APPENDIX C
THE STORY OF THE ORIGIN OF YOSEMITE VALLEY
A short lecture given at the Geology Models in the Yosemite Museum
by C. P. Russell, Approved by F. E. Matthes

Several times each day groups of visitors gather in this room to hear brief explanations of how this amazing Yosemite chasm came to be. I know that you do not care to stand here for too long a time and for that reason the lecture that will be given is to be concise and to the point. These models have been built that we may make clear our explanation with the fewest words possible.
Prior to 1913 no one had ever made a systematic study of the geology of the Yosemite region, and, as a consequence, ideas regarding the origin of the valley were very hazy, There were about as many theories as there were geologists who had visited the Valley. In 1913, however, at the instance of the Sierra Club, the U. S. Geological Survey sent a party of scientists to begin a systematic and detailed investigation. These men were F. E. Matthes and Frank C, Calkins. The former was to study especially the history of the development of the Yosemite Valley; the latter to study the different types of rock. In the years that have elapsed Dr. Matthes has carried his investigations over the entire Yosemite region and considerable areas to north and south in the Sierra Nevada, and thus he has worked out quite definitely, and all the way back to its beginning, the story of the origin of the Yosemite and the other valleys of the same type in the Sierra Nevada. His conclusions have stood the test of criticism by other members of the Geological Survey and are now about to be published by the Government. Those visitors who wish to carry away a simple statement of the story in written form will probably find the brief preliminary sketch by Mr. Matthes, which is printed on the back of the map of Yosemite Valley, most enlightening . The map is available at the sales counter here in the museum. Understand it is a Government publication, and the price of 10 cents is to cover the cost of production.
We will begin our explanation with that period nearly 200 million rears remote, when the entire Sierra region formed part of the floor of an ancient ocean. While thus submerged great quantities of sand and silt, brought down by the streams of the neighboring continent, were deposited on this ocean floor in successive beds to a depth of several thousand feet. Then there came a change which caused the ocean floor to be lifted and be transformed into dry land. At the same time the beds of sand and silt, hardened to rock, were compressed into gigantic folds, or wrinkles, thousands of feet high, and thus was created a mountain system composed of many parallel ridges with northwesterly trend. Under the arches of this mountain system welled up in viscous molten form the granite which you see here on every hand. During the long periods of time that followed (more than 100 million years) these ancient earth wrinkles were slowly worn down by streams, rain wash, and other agents, until at last the sedimentary rocks were completely removed, and the granite was exposed over large areas.
For a long time thereafter the Sierra region was a lowland, and then it began slowly to bulge up, together with the country to the east. Finally, more violent convulsions in the interior of the earth caused the crust to break into huge blocks. Some of these blocks were pushed or tilted and began to stand out as mountain ranges, others settled back and gave rise to intermediate valleys. Thus came into existence the ranges and intermediate valleys of what is termed the Great Basin, which includes most of Utah, Nevada, and eastern California.
The largest and westernmost of these many tilted blocks is this mountain range upon which we are now. The Sierra Nevada, like some of the others, has been rotated on its long axis, and the method of uplift is very well demonstrated by this first model. (Lecturer at this point may operate lever which causes “Sierra block” to lift.) This tilting did not take place in one gigantic upheaval as the model might lead one to believe. There were many gradual upward movements, separated by great lapses of time, and finally two distinct periods of more rapid upthrusting. Since then there have been only occasional minor disturbances, and the range as a whole has been essentially stable. The last noticeable movement occurred in 1872, at the time of the Inyo earthquake. The range then lifted twenty feet, and Owens Valley on the east dropped twenty feet. At any rate, a fresh break twenty feet high is visible for miles along the eastern foot of the Sierra Nevada.
With the beginning of the uplift the streams which had found their ways about the surface of the region naturally rearranged themselves on the Sierra block. The long westward slant resulted in master streams sing down the slope to the Pacific. With successive uplifts, the slope became steeper, and the steeper the slope became, the more rapidly the rivers flowed. The greater the velocity of the streams, the deeper the canyons they cut. The Merced and the Tuolumne Rivers, two of the master streams that are now within Yosemite National Park, were affected exactly as were the others. The powerful rush of their waters from the heights carried boulders, gravel, and sand which added scouring forces and rapidly deepened their beds. Dr. Matthes has determined in his engineer’s way that the Merced (indicate on model) in the region of Yosemite Valley had in this manner cut its canyon to a depth of more than 2,000 feet. Like all water-cut canyons, the Merced gorge was V-shaped in cross sections.
Then came a change in climate. Winter conditions prevailed on the mountain tops for many thousands of years. It was at this time that the great Ice Age came on. On the Sierra, snow accumulated to the depth of more than a thousand feet. The weight of this mass caused it to compact to form granular ice. Such ice, present in great fields, is plastic like tar. Under the action of gravity, long tongues or rivers of it flowed down from the continuous fields on the heights, and ground with irresistible force into the V-shaped canyons that the streams had cut. Such a glacier flowed into the Merced Canyon and occupied the section which is now Yosemite Valley. This ice invasion, while not forming part of the great continental ice sheet of the north, was nevertheless contemporary with it. The Ice Age, as a whole, comprised really four successive epochs of glaciation. In the Yosemite region Dr. Matthes found thus far clear evidence of two gIaciations and some indication of a possible third, still earlier one.
On this next model (moving to Model No. 2) we have indicated the extent of the ice of the last glacial stage over that section of Yosemite National Park that is drained by the Merced River. This model is made to scale and is a true relief of Yosemite National Park. The model first considered, I believe, was understood by all to be diagrammatic and served chiefly to convey the idea of the uplift of the Sierra block. Here (indicate on relief model) is the crest of the range. Note that it serves as the eastern boundary of Yosemite National Park. The steep eastern scarp is readily recognized here, and this great expanse of park area is all on the long western slope. This drainage system of the southern section of the park is the Merced, and here is Yosemite Valley within the Merced Canyon proper. In the north is the Tuolumne system, and Hetch Hetchy Valley, in the canyon of the Tuolumne, was, before man made it a water reservoir, similar in many respects to the Yosemite Valley.
On this model we may point out definitely the extent of the ice during the last invasion. Dr. Matthes has traveled about over this area drained by the Merced and has carefully searched out and mapped the limits reached by the ancient glaciers. His maps enable us to show the extent of the glaciers of each of the two clearly attested ice invasions. When this model is complete, the areas covered by the earlier, more extensive flow will be indicated in a distinctive color.
Here in the Tuolumne country similar ice masses pushed down from the heights and occupied canyons at lower levels. Dr. Matthes’ field work has not yet included this area and, consequently, our model cannot show it.
From this map, however, one may obtain a comprehensive idea of how the Merced ice formed on the mountain heights and flowed into the canyons below. During the earlier stages of the ice age the Yosemite Valley was filled to its rim. Before passing on to the next model, let me point out that during the last invasion the Merced glacier did not completely fill the gorge but reached only as far as the Bridalveil Meadows, a short distance below the Bridalveil Fall. With that point in mind let us consider the next model.
(At Model No.3)
Here we have the Yosemite Valley modeled on a much larger scale than were the two preceding models. It represents that period of some 20,000 years ago when the ice was melting back, and upon it we may study the effects of glacier action within the famous Valley itself.
I believe any of us, whether or not we have given much consideration to the science of geology, would expect to find a canyon that had been carved by ice rounded at the bottom. It is characteristic of glaciers to gouge out a trough that is U-shaped in cross section. When we examine the floor of Yosemite Valley, we find no suggestion of a U-shaped trough. The floor of the Valley is perfectly flat. Perhaps it was this fact that led J. D. Whitney, the first State Geologist of California, to stoutly declare that it was preposterous to suppose that ice had entered the gorge.
The key to the explanation of why the floor is flat is to be found in this ridge which extends across the gorge. (Indicate El Capitan moraine.) It will be remembered that we pointed out at the last model that the ice of the last invasion extended as far as El Capitan, and there melted as fast as it moved forward. In its crushing advance of miles through the upper Merced Canyon, much rock and earth debris was gouged up and incorporated in the ice mass. Material that had fallen from cliffs above had ridden upon the surface of the slowly-moving river of ice, and here at the snout of the melting glacier this debris was dumped to make a terminal moraine. It is readily seen today and my be identified as a glacial moraine by the fact that it contains many boulders derived from the High Sierra. If you care to visit it, drive to the El Capitan Bridge. You will find that the road which crosses the valley at that point is cut into the base of the moraine, throughout a good part of its length. That ridge served as a natural dam.
When the climate changed for the better, about twenty or thirty thousand years ago, the ice that had choked Yosemite Valley began to melt back. The water that accumulated from this melting, backed up behind the terminal moraine, and the lake shown on this model (ancient Lake Yosemite) resulted. Dr. Matthes has computed that the water was over 300 feet deep.
Immediately after the melting of the glacier, then, the original U-shaped bottom of the ice-carved trough was occupied by a lake.
From a geologist’s standpoint, it was but a short time before that lake disappeared. The Merced River, flowing from the melting ice above carried great quantities of sand and disintegrated rock. The many tributary streams, all flowing in torrential floods from melting ice above the rim also carried sand to the ancient lake. Here on the model we have indicated the beginning of the formation of a delta which displaced the water. Geologically speaking, that filling-in process took place quickly (probably several thousand years). We are walking today upon the flat sand fill, and were it possible to remove the sand, we would find the original U-shaped bottom of the Yosemite trough.
That same filling-in process may be witnessed in the park today. Mirror Lake, not a remnant of the old glacial lake, but formed recently as the result of a rock slide damming Tenaya Greek, is being destroyed exactly as was the ancient Lake Yosemite. If you visit Mirror Lake, will discover that a delta is pushing out into the upper end of the lake. Every year Tenaya Creek adds a great deal of sand to the delta, and, if you return twenty-five years from now, you might find that Mirror Lake had practically disappeared. All of our high country lakes, charming bodies of water occupying glacial basins in the rock, are undergoing the same change but at such extremely slow rates that we need not feel apprehensive about the possibility of their early disappearance.
Dr. Matthes has worked in considerable detail the depth to which valley had been cut by the Merced prior to the ice age. Hence, he can tell us how much additional deepening was done by the glacier. He finds that opposite El Capitan the preglacial chasm was 2,400 feet deep (measuring from the brow of El Capitan). The valley there is now 3,000 feet deep; hence, the glacier must have cut away fully 600 feet of granite. The depth of excavation increased steadily toward the head of the valley. Opposite Glacier Point the preglacial depth was about 2,100 feet. It is now 3,200 feet, and as there is at least 100 feet of sand filling the bottom of the Valley, it follows that the ice there cut away the granite to a depth of no less than 1,200 feet.
The glacier could not have achieved this astounding work of excavation, both in depth and in breadth, if it had not been for the fact that granite in its path was divided by many natural joint cracks (call attention to joints in cliffs). It was thus enabled literally to quarry the granite out block by block. Above the Yosemite and immediately below, on the other hand, the granite is very sparsely jointed. It was so solid that the glacier found but few blocks that it could pluck out, and its work there, consequently, was restricted to grinding and polish—very slow, ineffective processes in the case of hard rocks such the granite of the Yosemite region. That is the reason why the broad, spacious Yosemite Valley begins so abruptly at the mouths of Tenaya Canyon and the upper Merced Gorge and contracts so abruptly at its lower end.
That, briefly, is the accepted explanation of how this valley was formed. It was not the result of an earthquake, which opened up a tremendous rent in the earth, as was maintained by some early observers; not the result of “the bottom falling out, permitting a chunk of the earth’s crust this size and shape to settle,” as J. D. Whitney thought; not due to ice action alone, and not due to water action alone; but a combination of the forces of water and ice gave us this world’s most remarkable valley. Now, if you please, we will move on to the last of the models. Here we have the Valley as it is today, and upon it are shown all roads, trails, streams, and other details. We will take opportunity here to answer questions, whether they pertain to geology or to your hike in the morning.
(Some members of the party will move away from the group at this point, and it is well to precede questions with announcements of field trips. This is an especially good opportunity to emphasize the High Sierra Hike, for a part of the route may be indicated on the model.)
(Questions that may be expected are of wide variety. Those that pertain to geology and those, experience has shown, are most frequently asked, are dealt with in the geology section of general information in the Information Manual.)
It will be noted that in the story given above are gaps and paucity of details. Experience has shown that it is more desirable to emphasize the four simple points (1) uplift of Sierra block (2) the stream-cut canyon (3) ice action and (4) subsequent happenings, than to give a lecture that will keep a crowd standing before the models for too long a time. It is seldom, indeed, that this talk does not strike a spark. Numbers of especially interested individuals will remain to ask questions. All of the group, including those not especially interested, have remained long enough to get the brief, connected story. Had it been made a lengthy discussion, many listeners would have moved on to other exhibits before conclusion was reached.
Not infrequently, a fundamentalist in the group of listeners will interject questions and suggestions contemplated to be embarrassing to the lecturer. It is safe to say that sympathy is with the lecturer, and the best policy is to ignore tactfully the would-be trouble maker. An argument before a group of museum visitors is undignified, and in its last analysis nothing can be gained by an exchange of views.