Pacific Crest Trail: Northern California. Jeffrey P. Schaffer

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      Light Marble Mountain and dark Black Mountain, Section Q

      After plutonism ceased in California, late Cretaceous through early Tertiary faulting broke up the longer range and the Sierra Nevada became separated from the Klamath Mountains on the north, and the Coast, Transverse, and Peninsular ranges on the south. (This, and much that follows, cannot be deduced from the geologic section.) Before the breakup, the longer range was high, similar to today’s Andes, but with the faulting into smaller blocks there also was detachment faulting—the separation of upper crust from lower crust. This occurred when the lower continental crust, under tremendous pressure from the thick, overlying upper crust and from high heat flow below, started to flow laterally. The upper continental crust lacked sufficient heat and pressure to flow. Rather, this brittle layer detached at its base and was transported laterally, atop the flowing lower continental crust. Where the upper several miles of Sierran proper crust went is not yet known. In the southern Sierra, most of the upper crust was transported westward. Then, when the San Andreas fault system developed, it was transported northwest, slivering into linear blocks in the process.

      With the upper crust removed—more than 65 million years ago for most of the Sierra—the unburdened lower crust rose to heights that probably were a bit higher than today’s. In the ensuing millions of years, broad summits such as Mt. Whitney’s have been reduced through weathering and erosion by only a few hundred feet, if that. Back in those early days following detachment, the range already had achieved a largely granitic landscape, since most of the exposed lower crust was granitic. Because stepped topography develops in granitic rocks, it would have begun generating cliffs and benches as well as streams, with almost level reaches alternating with rapids, cascades, and even falls. Like the Sierra Nevada, the Peninsular Ranges and the Klamath Mountains in the northernmost sections of the PCT had also experienced a similar postplutonic history of uplift and erosion to expose their lower continental crusts. This also may have been true for the eastern and central parts of the Transverse Ranges, but they have been so disrupted by faulting, especially over the last 30 million years, that some additional uplift probably has occurred.

      Thirty million years ago was an important time. Roughly about then the climate began changing from one that was somewhat tropical to one that was drier and more seasonal. In the northern half of the Sierra Nevada the range was in part buried by extensive rhyolite-ash deposits. Furthermore, the San Andreas fault system was born, west of the modern coast of southern California. By 15 million years ago, California had acquired an essentially modern summer-dry climate; the northern half of the Sierra Nevada was buried under even larger amounts of andesitic deposits (burying the old, granitic river canyons); and the fault system was beginning to migrate eastward onto existing lands, thereby disrupting them. As today, lands west of any fault segment moved northward with respect to those on the east (right-lateral faulting). Also by 15 million years ago, the composite Sierra Nevada–Central Valley block had begun migrating from its location near the southwestern Nevada border, first west, then northwest, some 150–180 miles to its present location. Today on a very clear day, from Mt. Whitney’s summit you can see granitic Junipero Serra Peak, highest summit of central California’s outer coast ranges, about 170 miles west. Likewise, back then from the same summit, on a very clear day you could have seen the Grand Canyon plateau (no canyon yet), a similar distance east.

      Most of the volcanic deposits in the northern Sierra Nevada were readily eroded, but the new canyons cut in such deposits were inundated by additional sediments. About 10–9 million years ago several massive outpourings of lava flowed westward from faults near the present Sierran crest. These faults were created by extension of the Great Basin lands, which before widespread down-faulting had been a rugged, mountainous highland. The floor of the Owens Valley sank, but the already high Sierra Nevada did not rise; the opposite-direction arrows along the fault in the idealized geologic section indicate only relative movement, not absolute up or down. Note that the fault cuts the bedrock but not the lateral moraine (an accumulation of debris dropped off the side of a glacier), and this indicates that no faulting has occurred since the moraine was deposited (or else it too would have been disrupted).

      Significant parts of these lava flows still remain, and the remnants best preserved are those that lie directly atop old bedrock, as does the remnant of an andesite flow in the idealized geologic section. Such remnants stand high above the floor of today’s granite-walled canyons, which had been mostly exhumed of volcanic deposits before glaciation commenced. From this relation, geologists have concluded—incorrectly in my opinion—that major postflow uplift raised the flows to their present high positions, and that the steepened rivers then cut through thousands of feet of granite to their present low positions. According to this view, glaciers aided in the excavation, but misinterpretation of the field evidence has led geologists to infer major glacial erosion in some canyons, such as Yosemite Valley, and very little in others, such as the Grand Canyon of the Tuolumne River—two adjacent drainages both in Yosemite National Park.

      The Sierra Nevada first experienced major glaciation about two million years ago, although it could have had episodes of minor glaciation long before that. These first large glaciers eroded the layer of rough, fractured, weathered bedrock, then retreated to leave behind much smoother surfaces. Where the bedrock floor was highly fractured and/or deeply weathered (in Yosemite Valley, the most extreme example, tropical weathering had penetrated some 2000 feet down), glaciers could excavate quite effectively, leaving behind bedrock basins that quickly filled with water each time the glaciers retreated, creating a bedrock lake, or tarn. (In some canyons a lake formed behind a terminal moraine, although such a lake exists not so much because of a moraine dam, but rather because of impervious bedrock that is buried by the moraine.) On the resistant, smoothed and polished bedrock, succeeding glaciers could do very little, despite a century of claims by glaciologists.

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