One Hundred Hikes in Yosemite

Evolution of the Yosemite Landscape — Multiple Glaciations
  |  Gorp.com

By around 15 million years ago the modern Mediterra nean climate of California had set in, complete with the cold coastal current and relatively dry summers. In these last 15 million years both chemical and physical weathering have been minimal, and summits such as Yosemite Val ley's El Capitan and North Dome have experienced only a few feet of denudation. Unglaciated, granitic Sierran lands 15 million years ago had achieved topography nearly identical to today's. The main difference appears to have been slight canyon deepening by the major riv ers, such as the Tuolumne and the San Joaquin, whose glaciated canyons may have deepened by tens of feet, or perhaps even by 100 feet, but not much more.

Glacial landscapes developed best near the crest of the Sierra Nevada, but this was due more to mass wasting, particularly earthquake-induced rockfall, than to glacial erosion. The principal role of an alpine, or mountain, glacier in any of the world's glaciated ranges is to transport the products of mass wasting, not to erode by abrading and plucking, which is minimal where the bedrock is resistant, as in most of the Sierra Nevada. Indeed, glaciers failed to remove some pre-glacial volcanic flows and deposits on the floors and/or lower slopes of the South Yuba, North Stanislaus, Tuolumne, South San Joaquin and Middle Kings river canyons. Nevertheless, glaciers did remove loose rock, soil, and vegetation to expose vast tracts of fresh, generally light-colored bedrock-the final step in transforming the range into John Muir's Range of Light.

The range's earliest glaciation may have oc curred as early as 15 million years ago, when the climate cooled and Antarctica first developed a major ice cap. Glaciers may have been restricted to the heads of canyons below high peaks. In each glacial episode, cold climates would create myriad freeze-and-thaw cycles of ice, which pried rock from steep walls to collect below as talus. As the walls gradually retreated through rockfall, the heads of canyons became broader. The canyon heads are called cirques, the small glaciers that occupied them are called cirque glaciers, and today there are several recently formed ones in our area, the most notable one lying beneath the shady north slopes of Mt. Lyell. Not all of the early glaciers were small. For example, there were worldwide glaciations at about 5.6 and 5.2 million years ago, and during them valley glaciers could have extended many miles down the Sierrra Nevada's principal canyons.

Glaciation began in earnest by 2 million years ago, if not 1/2 million years earlier, and the upper lands may have been glaciated about 3 dozen times. There is very little evidence of most of these glaciations, and those occurring before 200,000 years ago collectively are called pre-Tahoe, since they preceded the well known Tahoe glaciation. On the east side of the range, the Sherwin glaciation, which lasted from about 900,000 to 800,000 years ago, produced the largest known glaciers. On the west side, the few old, remaining boulders left by glaciers probably were left by Sherwin glaciers.

By about 2 million years ago, the lands of the Sierra crest already may have experienced much gla ciation, which would have widened the heads of canyons into cirques. However, the range has"cirques" in unglaciated lands. Additionally, the cirque that holds Clark Fork Meadow, just north of Sonora Pass and the Park, had already formed when it was bur ied under volcanic deposits about 20 million years ago. These features suggest that the Sierra's cirques may be quite old and may not have been widened that much by glaciation. Gla cial deepening definitely was minimal, for had much occurred, the cirques would not hang above the floor of the canyon just beyond it, as most of them do.

The rate of mass wasting greatly increased late in the Cenozoic, judging from the excess amount of post-gla cial talus present today. In similar unglaciated landscapes, such as the southern Sierra's Dome Land Wilder ness, talus is nearly absent, although the area lies relatively close to east-side faults. The current rate of rockfall production in Yosemite Valley appears to be great enough to fill it to the rim in 20 million years (at today's volume of space between the rims above the sedimentary floor, not the bedrock floor). This great in crease in rate is due to several causes. First, and perhaps least important, was the Valley's gradual deepening over time, which led to taller cliffs that had more surface area from which rocks could fall. Second, during glaciations, there would have been some minor glacial abrasion of loose wall rocks. Third, with the development of east-side faults came earthquakes, which dislodged the Valley's wall rocks. Fourth, after each glacier left the Valley, post-glacial pressure release triggered accelerated exfoliation, generating a great amount of mass wasting. The most impressive slabs to fall were the ones that resulted in Royal Arches. Finally, the overabundance of talus in the Valley's deep recesses may be due to a misperception. Geoscientists have assumed that all of today's abundant talus formed after the last glacier left the Valley, about 14,000 years ago. However, rather than being carried away by glaciers, preexisting talus in these recesses was instead buried by them, and hence much, if not most, of the talus had already accu mulated before the last glaciation, not after it. Therefore its average age is much more than we have imagined; and its rate of production is much less.

Just before the first major glaciers developed in the Sierra Nevada about 2 million years ago, Yosemite Valley had widened, mainly through rockfall from its cliffs, to perhaps 90% of its present width. Since then, its walls, on average, have retreated as much as 150 or so yards, also mainly through rockfall, not glacial erosion. The resistant summits were only about 1 to 5 feet higher than today's. Where the pre-glacial bedrock floors were fractured, glaciers removed the loose rocks, and lakes formed when the glaciers retreated. The largest lake was one that formed in Yosemite Valley, presumably after the Sherwin glaciation. At the head of Merced Gorge, west of the Valley, it may have been 1900 feet thick; at the east end, about 2400. While enormous, this glacier still did not overtop Glacier Point or other parts of the Valley's rim, except for Royal Arches and Washington Column. When this glacier retreated from the Valley, it could have left a shallow 8-mile-long lake, one extending west to Pulpit Rock. After the Sherwin glaciation, the following glaciers were smaller, perhaps due in part to a rain shadow that began to develop in response to the rapid uplift beginning in central California's coastal ranges. These smaller glaciers deposited more sediments than they removed, so after each successive glaciation there was a thicker layer of sediment, and therefore a slightly higher Valley floor.

Two of these post-Sherwin glaciers were larger than the others, the Tahoe and Tioga glaciers, which existed, respectively, about 200-130,000 and 35-13,000 years ago. These two were larger than previously supposed, advanc ing to the lower part of Merced Gorge, about 6 miles beyond the last end moraine (a deposit left by the retreating Tioga glacier) by Bridalveil Meadow. Also, both glaciers were thicker than previously supposed, especially in Little Yosemite Valley, where they were about 2000 feet thick, not 1000. As in all other glaciated canyons studied so far, the Tahoe glacier was slightly larger than the Tioga glacier, although in Yosemite Valley the Tioga's ice sur face was slightly higher than the Tahoe's simply because some sedimentation had occurred between the two gla ciations, giving the Tioga a higher base, and hence a higher ice surface. After the Tioga glacier left the Valley about 14,000 years ago, there was at best a swampy floor, not a lake. The greatest change between then and now was the significant accu mulation of talus, generated mostly through depressuri zation of the Valley's lower slopes when the glacier rap idly left them, after having exerted force against them for some 20,000 years. Myriad slabs exfoliated over this time span, and even today there are thousands of cracks, many of them active, some even noticeably widening over just a few years, to the consternation of climbers. Yet without this depressurization and resulting crack formation, the glaciated Sierra Nevada would have far fewer climbing routes.


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