Spectacular water falls grace the sheer walls of Yosemite
National Park, Sierra Nevada Range. Photo by S.R. Jones.
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Although the Sierra Nevada and Cascade Range form a nearly continuous barrier along the western edge of the United States, the two ranges really have very little in common. They have been and continue to be formed by quite different geological forces and processes.
The rocks that form the backbone of the Sierra Nevada are mostly granitic rocks that formed during the Mesozoic Era, when dinosaurs roamed the Earth. At that time, an arc-shaped chain of volcanoes, similar to the present-day Cascade volcanic arc, erupted where the Sierra Nevada now stands. Rising through older Paleozoic rock, molten rock erupted at the surface as lava, but most solidified deep within the earth, forming the gray granitic rocks familiar to any Sierra traveler.
Although from a distance the Sierran rock looks quite similar, it is actually made up of many individual rock bodies that formed from repeated intrusions of magma over many millions of years.
Even as they grew, erosion was wearing away these Mesozoic Era volcanoes. By Late Cretaceous time, about 70 million years ago, the once-deep granitic rocks began to be exposed at the Earth's surface. By a few tens of millions of years ago, so much of the upper part had worn away that the surface of the ancient range had a low relief of just a few thousand feet.
It wasn't until quite recently, geologically-speaking, that the Sierra Nevada range as we know it today began to grow. During the Miocene Epoch, less than 20 million years ago, the continental crust east of the Sierra Nevada began to stretch in an east-west direction. The crust broke into a series of north-south-trending valleys and mountain ranges—the beginning of the Basin and Range province.
The entire Sierra Nevada can be thought of as an enormous tilted fault block.
Less than five million years ago, the range that we now know as the Sierra Nevada began to rise along its eastern margin. Through a combination of uplift of the Sierran block and down-dropping of the area to the east, the Sierra rose upward. Rising far more steeply to the east than the west, the entire Sierra Nevada can be thought of as an enormous tilted fault block with a long, gentle slope westward to California's Central Valley and steep eastern slope.
Not long after the Sierra uplift began, the Earth cooled, marking the beginning of the Pleistocene (Ice Age) Epoch. Glaciers grew in the Sierra highlands and made their way down former stream channels, carving U-shaped valleys. The sheer walls and hanging valleys of Yosemite National Park are a product of this chilly past.