Grace Meadow and adjacent Bigelow Peak within the Park, and between the peak and adjacent Bigelow Lake, just over the boundary in Emigrant Wilderness. Other ancient rocks, nearly as old, exist here in the Snow Lake pendant, and also about 3 miles to the southwest, in the Sachse Monument pendant, composed of small remnants between that summit and Lower Twin Lake.

A pendant is by definition a suspended object, which neither of these two is-nor are the several others located along the Park's periphery. Nevertheless the inappropriate term has stuck. There are a number of pendants in the range, and each is a part of a once quite extensive, but now mostly eroded away, terrane.

The oldest rocks of the Snow Lake and Sachse Monument pendants originally were sediments deposited on the ocean's floor off North America's west coast some 500+ million years ago. In addition to these sediments there are considerably younger ones, which like those at Highway 140's geologic exhibit are about 245 million years old. Between these are more than 250 million years of missing sediments. There are two other Park-borderland pendants with a similar age span, but they contain several units or formations of intermediate age. These are the Saddlebag Lake pendant, which extends north from Tioga Pass, and the northern Ritter Range pendant, which extends south from it. Their rocks have weathered to earth tones, which locally add color to our area's crest lands. Additionally, the mineral-rich soil they produce results in more plant species and in greater numbers than in adjacent granitic soil.


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