November 25, 2004
Professor James S. Aber
ES 767 Quaternary Geology
|Yosemite Valley was probably first viewed by early American explorers in 1833. This party of explorers were led across the Sierra Nevada between the Merced and Tuolumne Rivers by Joseph "Snowshoe" Walker. A member of that party remarked in his diary of seeing "water shooting out over precipices that appeared more than a mile high". The official date of dicovery is 1851 when the Mariposa Battalion led by Major James Savage entered Yosemite Valley. The Mariposa Battalion entered the valley in pursuit of Indians.
Early visitors quickly realized that Yosemite Valley was something very special and preservation was paramount. In 1864, President Abraham Lincoln signed legislation granting Yosemite Valley to the State of California. Additional land surrounding the valley was set aside as a national park in 1890. Yosemite Valley, the state grant, was blended with surrounding national park land and the current Yosemite National Park was intact by 1906.
Two prominent Californians had a disagreement over the formation of Yosemite Valley. The two important people were Josiah Whitney and John Muir. Josiah Whitney, a trained geologist, visited the valley in 1863 as head of the Geological Survey of California. Whitney's observations led him to the belief that the great valley was created by faulting. He felt that the valley was part of a large black that had simply dropped down between two faults. Whitney went on to state that glaciers had not played a role in forming Yosemite Valley. John Muir, an early environmentalist but not a trained geologist, first entered the valley in 1868. After living in and exploring the Yosemite region for several years Muir became a proponent of a glacially formed valley. Muir had traveled to Alaska and saw first hand glaciers and the landforms they created. After seeing the Alaskan glaciers Muir was thoroughly convinced that glaciers had carved Yosemite Valley. Muir actually went too far with his glacial theory. He felt that glaciers had descended from the Sierra Nevada and carved the Central Valley of California.
The United States Geological Survey sent a team of geologists to study Yosemite and settle the long running dispute. In 1913, Francois Matthes and Frank Calkins began a very long study of the Yosemite region. In 1930, their observations and conclusions were published. Their findings concluded that Yosemite Valley had been greatly modified by a series of glaciers. So, after sixty years the dispute was put to rest. John Muir had been correct with respect to glaciers helping to form Yosemite Valley. Unfortunately, John Muir had died 16 years earlier and was unable to celebrate his victory.
There are several key points that help explain the formation of Yosemite Valley. The process began roughly 25 million years ago when the Sierra Nevada mountains began a steady uplift. As the western slope increased, the Merced River carved a V-shaped canyon. Eventually the Sierra Nevada mountains were tall enough to catch their own precipitation. A cooling climate one to two million years ago combined with significant precipitation to form the first Sierra glaciers. An ice sheet developed on the Sierra crest. Valley glaciers descended into the preexisting river canyons. The glaciers straightened, widened and deepened the river valleys. This first glacial period, the Sherwin, lasted 300,000 years and ended one million years ago. The Sherwin produced the longest and most massive valley glaciers in Yosemite. The Merced River Glacier extended from the Sierra all the way to El Portal. Several classic landforms of Yosemite, like Halfdome and El Capitan, formed during the Sherwin glacial period.
The ancient Merced River canyon was straightened and widened by a valley glacier that formed today's U-shaped valley. Tributary streams with their own tributary glaciers flowed into the main Merced Glacier. For example, the Bridalveil watershed contained a glacier. This tributary glacier fed into the main Merced Glacier. When these glaciers retreated a hanging valley was left behind. Bridalveil fall descends from this hanging valley.
At the conclusion of the Sherwin glaciation, the ice retreated and Lake Yosemite formed. Lake Yosemite slowly filled with riverborn sediment. Over time sediments filled the lake and a meadow formed. This explains why Yosemite Valley does not have the classic U-shape. The bottom of the "U" is filled with sediment. What is left behind is a canyon with a wide, flat bottom and extremely steep cliffs. The cliffs are extremely steep because of vertically jointed rock. The rock walls that enclose Yosemite Valley happened to contain vertically oriented joints. So, as glaciers pushed along the walls granite sloughed off along these zones of weakness. The vertical cliffs were formed by the combination of joint orientation and glacial pressure.
The valley was invaded by glaciers at least three times in the last million years. The final glaciation, the Tioga, began approximately 60,000 years ago and ended 15,000 years ago. The Tioga glaciation produced a glacier that reached Bridalveil meadow and left behind a terminal moraine. This terminal moraine dammed up the Merced River creating the final in a series of Lake Yosemites. Sediments filled the lake forming the valley floor.
In the following sequence of scenes, the viewer will travel approximately 30 kilometers. The journey begins west of the small settlement called El Portal. El Portal is immediately west of Yosemite Valley. The trip concludes in Yosemite Valley by Mirror Lake. The journey begins at an elevation of 500 meters and ends at 1300 meters. Photographs for this journey were taken by Rich Gunther in October, 2004.
The Merced River is at an elevation of 500 meters in this location. Here the river has carved a v-shaped, curvilinear canyon. The valley glacier that modified Yosemite Valley did not reach this area. This area may represent how a preglaciated Yosemite Valley appeared over one million years ago.
The shape of the Merced River canyon at an elevation of 800 meters takes on a new appearance. The canyon is straighter and has a broader bottom. This evidence supports the idea that a valley glacier reached this area. Geologist believe that largest glacier, the Sherwin, reached this area almost one million years ago.
This area marks the beginning of Yosemite Valley. Within the last year an old humanmade dam was removed from this site. This area is significant to the floor of Yosemite Valley. Geologist believe that a natural dam formed in this area and formed the earliest Lake Yosemite. As the glacier retreated meltwater filled the trough left by the glacier and formed a natural lake. Eventually the lake filled with fluvial sediment and a level valley floor formed. In the center of this picture, the hanging valley forming Bridalveil fall can be glimpsed.
Traditional view of Yosemite Valley illustrates the valleys remarkable shape. El Capitan is the monolith on the left and Bridalveil Fall is in the hanging valley on the right. During the Sherwin glaciation ice engulfed most of this scene. Only the very top of El Capitan would have protruded above the ice. El Capitan was able to resist the ice because it did not possess any zones of weakness.
Just to the right of center there can be seen a hanging, U-shaped valley. Bridalveil Fall flows over this precipice during the wet season and into summer. However, in the fall and early winter, the fall is dry. This hanging valley formed when a tributary glacier flowed into the main valley glacier. The tributary was not massive enough to cut down to the base of main valley glacier. When the ice retreated, Bridalveil fall was left behind.
At the close of the last glaciation, the Tioga glacier began retreating up valley. During this retreat the climate temporarily cooled and the snout of the glacier stabilized at this point. Till was deposited in this meadow creating a recessional moraine. The moraine is approximately 5 meters tall. This moraine may have been the last dam that formed Lake Yosemite.
This image depicts Lake Yosemite as it may have appeared 15,000 years ago. Lake Yosemite filled with riverborn sediment and formed the horizontal valley floor of today.
Halfdome reaches up almost 1500 meters above the valley floor. Evidence collected from the face of Halfdome shows that glacial ice reached within 250 meters of the summit. The Sherwin glacier exploited the vertical joint deep within the rock to create the vertical face of Halfdome.
On the valley floor where this picture was taken sediment goes down approximately 500 meters before solid bedrock is reached. This bedrock was eroded by the Sherwin glacier. As the glacier retreated glacial sediment and river sediment filled the valley. The current floor of Yosemite Valley is very flat.
This moraine formed during the Tioga glacial period. The Merced River Glacier approached from the south and collided with the Tenaya Glacier which was flowing west. The right lateral moraine of the Merced River glacier combined with the left lateral moraine of the Tenaya Canyon Glacier to form this medial moraine. This moraine is about 15 meters tall and stretches from the Merced River to the base of Halfdome.
This view of Halfdome from "Mirror Lake" is nearly vertical. The picture does not adequately capture the 1500 meter displacement from base to summit.
Halfdome stands as silent testimonial to the forces on nature.