One Hundred Hikes in Yosemite

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

Yosemite Valley has been hailed as perhaps the premier example of what a glaciated canyon should look like. It certainly was glaciated. Its Tahoe glacier was about 1200 feet thick near El Capitan and about 1600 feet thick up-valley near Glacier Point, and was about 33 miles long. While large, it paled in comparison to the Tahoe glacier in the Grand Canyon of the Tuolumne River, which was about 3500 feet thick near Hetch Hechy's dam, over 4000 feet thick up-canyon around Pate Valley, and about 58 miles long (it may have flowed 2-3 miles past the Tuolumne/Cherry confluence). Despite the enormity of the glaciers, they barely widened and deepened the floor of their canyons. Most glaciated canyons in the Sierra and elsewhere do not look like Yosemite Valley. Most do not have nearly perfectly flat floors and most do not have steep-to-overhanging cliffs. The Valley's expansive, flat floor is easily explained: it is not bedrock, but rather is the top of sediments left by glaciers and by the Merced River between glaciations.

Yosemite Valley is unique because of the pattern of variations-in size, spacing and orientation-of its joint planes ("joints" for short). Joints are often linear, usually parallel fractures in bedrock. Most of the Sierra Nevada is granitic and possesses joints, as do other types or rocks such as massive sandstone, which in Utah's Zion National Park is cut by a rectangular grid of streams (and it has glacial features-such as cirques, hanging canyons, and broad floors-despite the absence of glaciation). But Yosemite Valley has a unique pattern of variations in its joint system.

Without the overabundance of joints in the bedrock floor of the Valley, deep subsurface, tropical weathing never would have occurred. The east end of the Valley would have been just like many other glaciated canyons having two major joining canyons-very little downcutting. Furthermore, Yosem ite Valley has steep-sided walls (with over 1000 difficult climbing routes up them) because its walls are governed by the presence of vertical joints. Glaciers didn't make the walls vertical-their contribution was minor; the walls were already quite steep long before glaciation.

In like manner, parts of Tenaya Canyon and of the Grand Canyon of the Tuolumne River may stay V-shaped almost indefinitely, for they lack this important control. I should note here that the spacing of joints is also important. If joints are closely spaced, the rock is more easily excavated-witness Indian Canyon and the Rockslides. In contrast, the hulking, vertical-walled monolith of El Capitan is essentially joint-free, although at one time it had been bounded by several major, vertical joints before it backwasted to its present state.

North of the Tuolumne River, Yosemite's back country is a fantastic landscape of straight, joint-controlled canyons. An easy way to visualize the effect of joints in controlling the development of a landscape is to drive up the Glacier Point Road and get a bird's eye view of Yosemite Valley and its environs. Your first stop should be at Washburn Point, from which you look directly across at Half Dome. Note that it is not really half of a dome, as its name implies, for it is rather symmetrical, the southeast face being about as tall and as steep as the northwest face. The steepness of these two faces are in large part controlled by vertical, northeast-trending joints. These vertical joints were more closely spaced along the northwest face, and glaciers took advantage of this, cutting the face back by about 500 feet. However, contrary to what Josiah Whitney had proclaimed in the 1860s, at no time in the dome's history did a northwest half of the dome fall into Tenaya Canyon.

Below and to the right of Half Dome you'll look down-from Washburn Point-on Mt. Broderick and Liberty Cap, both also bounded by vertical, northeast-trending joints. Glaciation has substantially enlarged the joint that separates the two monoliths, although it is instructive to note that although these two features were likely glaciated perhaps three dozen times, neither was eroded away nor was the gap between them transformed into a broad canyon. Had they been laced with joints, they would not be standing today.

The same joint that governs the southeast face of Liberty Cap also governs the cliff that Nevada Fall leaps over. Vernal Fall is also governed by a vertical joint, but it trends northwest, not northeast. If such vertical joints had been closely spaced, then glaciers would have been able to cut through them, creating cascades, not falls. Like Half Dome, Mt. Broderick and Liberty Cap, Panorama Cliff is governed by vertical, northeast-trending joints, and if you were way down on the trail to Vernal Fall, you could look toward our viewpoint and identify many major, parallel joint planes between Illilouette Fall and Glacier Point.

Before driving on to Glacier Point, look at Mt. Starr King, on the horizon east-southeast of you. It is rounded on all sides-a true dome, unlike most so-called domes. If it is so rounded, does that mean it is not controlled by joints? Not really. Mt. Starr King appeared on the Yosemite landscape by 65 million years ago, if not earlier. It was almost certainly bounded by vertical, intersecting joint planes, and back in its early days it may have looked somewhat like Mt. Broderick, below you, looks today. Time and weathering, however, have taken their toll, weathering faster at the edges than at the faces, and even faster at the corners than at the edges.

From Washburn Point continue down to the Glacier Point parking lot and walk out to unglaciated Glacier Point. The 1200-foot-dead-vertical cliff below you is governed by a major joint, but the curving Glacier Point Apron, below it, lacks joints and has been very resistant to erosion. It's been planed down by glaciers perhaps a maximum of several hundred feet, or perhaps only tens of feet.

From Glacier Point you can easily see how Half Dome got its name, for it certainly looks as if there had been a northwest half of the dome, which would have fallen into Tenaya Canyon. Looking up Tenaya Canyon, note that it is distinctly V-shaped in cross profile, not U-shaped, as a glaciated canyon is supposed to be. However, there are no major vertical joint planes in this canyon, so a"Yosemite Valley" never developed. Above the canyon stand North Dome and Basket Dome, both rounded like Mt. Starr King and both having a similar history.

Below North Dome is Washington Column, which is distinctly separated from Royal Arches by a large, vertical fracture. Royal Arches is a beautiful anomaly. An arch can form on a granite face when an exfoliating sheet of granite breaks loose, leaving a curved scar at the place of attachment. However, more often than not, an arch does not form. On Half Dome's southwest slope, facing us, you'll see a giant half arch. Looking northwest, you'll see a light-colored scar just west of the lower part of Upper Yosemite Fall. That rockfall, perhaps 1000 tons in mass, occurred in June 1976, leaving no arch, though a considerably older one, above and right of it, did. The reason for the massiveness of the uppermost arch of Royal Arches seems to be that the joint planes are widely spaced. Had they been closely spaced, only thin slabs of rock would fall or be quarried away by a former glacier.


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