document.write=function() {}; document.writeln=function() {};
Home Archived Aug 4, 2016
(i)

USGS Geology in the Parks

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.

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.

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

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.
Skip the terranes of the Western Domain and go on to the Metamorphic Core Domain

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.

Accessibility FOIA Privacy Policies and Notices

Take Pride in America logo USA.gov logo U.S. Department of the Interior | U.S. Geological Survey
URL: http://geomaps.wr.usgs.gov/parks/noca/nocageol5.html
Page Contact Information: Webmaster
Page Last Modified: 02-Oct-2014@12:35