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Yosemite Nature Notes 46(2) (1977)


Placing the Geologic and Human History of Yosemite in Perspective

Richard Balogh

Most people will agree that 100 years is a long time. But compared to the time that Yosemite’s landscape has been evolving, 100 years represents less than one minute of one human lifetime. Since our lifetime does not span the millions or billions of years necessary for a landscape to evolve, man cannot grasp the relative importance of such large numbers. For example, if you were one billion seconds old, how many years old would you be? Most of the Park visitors asked this question by the author have replied five or ten years. But the real answer is 32 years. Similarly, when the geologic history of Yosemite is explained in terms of actual time, one does not appreciate the relative time spans involved. The following simplified geologic history of Yosemite presents the notion of geologic time in a way that may be easier to understand.

Since everyone is familiar with one year of time, let’s condense the geologic history of Yosemite into just one year. If January 1 represents the time of formation of the oldest rock in the Park, then today’s date is represented by December 31, just before midnight, and all the events in the formation of the landscape will take place some time in between. Since the oldest rocks in the Park were formed over 500 million years ago, we are condensing about 500 million years into one year and, as a consequence, each day represents more time than usual. In fact, one day represents 1.4 million years, one hours represents 58,000 years, one minute represents 970 years, and one second represents 16 years.

From January 1 to September 21, sediments were being deposited on the floor of a large inland sea that covered what is now the Sierra Nevada. At the end of this period, the layers of deposits accumulated to a thickness of approximately 11 miles. The weight of overlying layers compressed and lithified the sediments into rocks of various types — sandstone, claystone, and limestone, to mention only three. The top layers of this rock were pushed above sea level and folded during a period of five days, September 21 to September 26, and formed the ancient Sierra Nevada Range. Geologists believe this ancient range resembled the Appalachian Range of today in that the rocks were folded into northwest-trending ridges. From September 26 to October 1, all the granitic rock of the Yosemite region cooled and solidified from a hot liquid state while covered with an estimated five miles of rock overburden. The granite slowly melted its way upward owing to its lighter density but never flowed as a liquid on the earth’s surface. Only an occasional volcano, composed of a rock related to granite, hinted of the large mass of granite cooling below. Heat lost by the cooling granite changed the older sandstone, claystone, and limestone into quartzite, slate, and marble, respectively. Erosion was dominant from October 1 to November 20, as rivers slowly removed almost all of the five miles of rock capping the granite, and by November 20 had exposed the granite itself. November 20 to December 31 was a period of major uplift and more erosion as the Sierra attained its present height. Since the Sierra was uplifted as a single block which ruptured along a fault system on the east side and only warped the rocks on the west side, erosion in the Yosemite region was dominated by westward-flowing rivers such as the Merced. By December 31, Yosemite Valley had been eroded by the Merced River, the hanging valleys of Yosemite Creek and Bridalveil Creek had formed due to the slower erosion rate of north-south oriented streams compared to the steeper and faster eroding Merced River. All large valleys that are present today had formed by December 31 but their shape in cross section was “V”-shaped - a characteristic of canyons eroded by running water. It was not until glaciers crept down those valleys, during a period of slightly less than 17 hours on December 31, that their cross sections assumed the “U” shape of glacially modified valleys.

Although the ancient glaciers of the Yosemite region were responsible for widening the valleys, they were only here for a short time compared to the age of the rocks. They may be likened to icing on a cake where the cake had been baking for over 364 days and the icing was added on the 365th day. The last of the four successive ice ages, which collectively lasted for almost 17 hours, came to an end with warming temperature at six minutes before midnight on December 31. Four minutes before midnight began the “little ice age” when smaller glaciers of the Sierra were born which have lasted to the present time.

Thus far, we have reviewed the major stages in the geologic history of Yosemite without mention of man-made alterations to the landscape. At least by December 31, four minutes before midnight, Indian communities existed in the Park area but their impact on the landscape was minimal. However, from the time that Yosemite was discovered by the first white man on December 31, nine seconds before midnight, man has done much to change the landscape. Cultural additions are almost insignificant when compared to such giant features as Half Dome and El Capitan. Even so, roads and buildings are evident in the landscape, particularly in Yosemite Valley.

Perhaps the time period that is most striking is the last nine seconds, because in that time man has changed the landscape to a degree that is admittedly small, but second in magnitude only to the changes brought on by Mother Nature herself.



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