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USGS Geology in the Parks

Domains of the North Cascades

SIDE TRIP C - North Cascade Highway (State Route 20)

Ruby Mountain

Ruby Mtn
Ruby Mountain viewed from Ross Lake. 4th of July Pass is off the picture on the right skyline.

A long climb and a huge fold

The summit of Ruby Mountain is a marvelous viewpoint for North Cascades National Park, but the crude steep trail (about 2.5 miles from Fourth of July Pass) and 4,000 foot climb entice few visitors. Rocks on the summit are fine-grained amphibolite (metamorphosed basalt) and mica quartz schist (metamorphosed chert), as well as pods of ultramafic rock (metamorphosed mantle), commonly colored red-orange by weathering. This is the typical assemblage of oceanic rocks (Napeequa Schist) of the Chelan Mountains terrane, which are here perched atop gneiss and schist of the Skagit Gneiss Complex.

Cross sectional sketch of giant fold Metamorphic Core Domain.
Cross sectional sketch of giant fold exposing migmatites in the Metamorphic Core Domain (Chelan Mountains terrane). Flanks of the fold are faulted.

Beyond Ruby Mountain to the northeast is the Ross Lake Fault, and beyond it is relatively unmetamorphosed rock of the Little Jack and Hozomeen terranes. Geologists, and others too, may find it useful to think of the metamorphic core of the North Cascades as a huge, albeit complex, fold that is faulted on the sides. Almost a mile below the top of Ruby Mountain, once-deeply-buried Skagit migmatites are exposed along Highway 20. The piece of the Napeequa Schist on Ruby Mountain is a remnant of the core’s less-metamorphosed husk, which is preserved here up on the northeastern side of the huge fold.

On to Ross Lake Overlook

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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