One Hundred Hikes in Yosemite

Evolution of the Yosemite Landscape — Evolution of Yosemite Valley
  |  Gorp.com





At the start of the Cenozoic era 65 million years ago, the dinosaurs had met their demise, and small, primitive mammals, including our ancestors, had inherited the gi ant-free kingdom. At this time the granitic Yosemite Valley was continuing to evolve under warm, wet climates. These generated intense chemical weathering, which causes deep subsurface weathering, such as in the highly fractured bedrock floor of Yosemite Valley. However, weathering was minimal on essentially joint-free monoliths such as El Capitan and Half Dome, which had already come into existence. Some 65 million years ago, the main difference from today's lands was that the river canyons were shallower. In contrast the topographic features above the canyon rims resembled their modern equivalents.

By around 65 million years ago the Merced River may have cut down to about 5600 feet elevation (versus about 4000 feet today). The river, having encountered joint-free granitic bedrock in the upper Merced Gorge west of the Valley proper, greatly slowed its rate of incision. In the Valley significant subsurface weathering should have begun in its highly fractured bedrock floor. If so, then the river would have had a low gradient and it would have meandered across the Valley's river sedi ments, which lay atop decomposed bedrock. Even back then the Valley was develop ing a generally broad U-shaped cross pro file of steep slopes and a flat floor-one characteristic of today's equatorial and tropical gra nitic ranges.

If the Merced River bed's estimated elevation is more or less correct, then at the start of the Cenozoic era the summit of El Capitan would have been about 2000 feet above the west end of the Valley, while the summits of Sentinel Dome, Glacier Point, and Half Dome would have been, respectively, about 2500 feet, 1600 feet, and 3200 feet above the east end. These estimates suggest that the Valley had achieved at least half of its present depth by then. The lower part of Tenaya Canyon would have achieved a similar depth. All of the Valley's major features that today rise above 5600 feet elevation were recognizable, and the slopes of these features were about as steep as those of today's, governed by similar joint patterns. Lower Brother and Lower Cathedral Rock barely protruded above the Valley's floor.

Since this time, weathering and erosion have removed only about 100 feet of bedrock from the most resistant summits. Broad uplands, where forested, had a higher rate of denudation. Consequently, 65 million years ago they would have been perhaps 200 to 500 feet higher than today's, so the domes would not have projected as much above their surrounding lands as they do today. Just as the Merced River should have encountered resistant bedrock in Yosemite Valley around this time, it also should have en countered resistant bedrock in Little Yosemite Valley. Then from that valley's west end the river would have plunged down to the east end of Yosemite Valley via two small falls-the in cipient Nevada and Vernal falls-and a series of rapids, cascades. Other Valley falls existed, es pecially Upper Yosemite Fall, which could have had about two thirds of its present 1410-foot height. Also, today's 1612-foot Ribbon Fall back then may have achieved a similar proportion. In like manner, Hetch Hetchy Valley had achieved similar proportions and had developed incipient Tueeulala and Wapama falls. The Grand Canyon of the Tuolumne River was not so grand, but east of it the broad, flat Tuolumne Meadows area resembled its modern equivalent.

Throughout the first half of the Cenozoic era Yosemite Val ley continued to deepen, but only slowly, since the Merced River was quite ineffective at incising through the mas sive bedrock west of the Valley. The jointed bedrock floor of the Valley weathered over millions of years, eventually producing deep basins of decomposed end-products of the granitic bedrock. Both immediately above and below the Valley the Merced River's gradient was much higher, and its gradient was irregular (stepped). Also, during this time, the slopes and cliffs of Yosemite Valley were subject to slow retreat due to mass wasting, which was governed by the spacing and pattern of joint planes. The rate of rockfall generally was too slow to produce talus slopes.

All of the falls in Yosemite Valley proper should have achieved their full heights by the time the climate changed profoundly 33 million years ago. Overall the climate became cooler and drier, and rates of chemical weathering de creased substantially. By this time, Yosemite Valley may have reached 80% of its width and more likely reached its full depth and then some-the floor is now higher thanks to a couple hundred feet of glacial deposits. Resistant summits such as El Capitan, Middle Cathedral Rock, Sentinel Dome, North Dome, Half Dome, and Mt. Starr King stood only about 10 to 30 feet higher than today's summits. The same would apply to lesser features, such as Mt. Broderick and Liberty Cap, which now would have achieved close to their modern size, and their southwest faces would have projected perhaps some 200+ yards beyond their present location. Similar reconstructions apply to other similar faces. The forbidding, exfoliating northwest face of Half Dome, instead of being slightly concave, would have been slightly convex, much like its back side. Royal Arches was an ordinary cliff, some what like its western continuation, neither hav ing significant arches.

Over the 32 million years that had elapsed since the start of the Cenozoic era, the Merced River had cut down through resistant bedrock to perhaps about 3800 feet elevation just west of Yosemite Valley. In the Valley the river meandered across a flat floor, at perhaps 3850 to 3900 feet elevation, so the Valley was slightly deeper than today's, and slightly nar rower. In modern tropical lands, subsurface weathering can extend 2000 feet down through bedrock, and beneath the Merced River such weathering may have locally extended to this depth by this time.

Virtually all of the Valley's major features ex isted, including Lower Yosemite and Bridalveil falls. Back then you could have used today's topographic map of the Valley to navigate along its forested floors, up its forested recesses and side canyons, which would have lacked talus slopes, and across its forested uplands. The same applies to the rest of the Sierra Nevada from about the northern border of Yosemite National Park south ward. The lands have evolved only a relatively minor amount over the last 33 million years, and you could have used today's topographic maps without getting lost. North of the Park's border, violent rhyolitic eruptions were beginning to occur, and in time most of the north ern Sierran landscape would become buried under vari ous volcanic deposits.


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