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North Cascades Geology

Forbidden Peak
Forbidden Peak.

World Class and Close to Home

The North Cascade Range in Washington State is part of the American Cordillera, a mighty mountain chain stretching more than 12,000 miles from Tierra del Fuego to the Alaskan Peninsula, and second only to the Alpine-Himalayan chain in height and grandeur. Although only a small part of the Cordillera, mile for mile, the North Cascade Range is steeper and wetter than most other ranges in the conterminous United States. In alpine scenery and geology, the range has more in common with the coast ranges of British Columbia and Alaska than it does with its Cordilleran cousins in the dry Rocky Mountains or benign Sierra Nevada. Although the peaks of the North Cascades do not reach great elevations (high peaks are generally in the 7,000 to 8,000-foot range), their overall relief, that is, the relatively uninterrupted vertical distance from valley bottom to mountain top, is commonly 4,000 to 6,000 feet, a respectable height in any world-class mountain range. Much of the range is roadless wilderness preserved from commercial exploitation by inclusion in North Cascades National Park, the Ross Lake and Lake Chelan National Recreational Areas, and several dedicated wilderness areas managed by the U.S. Forest Service.
Location of the North Cascades.
Location of the North Cascades. This guide covers the lined area.

Rocks of the North Cascades record at least 400 million years (see geologic time) in the history of this restless Earth-time enough to have collected a jumble of different rocks. The range is a geologic mosaic made up of volcanic island arcs, deep ocean sediments, basaltic ocean floor, parts of old continents, submarine fans, and even pieces of the deep subcrustal mantle of the earth. The disparate pieces of the North Cascade mosaic were born far from one another but subsequently drifted together, carried along by the ever-moving tectonic plates that make up the Earth's outer shell. Over time, the moving plates eventually beached the various pieces of the mosaic at a place we now call western Washington.


As if this mosaic of unrelated pieces were not complex enough, some of the assembled pieces were uplifted, eroded by streams, and then locally buried in their own eroded debris; other pieces were forced deep into the Earth to be heated and squeezed, almost beyond recognition, and then raised again to our view.

About 35 million years ago a volcanic arc grew across this complex mosaic of old terranes. Volcanoes erupted to cover the older rocks with lava and ash. Large masses of molten rock invaded the older rocks from below. The volcanic arc is still active today, decorating the skyline with the cones of Mount Baker and Glacier Peak.

The deep canyons and sharp peaks of today's North Cascades scene are products of profound erosion. Running water has etched out the grain of the range, landslides have softened the abrupt edges, homegrown glaciers have scoured the peaks and high valleys and, during the Ice Age, the Cordilleran Ice Sheet overrode almost all the range and rearranged courses of streams. Erosion has written and still writes it own history in the mountains, but it has also revealed the complex mosaic of the bedrock. There is much to be learned about the processes of nature in this special place. Before setting out into the rather complex terrain of North Cascade geology, the reader had best be equipped with some basic geologic vocabulary and conceptual tools. The following pages introduce these words and tools and includes briefings on minerals and rocks, geologic time, and the fundamental geologic structure of the North Cascades. The part of the website also introduces readers to the theory of plate tectonics. Some understanding of this "unified field theory" of geology makes it possible to place the North Cascades in the big picture of geologic processes which operate on a worldwide scale.

Something extra: Catching the Clouds
Something extra: Early Encounters with the Rocks

On to Rocks and Minerals of the North Cascades
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Material in this site has been adapted from a book, Geology of the North Cascades: A Mountain Mosaic by R. Tabor and R. Haugerud, of the USGS, with drawings by Anne Crowder. It is published by The Mountaineers, Seattle.

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