Geological Survey Professional Paper 160 Geologic History of the Yosemite Valley

ANTIQUITY OF THE SIERRA NEVADA AND THE YOSEMITE VALLEY

The story of the evolution of the Yosemite Valley is so intimately interwoven with the history of the rise of the Sierra Nevada that it can scarcely be told with out reference to that larger history. Indeed, every stage in the development of the valley corresponds to a definite epoch in the growth of the range and assumes its real significance only when considered in the light of that relationship. It is but logical, therefore, to begin with a brief sketch of the history of the Sierra Nevada.

At once arises the question of the length, of time involved. How far back does this history take us? Mountains are by many people supposed to stand forever, permanent and unchanging. "As old as the eternal hills" is a familiar phrase. Yet to those who study the earth nothing is more evident than that mountains have limited, measurable spans of life. They come into being by upheaval and, through the erosive action of streams, glaciers, and atmospheric agencies, are worn down by degrees, sometimes to mere hills or even to plains. The Sierra Nevada is decidedly a new range in the geologic sense, a still youthful feature of the continent. Its youth is manifest from the fact that it has not yet lost significantly in height by erosion. Though deeply furrowed by canyons, it still ranks with the highest ranges in this country and bears summits that retain in part the subdued outlines which they had acquired prior to their elevation. The Sierra Nevada has stood with its present height and form about a million years—an astounding stretch of time compared with the few thousand years of recorded human history, yet very brief compared with the total length of geologic history, which runs into billions of years. Viewed in its proper historic perspective the Sierra Nevada is but the latest of several successive mountain ranges, or mountain systems, that have in turn occupied the same place. Each of these ancestral mountain systems must have been in existence a very long time, for each was reduced to ridges and hills of only moderate height, in part to an undulating lowland. The time required for their demolition was doubtless between 50,000,000 and 100,000,000 years for each system.

The major facts as to the character, structure, and history of at least two of these ancestral mountain systems are indicated by certain rock formations that are the roots, so to speak, of those earlier mountains, incorporated in the present range. And the approximate time of creation of each of these earlier mountain systems is indicated by the fossil remains that are preserved in their rocks, each geologic epoch being known by its characteristic life forms.

In the table of geologic time divisions on page 23 the outstanding events in the geologic history of the Sierra region are set forth in chronologic order, each referred to its proper era, period, and epoch as definitely as the knowledge at hand permits. The figures for the duration of the successive time divisions are taken from the table which the late Professor Barrell³⁰ compiled from calculations of the age of uranium minerals from different parts of the world. The age of these minerals is computed from the ratio of lead to uranium present in them, the rate at which uranium breaks down and is reduced to lead by atomic disintegration being accurately known. The figures given are really the means of the maxima and minima of Professor Barrell's table, these means agreeing closely with the results obtained from the more refined calculations made by Holmes and Lawson.³¹ These measures of geologic time are much greater than those that have been current among scientists in the past, but they doubtless afford much closer approximations to the truth than the shorter measures, for they are of an order of magnitude that is consistent with many geologic facts, notably with the extremely slow rate at which mountains are worn down.

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³⁰Barrell, Joseph, Rhythms and the measurement, of geologic time: Geol. Soc. America Bull., vol. 28, pp. 884-885, 1917.

³¹Holmes, Arthur, and Lawson, R. W., Factors involved in the calculation of the ages of radioactive minerals: Am. Jour. Sci., 5th ser., vol. 13, p. 342, 1927.

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It will be seen from the table that the first of the two ancestral mountain systems of which the roots are still recognizable came into being in the Permian epoch, near the end of the Paleozoic era, more than 200,000,000 years ago. It was formed by the uplifting and folding of a great series of layers of slate, shale, and sandstone—originally mud, silt, and sand derived from a land mass lying mostly to the west of the present border of the continent and laid down in an arm of the Pacific Ocean. Folded in with these sediments, which aggregated thousands of feet in thickness, were beds of lime (calcium carbonate) deposited presumably in shallow water at times when the land was low and the streams brought down but little sand or mud.

In the long stretches of time that ensued the wrinkles in the earth's crust thus produced were in large part worn away, and finally the region again sank below the level of the sea and became a place of deposition. For millions of years new layers of mud, silt, and sand, together with beds of volcanic material, accumulated upon the submerged remnants of the first mountain system, and then, at the end of the Jurassic period, about 130,000,000 years ago, there came another upheaval, the new strata were folded and crumpled and were invaded by molten granite from below, and there arose a second system of mountain ranges that occupied most of eastern California and large areas in adjoining States. Throughout the Cretaceous period, which followed upon the Jurassic, this second mountain system was being gradually worn down, until by the beginning of the Tertiary period only ridges of moderate height were left.

Sequence of mountain-building events in Sierra region
[Read from bottom up]

The present Sierra Nevada was not formed until a long time thereafter. It assumed its present height and form about the dawn of the Quaternary period; but throughout most of the Tertiary period, especially in the later half, the region was the scene of repeated disturbances and minor mountain-building movements that finally led up to the culminating uplift. It is with these preliminary happenings in the Tertiary period, which may be traced back with some confidence fully 20,000,000 years, that the history of the Sierra Nevada properly begins, for it was in consequence of those happenings that many features which are now part and parcel of the physiognomy of the range were developed—among others the Yosemite upland, the parallel crests of the High Sierra, and the course of the Merced River, including that part in which the Yosemite Valley was finally elaborated.

It will be helpful to the reader pursuing this story to have at the outset some insight into the structure and general make-up of the Sierra Nevada, and accordingly these will first be explained.