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North Cascades Geology

Recognizing the Mountain Mosaic: Major Terranes in the North Cascades

View of the Eldorado Orthogneiss.
View of the Eldorado Orthogneiss in the Metamorphic Core Domain as seen through a joint.

spacer image Division of the North Cascades into the Western, Metamorphic Core, and Methow Domains is a start at understanding North Cascade geology, but as each domain is a mosaic of several terranes, brought together along a variety of faults, the rock-alert traveler needs to know something about the numerous terranes. Many readers may want to skip over all these names, but, we have included some memory icons that will help sort them out.
spacer image Although we relate in this website some of the early events in the history of the terranes, before they were brought together in the North Cascades, we mostly describe the terranes in terms of where and what they are now. There are few clues to events that shaped these terranes before they assumed their present positions in the North Cascades mosaic. We have a better understanding of what happened to the terranes after the mosaic was established. We describe these later events metamorphism, faulting, burial under deposits of the Cascade Volcanic Arc, and erosion in later sections. Although in this website we discuss some of the early events in the history of the terranes, before they were brought together in the North Cascades, we mostly describe the terranes in terms of where and what they are now. The discussions of individual terranes include small iconic maps that are keyed to the larger geologic map and help identify their locations.

Rocks of the Western Domain

spacer image The Western Domain consists of a folded stack of terranes. From lowest to highest (but not oldest to youngest, as in a sedimentary pile) are:
The Nooksack terrane:
Rocks once deposited as conglomerate, sand, and mud in a submarine fan flanking a volcanic arc.
The Chilliwack River terrane:
Volcanic rocks and sedimentary rocks from a different volcanic arc some 100 million years older than the Nooksack deposits.
The Bell Pass mélange:
Bits and pieces of many things, including deep ocean deposits and pieces of ancient continental crust.
The Easton terrane:
The only well metamorphosed terrane of the Western Domain, derived from deep-ocean sands and muds and underlying basaltic ocean floor.

Continue to the Western Domain


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This site is a cooperative endeavor of the
US Geological Survey Western Earth Surface Processes Team
and the National Park Service.
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http://www.nature.nps.gov/grd/usgsnps/noca/nocageol5.html
This page was last updated on 12/1/99
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Material in this site has been adapted from a new 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