Chapter 4: Origin of Yosemite Valley
The Yosemite Problem
Yosemite Valley deserves a chapter of its own because it is one of the world's special places, with an interesting geologic story as well as an interesting human history. This chapter summarizes the development of ideas about the origin of Yosemite Valley and provides an example of how science progresses by the accumulation of knowledge by many different people. Because they aid historical understanding, some quotes are included even though they contain statements that are outdated; beware of taking quotes out of context.
Yosemite Valley is famous for its beauty, and tourism has thrived there since nonnatives "discovered" it in 1851 (fig. 30). What makes Yosemite Valley special is the pleasing juxtaposition of cliffs, waterfalls, and domes with a flat valley floor over which a lovely river meanders through wood and meadow. Visitors are naturally curious about the origin of such a magnificent place.
Figure 30. Yosemite Valley, looking east. The flat, forested, floor of the valley was formed by in-filling of a lake. Thick lake sediments mask the U-shape of the glacial valley.
Efforts to understand the origin of Yosemite Valley began in the 1860s with study by the California Geological Survey. In the 1870s the origin of the valley became the subject of a dispute, sometimes bitter, known as the "Yosemite Problem" or the "Yosemite Controversy." At first, debate centered upon the competing roles of faulting, advocated by Josiah D. Whitney, and glacial erosion, advocated by John Muir. Later study and debate concerned the relative importance of erosion by glacier and by river. In 1913 the U.S. Geological Survey assigned François Matthes the task of determining the valley's origin. Upon publication in 1930 his work was well received, and it ended the Yosemite Controversy. Knowledge gained since 1930 requires modification of some of Matthes's work, and these new ideas are briefly presented in this chapter. Finally a summary of the geologic development of Yosemite Valley is given.
The Bottom Dropped Out (Josiah Whitney)
Josiah D. Whitney thought Yosemite Valley was formed by down-dropping of a large block of rock between faults, and that glaciers were of no importance at all. This hypothesis had little evidence to support it, but it is interesting to examine as an example of how appearances can deceive.
Features of Yosemite Valley that most impressed Josiah D. Whitney were the steep, nearly parallel valley walls, with little talus at their bases, the sheer cutoff face of Half Dome, and the flat valley floor (fig. 31). Whitney knew these forms were not typical of glacial valleys in the Alps. The narrow, V-shaped gorge that exists along Tenaya Creek above Yosemite also argued against a glacial origin and suggested some special, local, process had been at work. Of Half Dome he wrote:
Figure 31. The Merced River in Yosemite Valley meanders on the nearly flat surface of in-filled Lake Yosemite.
It is an inaccessible crest of granite, rising to the height of 4737 feet above the valley, the face fronting towards Tenaya Creek being absolutely vertical for 2000 feet down from the summit.... It is an unique thing in mountain scenery, and nothing even approaching it can be found except in the Sierra Nevada itself. (Whitney, 1865, p. 416)
Whitney concluded that the unique forms of Yosemite Valley must have formed by down-dropping between faults:
[I]t appears to us probable that this mighty chasm has been roughly hewn into its present form by the same kind of forces which have raised the crest of the Sierra. . . . The Half Dome seems, beyond a doubt, to have been split asunder in the middle, the lost half having gone down in what may truly be said to have been "the wreck of matter and the crush of worlds." (Whitney, 1865, p. 421)
We conceive that, during the process of upheaval of the Sierra or, possibly at some time after that had taken place, there was at the Yosemite a subsidence of a limited area, marked by lines of "fault" or fissures crossing each other somewhat nearly at right angles. In other and more simple language, the bottom of the Valley sank down to an unknown depth. . . .
(Whitney, 1868, p. 77)
Lake Tahoe and the valley which it partly occupies we conceive also to be, like the Yosemite, the result of local subsidence.
(Whitney, 1868, p. 79)
Clarence King, who was familiar with glacial features in Yosemite Valley, thought Whitney was right, as did the prominent geologists Israel C. Russell and Andrew Lawson, who, from studies elsewhere in the Sierra, had concluded that glaciers did little eroding of the valleys through which they flowed.
Whitney knew some glacial features existed in the valley and that one moraine in particular
may have acted as a dam to retain the water within the valley, after the glacier had retreated to its upper end, and that it was while thus occupied by a lake that it was filled up with the comminuted materials arising from the grinding of the glaciers above, thus giving it its present nearly level surface. (Whitney, 1865, p. 423)
Writing about the lake, Whitney came close to grasping an important point that, if understood, might have caused him to recognize that glaciers were important in creating Yosemite Valley, thereby avoiding the Yosemite Problem, and might have caused history to be kinder to him. But nature tricked Whitney, as illustrated by figure 32. Thick lake deposits obscure an important part of the glacial evidence, the bottom of the U-shaped valley. What is left to be seen of the U, together with the flat valley floor formed by the lake deposits, is a good imitation of a down-dropped fault block, or graben. If lake sediments were not present, the U-shape of the underlying bedrock would be visible, as in many other Sierra valleys, Yosemite Valley would not resemble a graben, and Whitney would probably have recognized the importance of glaciers in its development.
Figure 32. A. In basic surface form, Yosemite Valley resembles a fault-block basin, or graben, more than it does a glacial valley. B. Beneath the thick lake sediments is the rest of a U-shaped glacial valley, not a block of rock that has dropped down between faults.
But how did Whitney miss other signs that point to the importance of glaciers? After all, is not Yosemite the finest example of a glacial valley anywhere? No, it is not, and this is a surprise for many people. There is much to praise about Yosemite Valley, but to praise it, as has been done, as a "sublime monument to glaciers" or as "nature's textbook on glacial erosion" is misleading. Many valleys in the Sierra Nevada display more characteristic glacial features than Yosemite Valley does. Yosemite abounds in unusual landforms, features not at all typical of glaciated valleys but which are more conspicuous than the glacial landforms present. The steep cliffs and somewhat straight, open valley are of glacier origin, but they are not uniquely so, because other geologic processes create similar features. Moraines and small areas of glacial polish are present but are not prominent, and most visitors neither notice nor photograph them.
By contrast, the outstanding landforms of Yosemite Valley, the trademark features that are photographed, gawked at, climbed on, painted, hiked to, admired, and that adorn T-shirts, book covers, and corporate logos are neither characteristic of, nor unique to, glacial erosion. Half Dome owes its special form to weathering, exfoliation, and a vertical joint. North Dome and Sentinel Dome formed by weathering and exfoliation, not glacial erosion. El Capitan is the imposing monument it is because the rock of which it is composed has few joints; its near-verticality is in part attributable to glacial erosion, but imposing cliffs are not unique to glacier erosion. Distinctive forms such as Royal Arches, Washington Column, and Three Brothers were shaped by jointing of diverse orientation and form, plus removal of rock by rockfall or glacier plucking, and it is not clear which process removed any given block of rock. The flat valley floor was produced by in-filling of a lake; yes, the lake was of glacial origin, but, once again, such features are not unique to glaciated areas. What is surprising is not that Whitney failed to see the hand of glaciers, but that Muir and others did!
Glaciers Excavated It (John Muir)
John Muir became champion of the idea that glaciers were responsible for excavating Yosemite Valley, but it should be noted that he was not the first to observe glacial features of the valley. In October of 1864, four years before Muir arrived in California, Clarence King of the California Geological Survey studied Yosemite Valley:
I gathered ample evidence that a broad sheet of glacier, partly derived from Mount Hoffmann and in part from the Mount Watkins ridge and Cathedral Peak, but mainly from the great Tuolumne glacier, gathered and flowed into the Yosemite Valley. Where it moved over the cliffs there are well-preserved scarrings. The facts which attest this are open to observation, and seem to me important in making up a statement of past conditions. (King, 1872, p. 165)
Up the Yosemite gorge, which opened straight before me, I knew that another great glacier had flowed; and also that the valley of the Illilouette and the Little Yosemite had been the bed of rivers of ice; a study, too, of the markings upon the glacier cliff above Hutching's house, had convinced me that a glacier no less than a thousand feet deep had flowed through the valley, occupying its entire bottom.
(King, 1872, p. 168)
King did not have opportunity to follow up on his early study of Yosemite Valley.
When John Muir first visited Yosemite in 1868 at age thirty he lacked formal credentials as a scientist, but he had attended a university for several years, was knowledgeable about many aspects of natural history, and was a careful and perceptive observer. He lived in Yosemite Valley for many years and had abundant opportunity to study glacial features and reflect on their significance. Muir saw no merit in the subsidence idea of Whitney:
The argument advanced to support this view is substantially as follows: it is too wide for a water-eroded valley, too irregular for a fissure valley, and too angular and local for a primary valley originating in a fold of the mountain surface during the process of upheaval; therefore a portion of the mountain bottom must have suddenly fallen out, letting the super-incumbent domes and peaks fall rumbling into the abyss, like coal into the bunker of a ship. This violent hypothesis, which furnishes a kind of Tophet for the reception of bad mountains, commends itself to the favor of many by seeming to account for the remarkable sheerness and angularity of the walls, and by its marvelousness and obscurity, calling for no investigation, but rather discouraging it. (Muir, in Colby, 1950, p. 18)
"Tophet" refers to a shrine near Jerusalem where human sacrifice was once practiced. Lack of an academic degree was no handicap to Muir; he was a formidable opponent to Whitney and the better writer, one who was later amazed that publishers would pay him money for his writings about Yosemite and the High Sierra.
Muir was no half-hearted glacier booster. He was certain that glaciers alone had excavated Yosemite Valley, that all of California was once covered by a great ice sheet (see chapter 2), and that throughout the Sierra Nevada, glaciers alone had excavated every valley and peak:
In the development of these, the Master Builder chose for a tool, not the earthquake nor lightning to rend and split asunder, not the stormy torrent nor eroding rain, but the tender snow-flowers noiselessly falling through unnumbered seasons, the offspring of the sun and sea. If we should attempt to restore the range to its pre-glacial unsculptured condition, its network of profound cañons would have to be filled up, together with all its lake and meadow basins; and every rock and peak, however lofty, would have to be buried again beneath the fragments which the glaciers have broken off and carried away. (Muir, in Colby, 1950, p.3)
Joseph LeConte of the University of California in Berkeley sought out Muir in 1870 to discuss the origin of Yosemite Valley, and wrote,
He fully agrees with me that the peculiar cleavage of the rock is a most important point, which must not be left out of account. He further believes that the valley has been wholly formed by causes still in operation in the Sierra—that the Merced Glacier and the Merced River and its branches, when we take into consideration the peculiar cleavage, and also the rapidity with which the fallen and falling boulders from the cliffs are disintegrated into dust, have done the whole work. The perpendicularity is the result of cleavage; the want of talus is the result of the rapidity of disintegration and the recency of the disappearance of the glacier. I differ with him only in attributing far more to preglacial action. (LeConte, 1870, p. 68) Cleavage
is what, today, we call joints
(fig. 33). Joints are fractures in rock produced by stress, such as occurs during uplift of a mountain range, release of pressure after removal of overlying rock, or cooling of a lava flow. Joints hasten destruction of rock by permitting entry of water, air, and acids from decomposing vegetation. In his article "Mountain Sculpture," Muir shows that he understood the importance of joints in erosion, though he often single-mindedly went ahead and stressed glaciers to the exclusion of other geologic factors (Colby, 1950).
Figure 33. Joints are cracks in rock that make rock more easily weathered and eroded by glaciers. Here, water freezing in joints has disrupted the rock. (Photo by F.E. Matthes, U.S. Geological Survey.)
Muir traced the pathway of glaciers that converged on Yosemite Valley: They number five, and may well be called Yosemite glaciers, since they were the agents by which beauty-loving nature created the grand valley, grinding and fashioning it out of the solid flank of the range, block by block, particle by particle, with sublime deliberation and repose.
The names I have given them are, beginning with the northmost, Yosemite Creek, Hoffman, Tenaya, South Lyell, and Illillouette [sic] Glaciers. They all converged in admirable poise around from north-east to south-east, welding themselves together into one huge trunk which swept down through the valley, filling it brimful from end to end, receiving small tributaries on its way from the Indian, Sentinel, and Pohono Cañons; and at length flowed out of the valley, and on down the range, in a general westerly direction.
(Muir, 1880, p. 553)
Muir happily undertook the task of educating the public about glaciers and was successful because of writing like this:
The work of glaciers, especially the part they have played in sculpturing the face of the earth, is as yet but little understood, because they have so few loving observers willing to remain with them long enough to appreciate them. Water rivers work openly where people dwell, and so does the rain and the dew, and the great salt sea embracing all the world; and even the universal ocean of air, though invisible, yet it speaks aloud in a thousand voices, and explains its modes of working and its power. But glaciers, back in their cold solitudes, work apart from men, exerting their tremendous energies in silence and darkness. Outspread, spirit-like, they brood above the long predestined landscapes, working on unwearied through unmeasured ages, until, in the fullness of time, the mountains and valleys and plains are brought forth, channels furrowed for the rivers, basins made for the lakes and meadows and long, deep arms of the sea, soils spread for the forests and the fields—then they shrink and vanish like summer clouds. (Muir, 1880, p. 557)
No writer combines science and poetry better than John Muir, and this in part explains why his readers came to love him and believe in his ideas. Although he was overzealous about the erosional ability of glaciers, their former extent, and their effects on California and the Sierra Nevada, Muir focused attention on the importance of glaciers in eroding landforms and won praise from François Matthes:
To one thoroughly at home in the geologic problems of the Yosemite region it is now certain, upon reading Muir's letters and other writings, that he was more intimately familiar with the facts on the ground and was more nearly right in their interpretation, than any professional geologist of his time. (Matthes, 1938, p. 10)
A Digression in Defense of Whitney
Most historical accounts present Whitney unfavorably because he was wrong about the origin of Yosemite Valley and ill-mannered and arrogant toward John Muir, a popular underdog. The two men differed in all respects. Whitney was a professional geologist, graduate of a fine university, a respected scientist, and a responsible state geologist. Muir was a good-hearted vagabond, a nature lover, not a geologist or even a college graduate, and he did not claim to be a professional scientist. Whitney spent much time and energy managing a survey and fighting with the state legislature for its survival, while Muir lived carefree in Yosemite Valley, observing and thinking about the valley every day. A century later we would have said Whitney was establishment and Muir was a hippie. That such a person might challenge his geologic ideas, and be successful in so doing, infuriated Whitney. Frustrated and hurt by the popularity of Muir and his eroding ice, Whitney wrote harshly and condescendingly about him, calling him an "ignorant sheepherder" with "absurd" ideas, among other things. In his zeal to combat the glacier menace he even repudiated his friend Clarence King and his own earlier writing by denying that ice had ever occupied Yosemite Valley!
A myth has developed to the effect that Whitney was a pompous windbag who did nothing right while Muir was a neat guy who did nothing wrong. This is unfortunate and unfair. Whitney made major contributions to understanding Yosemite, the Sierra Nevada, and California, and he played an important role in getting Yosemite Valley preserved as public land. He had reason to hypothesize faulting in Yosemite Valley and the Tahoe Basin, was right about Tahoe, and correctly maintained that Muir had some wrong ideas about glaciers, though this latter fact is often discreetly overlooked. It does not detract from Muir for us to treat Whitney with more respect.
Both River and Glacier Shaped the Valley (François Matthes)
The accepted version of the origin of Yosemite Valley gives somewhat equal importance to river and glacier in deepening
the valley, while glacier erosion is regarded as the primary agent for widening
the valley and creating the steep cliffs. In large measure, though not completely, it supports the ideas of John Muir.
In 1930 the United States Geological Survey published Professional Paper 160, Geologic History of the Yosemite Valley
, based on years of field study by François Matthes and others. This well-written and illustrated work has many fine photographs and drawings that have been reproduced in hundreds of publications throughout the world. It is out of print, which is regrettable even though some aspects of the book are out of date.
Matthes wrote about the ideas of Whitney and Muir:
The underlying idea of Whitney's hypothesis was, it should be added in all fairness, not so absurd as some of his opponents have intimated, for there are many well-authenticated instances of valleys that have been created by the subsidence of blocks of the earth's crust. Several such depressions are associated with the Sierra Nevada—notably Owens Valley and the basin occupied by Lake Tahoe. However Whitney's hypothesis rested on no tangible evidence, and nothing has been found thus far to substantiate it. (Matthes, 1930, p.4)
Muir, on his part, went too far in his claims for glacial erosion.... He maintained that the Yosemite and, indeed, all the great canyons of the range, thousands of feet in depth, had been gouged out entirely by the glaciers.
(Matthes, 1930, p.5)
Matthes saw the Yosemite Problem not as a debate about faults and glaciers, but about the relative effects of glacier and river. He was able to make a better study than was possible previously because a new, detailed, topographic map of the valley had recently been prepared (by Matthes himself, in his role as topographer), and because of new ideas about how landscapes develop:
[S]ince the end of the nineteenth century there had come to maturity a new branch of geologic science—geomorphology, or physiography, as it is also termed—which deals specially with the origin and development of the surface features of the earth and within whose scope a problem such as that of the Yosemite Valley largely belongs. (Matthes, 1930, p.6)
The new ideas were mostly those of William Morris Davis, often regarded as the founder of geomorphology. Armed with a good map, new ideas, and lots of time, Matthes took to the field.
The Depth of Yosemite Valley
Matthes undertook to determine the relative amounts of river erosion and glacier erosion in sculpturing Yosemite Valley. The result permits visualization of what Matthes believed Yosemite Valley looked like before and after glaciation (figs. 34, 35). Matthes calculated that, just before the Ice Age, the rivereroded canyon was about 2,000 feet deep below Glacier Point and about 2,400 feet deep below El Capitan. He estimated the amount of glacier erosion as
Figure 34. Yosemite Valley as François Matthes believes it appeared during his hypothetical Canyon Stage, just before the first Ice Age glaciation. (Drawing by U.S. Geological Survey; Matthes, 1930.)
Figure 35. Yosemite Valley as François Matthes believes it appeared immediately after the last Ice Age glacier withdrew, perhaps 15,000 years ago. Multiple glaciation widened, deepened, and straightened the former river valley. Lake Yosemite, in part dammed by moraines, may have been several miles long but was not very deep. (Drawing by U.S. Geological Survey; Matthes, 1930.)
the depth of glacial excavation decreases from a maximum of about 1,500 feet at the head to a minimum of about 500 feet at the lower end. The reason for this decrease in glacial deepening down the valley is found in the fact that during all phases of glaciation the ice was thicker and therefore had greater excavating power at the head of the valley than at the lower end, and during the maximum phases it plunged into the head of the valley in the form of a mighty cataract. (Matthes, 1930, p. 102)
|RC Ribbon Creek ||E Echo Peak ||SD Sentinel Dome |
|EC El Capitan ||C Clouds Rest ||G Glacier Point |
|EP Eagle Peak ||SM Sunrise Mountain ||SR Sentinel Rock |
|YC Yosemite Creek ||HD Half Dome ||SC Sentinel Creek |
|IC Indian Creek ||M Mount Maclure ||CR Cathedral Rocks |
|R Royal Arches ||L Mount Lyell ||BV Bridalveil Creek |
|W Washington Column ||F Mount Florence ||LT Leaning Tower |
|TC Tenaya Creek ||BP Bunnell Point ||DP Dewey Point |
|ND North Dome ||LY Little Yosemite Valley ||MR Merced River |
|BD Basket Dome ||B Mount Broderick || |
|MW Mount Watkins ||LC Liberty Cap || |
These numerical estimates are not as highly regarded today as they were in 1930, but the important conclusion endures: that river and glacier made somewhat equal contributions to the depth of Yosemite Valley.
How Matthes estimated the amount of downcutting done by the Merced River prior to glaciation is an interesting application of the geomorphic ideas of William Morris Davis. Even though the methodology is imperfect and based on some questionable assumptions, it will be summarized here because it is enlightening and of historic interest.
As the Sierra Nevada was being uplifted during millions of years prior to the Ice Age it was also tilting down to the west. In response, the Merced River deepened its valley, and did so faster than its tributary streams could, because, first, it was the larger stream and, second, it flowed west down the increasing slope of the range while most tributaries flowed north or south and were not directly invigorated by the westerly tilting. Matthes, following Davis, believed that mountain ranges were typically uplifted in jerks, with short periods of relatively rapid uplift followed by longer periods of stability. After each uplift the Merced River (supposedly) cut down to a new level of equilibrium, or baselevel, and a new landscape developed. The less powerful tributaries, unable to keep up with the rapid downcutting of the Merced, were left "hanging" above the master stream (fig. 36). The old slopes of the hanging tributaries still "point" to the former elevations of the Merced River (fig. 37).
Figure 36. The valley of Bridalveil Creek "hangs" several hundred feet above the Merced River and the floor of Yosemite Valley, creating Bridalveil Fall.
Figure 37. François Matthes re-created the preglacial landscape of the Merced River Canyon by extending to the Merced River gradients of various tributary streams. He attributed differences between the reconstructed valley and the present-day valley to erosion by glaciers. This method is based on some questionable assumptions and is not as valid as Matthes believed. (Drawing by U.S. Geological Survey; Matthes, 1930.)
By drawing profiles of numerous tributaries, Matthes re-created steps in the downcutting of the Merced River, as shown in figure 38. He named the reconstructed landscapes the "Broad-Valley," the "Mountain-Valley," and the "Canyon" Stages, each landscape supposedly being formed during a period of stability between uplifts. Matthes spoke of the Merced River as having a "three-story" profile, illustrated in his "Canyon Stage" of figure 38: each story, or form, corresponds (Matthes believed) to a single uplift and subsequent erosional episode. The erosion surfaces in the vicinity of Mount Whitney (chapter 3) appear to support this part of Matthes's work, but the support may not be valid, as will be explained later.
Figure 38. Stages in the development of the Merced River Canyon at the future site of Yosemite Valley as re-created by François Matthes. Today scenes such as these are regarded as single moments in a continually changing scene, not as separate "stages." (Drawing by U.S. Geological Survey; Matthes, 1930.)
The Width of Yosemite Valley
What makes Yosemite Valley special is its width, not its depth, which though striking is not exceptional. Matthes attributed virtually all of the widening of the valley to glacial erosion.
Where a glacier flows through a former river valley it usually widens and straightens the valley. Ice erodes both the sides and the bottom of a valley, whereas a river only erodes the bottom. This is why river valleys are commonly V-shaped in profile while glaciated valleys are commonly U-shaped and have steep valley walls. Also, ice is more viscous than water and does not flow around bends as well, so erosion is concentrated on bends and the valley becomes straighter and more open. The width and the steepness of the cliffs of Yosemite Valley, though especially well developed, are consistent with the character of glaciated river valleys elsewhere.
Dozens of valleys in the Sierra Nevada have been widened by glaciers. John Muir wrote and spoke of "yosemites" as a generic feature and argued that similar valleys were eroded wherever tributary glaciers joined to form trunk glaciers. He wrote, for example, of the "Kings River Yosemite" and the "Tuolumne Yosemite" (Hetch Hetchy Valley). But with all respect to Muir, millions of tourists each year vote with their feet and pocketbooks as they insist that there is only one Yosemite. The widening of Yosemite Valley by ice is a common process done exceptionally well. Why this is so is explained in the next section.
The importance that jointed rock played in the origin of Yosemite Valley will now be stressed. Studies by Russell, Johnson, and Lawson (see chapter 3) showed that where rock is jointed, glaciers erode effectively, but where joints are lacking, glaciers accomplish little beyond smoothing and polishing. Joints, widened by one or more types of weathering, permit ice to get a "hold" on the rock, so to speak, pull blocks loose, and carry them away; this pulling process is called plucking
, or quarrying
). LeConte and Muir agreed in 1870 that jointing was important in the origin of Yosemite Valley. Matthes included in Professional Paper 160 a section devoted to joints entitled "The Key to the Secret of the Yosemite's Origin." He wrote of the relationship that exists between the sculpture of the cliffs and their "inner structure," pointing out that Yosemite Valley is located
in an exceptional locality where many small bodies of relatively basic rocks—granodiorite, diorite, and gabbro—have been intruded into the otherwise vast, unbroken bodies of siliceous granite and monzonite that make up the central parts of the great Sierra batholith...also that these basic rocks are in general more closely jointed than the siliceous rocks. (Matthes, 1930, p. 91)
Explanation is in order. While it is often said that the Sierra Nevada is composed largely of granite, the statement is technically incorrect; here is why. Granite
is a coarsely crystalline igneous rock with a significant amount of the minerals quartz and potassium feldspar. A similar rock with less of these two minerals and more of the mineral plagioclase is named granodiorite
, or what Matthes called monzonite
, even though these rocks still resemble granite in appearance. Rock with essentially no quartz or potassium feldspar is named diorite
—the so-called "basic" rocks—in contrast to the "siliceous" rocks, granite and granodiorite. The basic rocks are dark in color and rather different in appearance from the lighter-colored siliceous rocks. As it happens, there is more granodiorite and other siliceous rocks in the Sierra than there is "true" granite. But to write and speak simply, all these light-colored, crystalline rocks are often referred to as granite to distinguish them from the dark-colored, crystalline basic rocks and the multicolored metamorphic rocks. Today it is common to speak and write of "granitic rocks," including those similar in appearance to granite though not necessarily granite in the strict sense.
What do these somewhat technical distinctions have to do with Yosemite Valley? Matthes says that compared to granitic rock, basic rock is more jointed, weathers more rapidly, and so is more easily eroded by glaciers. The local abundance of basic rock enabled the Yosemite Glaciers to be effective. If basic rock had not been there, the area would consist of "unbroken" granitic rock, glacial erosion would have been less effective, and Yosemite Valley might now resemble nearby Tenaya Canyon (fig. 39) rather than the wide, open, flat-floored valley we regard as special.
Figure 39. Despite the fact that large glaciers flowed through it several times, part of Tenaya Canyon is narrow and V-shaped, very different from Yosemite Valley just a few miles downstream. Lack of joints in the granitic rock prevented the glacier from widening the canyon here.
Studies of the bedrock in the Yosemite area were done by a colleague of Matthes, F. C. Calkins, who contributed a major section to Professional Paper 160. Huber presents a modern discussion of the bedrock of Yosemite National Park and explains how meanings of some rock names have changed over the years (1987).
Glaciers occupied Yosemite Valley at least three times, and after each retreat a lake probably formed in the valley. Each lake was soon filled with gravel, sand, and mud from the retreating glaciers, the re-forming Merced River, and Tenaya Creek. Matthes named the lake that formed after the most recent glaciation Lake Yosemite.
To the camper who pitches his tent in one of the Yosemite's shady groves it may not occur, perhaps, that the spot he has selected for his temporary abode once lay in the midst of a mountain lake of exceptional beauty—a sheet of water in which the cliffs of El Capitan and Half Dome as well as the sprays of the Yosemite Falls were reflected. Yet there can be no doubt that the present embowered, parklike floor of the valley replaces such a body of water. ... [T]he lake owed its existence not to the moraine dam alone; it occupied an elongated basin scooped out in the rock floor of the valley by the ancient Yosemite Glacier, and its depth was merely increased somewhat by the moraine dam which was situated on a broad rock sill at the lower end of the basin.
(Matthes, 1930, p. 103)
The last Yosemite Glacier, of Tioga age, was not as large as that of at least one earlier glaciation and did not erode to bedrock. Lake Yosemite was probably smaller and less deep than Matthes believed, though the total sum of river and lake deposits in Yosemite Valley is now known to be greater than he imagined.
Matthes identified the final event in the evolution of Yosemite Valley as a period of "dismantling." Dismantling refers to significant changes that the cliffs of Yosemite Valley have experienced since they were last eroded by a glacier (fig. 40). The dominant means of dismantling is rockfall, which can be caused by water freezing in joints, by earthquake shaking, or by the undramatic process of weathering. The Owens Valley earthquake of 1872 produced numerous rockfalls in Yosemite Valley that were witnessed by John Muir.
Figure 40. Royal Arches and Washington Column. Most details of Yosemite cliffs reflect the spacing and shape of joints in the granitic rock and are not unique to erosion by glaciers.
The fatal rockfall from Glacier Point in July 1996 is but the most recent of many such events that have occurred, and that will continue to occur for many millennia. So long as large numbers of people live and camp in Yosemite Valley there is potential for great loss of life and injury from a really big rockfall. Such a threat might justify removal of cabins, stores, and campgrounds from the valley, as was done in Lassen Volcanic National Park when it was recognized that a large campground, cabins, and the visitor center were built on old rock-slide debris of considerable dimension.
Dismantling has created a difference in appearance between Hetch Hetchy Valley and Yosemite Valley. Hetch Hetchy, on the Tuolumne River 18 miles northwest of Yosemite Valley, was one of Muir's beloved "yosemites" before it was dammed and turned into a reservoir in 1923. There was a flat valley floor, waterfall, and steep cliffs, reminiscent of Yosemite Valley. John Muir and the Sierra Club fought hard to prevent the dam, but lost. Some people believe that losing Hetch Hetchy broke Muir's heart and shortened his life.
N. King Huber points out a significant difference between Hetch Hetchy and Yosemite:
During each major glaciation, including the Tioga, which probably peaked only about 15,000-20,000 yr. ago, the Tuolumne canyon was filled to the brim with ice at least as far west as Mather, some 10 km beyond Hetch Hetchy... Thus, Hetch Hetchy has been glacially scoured "recently." Yosemite Valley, however, has not been filled with ice for at least 750,000 yr., the minimum age of the Sherwin glaciation.... Thus, the major excavation of Yosemite Valley, including the bedrock basin beneath the valley floor, had to have been accomplished by that time, and since then, freeze-thaw cycles have promoted spalling of rock slabs, steepening the cliffs, and forming the recessed alcoves into which waterfalls such as Bridalveil now leap. For this reason, Hetch Hetchy Valley is in many ways a "fresher looking" glaciated valley than Yosemite Valley, long considered a classic glacially carved valley. (Huber, 1990, p. 114)
It should be emphasized that Yosemite Valley has been glaciated since the Sherwin advance but has not been filled with ice since that time. Filled is the key word, for only then could ice shape the walls significantly. The overall shape of Yosemite's cliffs is of glacier origin but their present appearance in detail mostly is not.
Studies Since Matthes
François Matthes showed that Yosemite Valley is the combined result of erosion by the Merced River and glaciers of various ages. This conclusion remains the accepted version of the origin of Yosemite Valley, but we have learned things since 1930 that require modification of some of Matthes's work. Modification number one:
In 1935 and 1937 seismic studies were conducted in Yosemite Valley to determine the depth of unconsolidated sediments that overlie bedrock. The results show that the depth to bedrock at the east end of the valley is about 1,800 feet, much greater than Matthes estimated:
The upper layer with a maximum thickness of about 150 m extends from Mirror Lake to the Wisconsin end moraines near Bridalveil Meadow....The intermediate and basal layers have maximum thicknesses of 220 and 300 m respectively, and the intermediate layer lies in a U-shaped trough seemingly gouged out of the basal layer. Both layers are thought to be remnants of earlier lake fillings, and at least the basal layer is pre-Wisconsin. The greatest thickness of fill, about 600 m, is near the head of the valley between Ahwahnee Hotel and Camp Curry. Modification number two:
... The amount of glacial excavation on the bedrock floor, essentially double the 450 m previously estimated, is attributed wholly to pre-Wisconsin glaciation. (Gutenberg, Buwalda, and Sharp, 1956, p. 1051)
The old landscapes that Matthes re-created may not have formed as he imagined. As described in chapter 3, Andrew Lawson identified several erosion surfaces in the vicinity of Mount Whitney that he thought marked erosion-to-baselevel during periods of stability that interrupted the uplift of the Sierra Nevada. The old landscapes named by Matthes in the Yosemite area—Broad Valley, Mountain Valley, Canyon—follow this lead. But Clyde Wahrhaftig of the University of California at Berkeley gives reason to suspect that some or all of these old erosion surfaces formed not at a baselevel reached by erosion during a period of stability but behind massive granitic outcrops that do not weather so rapidly as other rock.
[A] bench, summit flat, or nickpoint can develop in granitic terrane at any altitude at any time. The only requirement is the appearance of solid granitic outcrops to act as local base levels. Once formed, these summit flats are long-persisting and may accumulate ephemeral alluvial deposits at any time after their origin. Other evidence must be used to establish that granitic terranes were once nearly graded plains close to sea level. Modification number three:
(Wahrhaftig, 1965, p. 1186)
An assumption of William Morris Davis's that Matthes followed is no longer well regarded by geomorphologists. This is the idea that mountain ranges commonly, or normally, undergo short episodes of uplift followed by long periods of stability during which rivers may erode to baselevel before another uplift begins. In particular, it was not the case with the Sierra Nevada, as is shown by studies by N. King Huber, author of a recent book about the geology of Yosemite National Park (Huber, 1987). This book is written in non-technical language and is beautifully illustrated. The scope of the book is more than glaciers and Yosemite Valley, and it is a good summary of the rocks and geologic history of Yosemite Park. Huber writes about uplift of the Sierra Nevada during the last 25 million years.
The uplift began slowly and accelerated over time. The range certainly is still rising—and the rate may still be accelerating. The estimated current rate of uplift at Mount Dana, less than 1 1/2 inches per 100 years, may appear small, but it is greater than the overall rate of smoothing off and lowering of the range by erosion. Thus, there is a net increase in elevation. Estimates of uplift amount and rate are based on studies of lava flows and stream deposits thought to be nearly horizontal when formed, but which are now tilted westward toward the Central Valley. Progressive tilt is indicated by older deposits with greater inclinations than younger ones. François Matthes inferred from his studies that the late Cenozoic uplift occurred in a series of three pulses interrupted by pauses in uplift. In his view each pulse initiated a new cycle of erosion and thus produced a stage of landscape incision characterized by successively greater relief: Matthes' broad valley, mountain valley, and canyon stages. More recent studies show that fortuitous correlation and the commonly local control of erosion weaken Matthes' case for three distinct pulses of uplift. This does not mean that the uplift was entirely uniform—few things in geology are—but rather that uplift, once initiated, was more nearly continuous than he envisioned. (Huber, 1987, p.28) Modification number four:
As described previously, Matthes believed that local abundance of highly jointed basic rock (diorite and gabbro) permitted glaciers to be effective at erosion at Yosemite Valley. Huber points out that most all of the basic rock present in Yosemite Valley is fine grained, rather than coarse grained, and that abundant joints are associated with fine-grained rocks of whatever mineral composition, granitic or basic.
Summary of the Origin of Yosemite Valley
The geologic story of Yosemite Valley began about 25 million years ago, as slow uplift and westward tilting of the Sierra Nevada enabled the Merced River to erode a valley into the broad western flank of the range. Continued uplift, tilting, and river erosion resulted in an increasingly deep valley, perhaps 2,000 feet below the divides between river valleys. About 1.5 million years ago the Sierra Nevada was high enough, and the climate was suitable, for glaciers to form. At least three times—probably more—glaciers formed along the Sierra crest and flowed along two or more paths into Yosemite Valley to form a trunk glacier. The several Yosemite glaciers widened, straightened, and deepened the river valley into a U-shaped glacial valley with many near-vertical cliffs. Rock with many closely spaced joints was exploited by the glaciers to give the valley its width and depth. Massive rock with few joints endured to form steep, bold cliffs. The most extensive glaciation of which we know (Sherwin) occurred about a million years ago. During this time the maximum depth of glacial erosion was achieved at the east end of Yosemite Valley, and ice completely filled the valley and spilled over the surrounding upland, as at Glacier Point, for example. Widening and deepening of the valley by this largest of Yosemite glaciers produced oversteepened valley walls and hanging valleys where small tributary ice streams could not keep pace. Retreat of this glacier probably left waterfalls and a large, deep, lake that became in-filled with river-transported sediment, burying the lower part of the U-shaped glacial valley. A Yosemite Glacier re-formed at least twice since then but never again became nearly so large as that of a million years ago. The most recent glaciation was at a maximum only about 20,000 years ago, but the glacier was not very large. The ice extended only as far as Bridalveil Meadow and did not erode high on the valley walls. Retreat of this last glacier left moraines; Lake Yosemite; and waterfalls at hanging valleys, such as Yosemite Falls and Bridalveil Fall; and at glacial steps, such as Vernal Fall and Nevada Fall. Moraines and waterfalls remain, but Lake Yosemite was filled with river-borne sediment to form the nearly flat valley floor on which the Merced River meanders today. During all of this history slow uplift of the Sierra Nevada continued, punctuated by occasional earthquakes that caused rockfalls from the glacially oversteepened walls of Yosemite Valley. Rockfalls continue to fashion cliff details, such as the Royal Arches, as they have done since glacier ice last completely filled the valley, about 1 million years ago. One rockfall created Mirror Lake, now mostly filled in, at the east end of the valley. The rockfall of July 1996 and the flooding of the Merced River in January 1997, so destructive of human "improvements," show that powerful geologic forces remain at large and continue the long evolution of Yosemite Valley.
Colby, William F. 1950. John Muir's studies in the Sierra
. San Francisco: Sierra Club.
Gutenberg, B., J. P. Buwalda, and R. P. Sharp. 1956. Seismic exploration on the floor of Yosemite Valley, California. Geol. Soc. of Amer. Bull.
Huber, N. King. 1987. The geologic story of Yosemite National Park
. U.S. Geol. Survey Bull. 1595. (This U.S.G.S. Survey Bulletin is out of print, but a reprint by the Yosemite Association is available in many bookstores.)
_____. 1990. The late Cenozoic evolution of the Tuolumne River, central Sierra Nevada, California. Geol. Soc. of Amer. Bull.
King, Clarence. 1872. Mountaineering in the Sierra Nevada
. Boston: James R. Osgood and Co.
LeConte, Joseph. 1870; revised 1900 1971. A journal of ramblings through the High Sierra of California
. Reprint, New York: Sierra Club/Ballantine Books.
Matthes, François E. 1930. Geologic history of the Yosemite Valley
. U.S. Geol. Survey Prof. Paper 160.
_____. 1938. John Muir and the glacial theory of Yosemite. Sierra Club Bull.
Muir, John. 1880. Ancient glaciers of the Sierra, California. Californian
Wahrhaftig, Clyde. 1965. Stepped topography of the southern Sierra Nevada, California. Geol. Soc. of Amer. Bull.
Whitney, J. D. 1865. The High Sierra. Chap. 10 in Report of progress, and synopsis of field work from 1860 to 1864
. Vol. 1 of Geology
. Philadelphia: Geol. Survey of California.
_____. 1868. The Yosemite book. Geological survey of california
. New York: Julius Bien.