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U.S. Geological Survey Scientific Investigations Map 3144

In cooperation with Portland State University, Shannon and Wilson, Inc., and the Tri-County Metropolitan Transportation District of Oregon

A Tunnel Runs Through It—An Inside View of the Tualatin Mountains, Oregon

By Ken Walsh, Gary L. Peterson, Marvin H. Beeson, Ray E. Wells, Robert J. Fleck, Russell C. Evarts, Alison Duvall, Richard J. Blakely, and Scott Burns


Thumbnail of and link to report PDF (13.1 MB)

The Tualatin Mountains form a northwest-striking ridge about 350 m high that separates Portland, Oregon, from the cities of the Tualatin Valley to the west. Known informally as the Portland Hills, the ridge is a late Cenozoic anticline, bounded by reverse faults that dip toward the anticlinal axis. The anticline is a broad, open fold consisting chiefly of Miocene Columbia River Basalt Group, with remnants of Miocene-Pliocene Troutdale Formation and Pleistocene basalt of the Boring Volcanic Field on the flanks of the anticline. Anticlinal structures similar to the Tualatin Mountains are characteristic of the northern Willamette Valley, where the structures accommodate margin-parallel shortening of the Cascadia fore arc. Global Positioning System (GPS) results indicate that the shortening is due to the northward motion of Oregon at several millimeters per year with respect to stable North America. Some of the uplifts may contain active faults, but the structures are poorly exposed and are overlain by thick Pleistocene loess and Missoula flood deposits. Between 1993 and 1998, construction of the 3-mile-long (4500-m-long) TriMet MAX Light Rail tunnel through the Tualatin Mountains provided an unusual opportunity to investigate the geological structure and history of the Tualatin Mountains. This report is a collaborative effort among the tunnel geologists and the U.S. Geological Survey (USGS) to document the geologic story and quantify late Cenozoic and Quaternary deformation rates of the Tualatin Mountains.

Last modified July 29, 2011
First posted March 29, 2011

  • This report is available only on the Web.

For additional information:
Contact Information, Western Region Geology and Geophysics Science Center—Menlo Park
U.S. Geological Survey
345 Middlefield Road, MS-973
Menlo Park, CA 94025-3591

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Suggested citation:

Walsh, K.P., Peterson, G.L., Beeson, M.H., Wells, R.E., Fleck, R.J., Evarts, R.C., Duvall, Alison, Blakely, R.J., and Burns, Scott, 2011, A tunnel runs through it--An inside view of the Tualatin Mountains, Oregon: U.S. Geological Survey Scientific Investigations Map 3144.

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