explain under construction USGS home NPS home USGS/NPS banner
geologic provinces banner

Pacific Mountain System


spacer image The Pacific Mountain System straddles the boundaries between several of Earth's moving plates—the source of the monumental forces required to build the sweeping arc of mountains that extends from Alaska to the southern reaches of South America. This province is subdivided into several subprovinces, including the active and sometimes deadly volcanoes of the Cascade Range and the young, steep mountains of the Pacific Border and the Sierra Nevada.

Spectacular water falls grace the rocky cliffs of Yosemite National Park
Spectacular water falls grace the sheer walls of Yosemite National Park, Sierra Nevada Range. Photo by S.R. Jones.

Cascade-Sierra Mountains

spacer image Although the Cascade Range and Sierra Nevada 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.

Sierra Nevada

spacer image 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.
spacer image 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.
spacer image 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.
spacer image 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.

Spectacular water falls grace the rocky cliffs of Yosemite National Park
The entire Sierra Nevada can be thought of as an enormous tilted fault block.

spacer image 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.
Yosemite Valley
Mists enshroud glacially-carved Yosemite Valley, Yosemite National Park. Photo by S.R. Jones.

spacer image 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.

Continue north to the Cascades Volcanoes

Dig deeper
-Subprovince index map
-Cascade-Sierra Mountains
-Pacific Border Province
-Lower California Province
Visit the USGS Cascades Volcano Observatory
Seattle region earthquake hazards
Maps and illustrations
Shaded relief with National Park locations
Shaded relief with major and subprovince boundaries
Shaded relief
Image gallery
Lassen Volcanic National Park
Yosemite National Park
North Cascades National Park

| Pacific Mountain System| Columbia Plateau | Basin and Range |
| Interior Highlands | Appalachian Highlands | Atlantic Plain |
| List of parks by province or plate tectonic setting|

horizontal bar

This site is a cooperative endeavor of the
US Geological Survey Western Earth Surface Processes Team
and the National Park Service.

Please share your comments and suggestions with us!

This page was last updated on 10/10/00