The story of Yosemite began about 500 million years ago when the Sierra Nevada region lay beneath an ancient sea. Thick layers of sediment lay on the sea bed, which eventually was folded and twisted and thrust above sea level. Simultaneously molten rock welled up from deep within the earth and cooled slowly beneath the layers of sediment to form granite. Erosion gradually wore away almost all the overlying rock and exposed the granite. And even as uplifts continued to form the Sierra, water and then glaciers went to work to carve the face of Yosemite. Weathering and erosion continue to shape it today.
Tuolumne Meadows and the High Country - This section of Yosemite has some of the most rugged sublime scenery in the Sierra. In summer the meadows, lakes, and exposed granite slopes teem with life. Because of the short growing season, the plants and animals take maximum advantage of the warm days to grow, reproduce, and store food for the long, cold winter ahead.
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The Tioga Road (California 120), crosses this area. This scenic highway, originally built as a mining road in 1882-83, was realigned and modernized in 1961. The road passes through an area of sparkling lakes, fragile meadows, domes, and lofty peaks that only 10,000 years ago lay under glacial ice. Scenic turnouts along the road afford superb views. At Tioga Pass the road crosses the Sierra's crest at 9,945 feet, the highest automobile pass in California.
Tuolumne Meadows (at 8,600 feet) is the largest sub-alpine meadow in the Sierra. It is 55 miles from Yosemite Valley via the Tioga Road. Long a focal point of summer activity, it is also growing in popularity as a winter mountaineering area. In the summer Tuolumne Meadows is a favorite starting point for backpacking trips and day hikes. The meadows are spectacular in early summer, abounding in wildflowers and wildlife.
Giant Sequoia Groves - The Mariposa Grove, 35 miles south of Yosemite Valley, is the largest of three Sequoia groves in Yosemite. The Tuolumne and Merced Groves are near Crane Flat. Despite human pressures, these towering trees, largest of all living things, have endured for thousands of years. Only in recent years, however, have we begun to understand the Giant Sequoia environment. During the last 100 years protection has sometimes been inadequate and sometimes excessive. For example, in the late 1800s tunnels were cut through two trees in the Mariposa Grove. Conversely, good intentions created another problem, protection from fire has resulted in adverse effects.
Sequoias are wonderfully adapted to fire. The wood and bark are fire-resistant. Black scars on a number of large trees that are still prospering indicate they have survived many scorching fires. Sequoia reproduction also depends on fire. The tiny seeds require minimal soil for germination, and seedlings need sunlight. Historically, frequent natural fires opened the forest, thinned out competing plant species, and left rich mineral soil behind. But years of fire suppression have allowed debris, such as fallen branches, to accumulate, stifling reproduction and allowing shade-tolerant trees to encroach. Prescribed fires, intended to simulate natural fires and improve the health of the forest, are now set by the National Park Service.
As you look at these trees, keep in mind that they have been here since the beginning of history in the western world. The Mariposa Grove's Grizzly Giant is 2,700 years old and is thought to be the oldest of all Sequoias.
Yosemite Valley - "The Incomparable Valley", it has been called, is probably the world's best known example of a glacier-carved canyon. Its leaping waterfalls, towering cliffs, rounded domes, and massive monoliths make it a preeminent natural marvel. These attributes have inspired poets, painters, photographers, and millions of visitors beginning with John Muir for more than one hundred years. Nowhere in Yosemite is the sense of scale so dramatic.
Yosemite Valley is characterized by sheer walls and a flat floor. Its evolution began when alpine glaciers lumbered through the canyon of the Merced River. The ice carved through weaker sections of granite plucking and scouring rock but leaving harder, more solid portions—such as El Capitan and Cathedral Rocks—intact and greatly enlarging the canyon that the Merced River had carved through successive uplifts of the Sierra. Finally the glacier began to melt and the terminal moraine left by the last glacial advance into the valley dammed the melting water to form ancient Lake Yosemite, which sat in the newly carved U-shaped valley. Sediment eventually filled in the lake, forming the flat valley floor you see today. This same process is now filling Mirror Lake at the base of Half Dome.
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In contrast to the valley's sheer walls, the Merced Canyon along California 140 outside the park is a typical river-cut, V-shaped canyon, for the glaciers did not extend this far. Back from the rim of the valley itself, forested slopes show some glacial polish. But for the most part these areas also were not glaciated.
The valley is a mosaic of open meadows sprinkled with wildflowers and flowering shrubs, oak woodlands, and mixed-conifer forests of ponderosa pine, incense-cedar, and Douglas-fir. Wildlife from monarch butterflies to mule deer and black boars flourishes in these communities. Around the valley's perimeter, waterfalls, which reach their maximum flow in May and June, crash to the floor. Yosemite, Bridalveil, Vernal, Nevada, and Illilouette are the most prominent of these falls, some of which have little or no water from mid-August through early fall.
The question about Yosemite's origin proved difficult to answer. Controversy raged over several theories. Josiah D. Whitney, director of the California State Geological Survey in the 1860s, theorized that a cataclysmic event caused the valley floor to drop. He believed that this left behind a valley with steep walls, Yosemite Valley.
John Muir had a different idea. He developed his theory after discovering remnants of glacial action and a remainingglacierin the high Sierra. John theorized that these were the remains of glaciers that once gouged out Yosemite Valley.
In 1913, geologist Francois Matthes began a field study of Yosemite Valley's origins. His book,Geologic History of the Yosemite Valley, was printed in 1930. He confirmed John Muir's hypothesis of glacial formation but altered many of its details. More recently, geologist King Huber updated the geologic information concerning Yosemite's origins with more current data (The Geologic Story of Yosemite National Park, 1989).
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Event Years Before Present
Beginning of the Earth 4.5 billion years before present
Unicellular life 3.4 billion years before present
Multicellular life 700 million years before present
Animals with backbones first appear 500 million years before present
Oldest rocks in Yosemite 500 million years before present
Fish are major life form 400 million years before present
Age of Dinosaurs begins 245 million years before present
Breakup of Pangaea, continents begin to move 200 million years before present
Subduction begins off coast of what is now California. Ancient 200 million years before present
Sierra Nevada range starts to form.
Formation of the granite rock mass (batholith) begins. The 195 million years before present
Batholith would eventually become the Sierra Nevada range
First Birds appear on Earth 175 million years before present
Flowering Plants first appear 150 million years before present
Age of Mammals begins/Dinosaurs die out (become extinct) 65 million years before present
Ancient volcanic Sierra Nevada range is eroded down to rolling hills 65 million years before present in Yosemite, exposing the granite batholith
Movement at the plate boundaries off California changes 60 million years before present from subduction to a strike-slip movement
Uplift and tilting of the Sierra Nevada range begins 25 million years before present
Horses, apes, dogs, and cats appear 10 million years before present
Hominids (human ancestors) 3.5 million years before present
Significant uplift of Sierra Nevada. Rivers begin 5 million years before present to cut a V-shaped Canyon in Yosemite
Cooling throughout the Northern Hemisphere resulting 2.5 million years before present in the great ice age
V-shaped canyon widened to a U-shaped Yosemite valley 1.2 million years before present by glaciers
Humans appear on earth 100,000 years before present
End of most recent ice age; Last glaciers retreat from Yosemite 10,000 years before present
leaving behind Lake Yosemite
Eastern side of the Sierra Nevada continues Present to uplift causing occasional earthquakes
V-Shaped ValleyThe story begins some 200 million years ago with thesubductionof the oceanicplateand the continental plate off the coast of what is now California. An ancient volcanic Sierra Nevada range was forming.
This was followed 5 million years later by formation of a large granite rock mass orbatholith. Thisgranitebatholith was exposed 65 million years ago after erosion of the overlyingvolcanicSierra Nevada range.
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About 60 million years ago, movement at the plate boundaries off California U-Shaped Valleychanged fromsubductionto a strike-slipmovement. This led to changes that would causeupliftand tilting of the Sierra Nevada range 25million years ago. This in turn caused rivers and streams to flow faster, cuttingV-shapedriver canyons.
About 2.5 million years ago, the world's climate cooled, and 1.2 million years agoglaciersformed throughout the Sierra Nevada range. These Plate Subductionglaciers carvedU-shapedvalleys.Erosionandweatheringcontinue to change the face of Yosemite today.
Overview: Formation of the Sierra Nevada Range
Granitic rocksare a type ofigneousrock. Igneous rock is formed by the cooling and solidification of molten material (magma). There are two types of igneous rock:volcanicand plutonic. Yosemite was formed from plutonic rock that cooled deep beneath the surface of the Earth. The bedrock of much of the Sierra Nevada range is part of a vast field of rock that extends downward many miles.
This field of rock is called abatholith(from the Greek word bathos, meaning deep, and lithos, meaning rock). The batholith is not one uniform mass ofgranite. It is made up of a group of individual masses of rock calledplutons(named for Pluto, god of the underworld). Plutons can vary in size from 1.6 kilometers (1 mile) to many kilometers in extent. Some plutons are next to each other. Other plutons are separated by areas ofmetamorphicrock. The Sierra Nevada batholith is a collection of granite plutons.
The image at right contains an animation depicting this process. Click the right arrow to begin the animation. When the right arrow reappears, click it again to see the next step. (You can also download theFlash Playerif you need it to play the animation.)
The plutons formed during separate episodes of magma intrusion. Molten rock, lighter than the older host rock surrounding it, rose slowly and intruded into these host rocks. As the magma came within a few kilometers of the surface, it began to cool. After a long time the magma cooled completely. Themineralsin the magma crystallized and solidified to form the igneous granitic rock of the Sierra Nevada.
Overview: Granitic Rocks of Yosemite
Over 95% of therockin Yosemite isgranitic. Most people knowgraniteas a hard gray rock with a salt-and-pepper appearance. lndividual grains of light and dark minerals make up the rock, giving Click to see Granite Close-upit a salt-and-pepper look.
There are many different types of granitic rock in Yosemite, but they all contain two basic light-coloredmineralfamilies:feldsparandquartz. They can also have smaller amounts of the darker mineralsbiotite micaandhornblende.
Yosemitegranitic rocksare grouped into different types based on the relative proportion of the minerals found in them and on the size of their mineral crystals.
Sierra Nevada UpliftOverview: Sierra Nevada Uplift
Large amounts of sand, silt, and mud eroded from ancient mountain ranges surrounding the ancient sea and settled to the sea floor in layers, eventually becomingsedimentaryrock. Great forces deep inside the Earth's crust warped these sedimentary rock layers, lifted them above sea level, and folded them into a mountain range extending northeast to southwest.
These forces changed thesedimentaryrock intometamorphicrock. Metamorphic rocks can still be found along highway 140 in the El Portal area. As the mountains rose, molten rock began to form beneath them. The molten rock eventually cooled and solidified into thegranitic rockswe see in Yosemite today. Over millions of years,weatheringanderosionstripped away the overlyingmetamorphicrocks and created a landscape of rolling hills, broad valleys, and meandering streams.
Overview: Valley Formation
Movement at the San Andreas Fault stretched the land east of the Sierra Nevada. As this region expanded, the lightweight Sierra crust began to rise and tilt to the west while at the same time the basin to the east dropped down.
On the western slope of the Sierra Nevada, a once gently rolling landscape became steeper, causing meandering streams to flow faster. These faster flowing streams cut more deeply through the landscape, carvingV-shapedcanyons up to 2,000 feet deep.
The processes ofuplift, tilt, anderosioncontinue along the steep eastern border of the Sierra Nevada range although at a less dramatic rate. Uplift of the Sierra Nevada is approximately 3.8 cm (1½ inches) per 100 years in the Yosemite region; erosion is wearing down these mountains at about the same rate.
Overview: The Coming of the Glaciers
MorainesAn early glaciation 1.2 million years ago filled Yosemite Valley to the brim and excavated the valley. Thisglacieradvanced down the deep, narrow canyons created by the rivers. In places such as Yosemite Valley, glacial ice traveling through was thousands of feet deep. Half Dome stood 900 feet above the ice, but many peaks to the north were engulfed.
The grinding, gouging action of the heavy river of ice eroded the canyons and valleys and widened and deepened them intoU-shapedtroughs.Jointsand cracks in thegraniteallowed the glacier to erode out great blocks of granite at vulnerable points and carry Moraine Near Brialveil Fallthem away.
In other areas the glacier merely scraped, buffed, and polished the granite surface. Later glaciations did not fill the valley as much as this early glacier, and did little to further change the valley, leaving spires such as the Lost Arrow and Sentinel Rock. These formations would have been destroyed if glaciers had filled the valley to its rim. The spires were formed byweatheringprocesses over the last million years, long after the end of the extensive glaciation that filled the valley to its rim.
The last period of glaciation in Yosemite Valley, called the Tioga Glaciation, began 30,000 years ago and ended about 10,000 years ago. The glacier'sterminal moraine(rock and rubble deposited in front of the glacier) dammed the valley near the narrow western end, and the glacier's subsequent melting created ancient Lake Yosemite. Thismorainecan still be seen extending across the valley as a broad hill between El Capitan and Bridalveil Fall. This was probably the last of many Lake Yosemites that formed following periods of glaciation.
Eventually, enough sediment accumulated in Lake Yosemite to fill it completely. The process of succession describes how such lakes eventually become wetlands, then meadows, then forests. This process continues today.
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500—200 million years ago
The Sierra Nevada region was once submerged beneath a sea. Sediments slowly accumulated on the ocean floor, growing thousands of feet thick and compressing the layers into sedimentary rock.
200—80 million years ago
A slab of the Earth's crust, called a plate, began to slide under the North American continent in a process calledsubduction. Deep within the earth, tremendous heat and pressure caused the downgoing plate to melt into magma. The magma rose toward the surface, where some of it erupted to form a mountain chain of volcanoes. Much of the magma cooled underground to become granite.
60 to 10 million years ago
After subduction ceased, prolonged erosion stripped away the overlying volcanic rocks and exposed the granite. The land destined to become Yosemite National Park was made up of low mountains with shallow river-cut canyons. Hardwood forest flourished.
10 million years ago
Over the next 5 million years, the Sierra Nevada, California's "backbone," rose. The Sierran block uplifted, tilting westward, increasing the Merced River began to carve a narrow canyon. Redwood forests flourished.
3 million years ago
The Merced River carved its canyon deeper, while its tributaries cut the land more slowly. Forests began to thin as the Ice Age approached.
1—2 million years ago
A series of large glaciers flowed from the crest of the mountain range into the river canyons. Glaciers repeatedly filled the "V"-shaped Yosemite Valley, widening, deepening and carving it into a "U" shape, forming hanging valleys from which waterfalls now cascade.
15,000 years ago
Temperatures warmed and the last glacier retreated from Yosemite Valley. Its terminal moraine (rock debris) dammed the Valley 14,000 to 15,000 years ago, creating a shallow lake. Sediment eventually filled the lake, which became the flat valley floor we see today.
The same process of sedimentation continues at Mirror Lake, which is even smaller today than in this photo. Soon, sediment will completely fill in the lake, creating a meadow. Water and gravity continue to shape Yosemite's landscape. Events like the Middle Brother rockfall of 1987 and the flood of January 1997, were the most dramatic in the recorded history of the park. Be aware that rocks can fall at any time.