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U.S. Geological Survey
Geologic Investigations Series I-1963

Geologic Map of the Skykomish River 30 x 60 Minute Quadrangle, Washington

By R.W. Tabor, V.A. Frizzell, Jr., D.B. Booth, R.B. Waitt, J.T. Whetten, and R.E. Zartman


Photograph showing view from Skykomish River valley, north Cascades mountain range in the Pacific Northwest region of the United States
Looking east from Mount Index across the Skykomish River valley to the peaks of Mount Baring underlain by large erosion resistant tectonic slices of amphiolite and gneiss in the less resistant sedimentary rocks of the eastern melange belt. The lower, more rounded mountain to the right is Grotto Mountain composed of granodiorite of the Grotto batholith. Mountains on the skyline beyond are carved from rocks of the Nason terrane, predominantly the Chiwaukum Schist and rocks of the Mount Stuart batholith. The forground rocks are metagabbro, another tectonic slice in the eastern melange belt.


From the eastern-most edges of suburban Seattle, the Skykomish River quadrangle stretches east across the low rolling hills and broad river valleys of the Puget Lowland, across the forested foothills of the North Cascades, and across high meadowlands to the bare rock peaks of the Cascade crest. The Straight Creek Fault, a major Pacific Northwest structure which almost bisects the quadrangle, mostly separates unmetamorphosed and low-grade metamorphic Paleozoic and Mesozoic oceanic rocks on the west from medium- to high-grade metamorphic rocks on the east. Within the quadrangle the lower grade rocks are mostly Mesozoic melange units. To the east, the higher-grade terrane is mostly the Chiwaukum Schist and related gneisses of the Nason terrane and invading mid-Cretaceous stitching plutons. The Early Cretaceous Easton Metamorphic Suite crops out on both sides of the Straight Creek fault and records it's dextral displacement.

On the south margin of the quadrangle, the fault separates the lower Eocene Swauk Formation on the east from the upper Eocene and Oligocene(?) Naches Formation and, farther north, its correlative Barlow Pass Volcanics the west. Stratigraphically equivalent rocks of the Puget Group crop out farther to the west. Rocks of the Cascade magmatic arc are mostly represented by Miocene and Oligocene plutons, including the Grotto, Snoqualmie, and Index batholiths. Alpine river valleys in the quadrangle record multiple advances and retreats of alpine glaciers. Multiple advances of the Cordilleran ice sheet, originating in the mountains of British Columbia, Canada, have left an even more complex sequence of outwash and till along the western mountain front, up these same alpine river valleys, and over the Puget Lowland.

Download the Skykomish geologic map as a 78 x 34-inch PDF file (77.8 MB)

Download the accompanying pamphlet as a 67-page PDF file (772 KB)

Jump to USGS Data Series 222: Database for the Geologic Map of the Skykomish River 30-Minute by 60-Minute Quadrangle, Washington (I-1963)

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Maintained by: Michael Diggles
Created: August 28, 2001
Last modified: July 9, 2007 (mfd)
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