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Terranes of the North Cascades Rocks of the Methow Domain

Sandstone beds in the Methow
Late summer snow highlights beds of sandstone from the Methow Ocean (Harts Pass Formation). Looking south along the Cascade Crest from Mount Winthrop.

The east-bound traveler, leaving the canyons that drain the hard rocks of the Metamorphic Core Domain, and looking out across the broad valley of the Methow, is struck by the change in scenery. From alpine to pastoral, from cascading water to broad meandering river, and from rocky peaks sheltering perpetual snowfields and glaciers to rocky but rounded and dry hillsides. Climate is the main actor in this scenic transformation. By the time previously water-rich clouds arrive over the valleys of the Methow River area, they have dropped much of their moisture as rain and snow in the process of rising over the North Cascades.

But bedrock also plays an important supporting role. Much of the Methow Domain consists of unmetamorphosed marine sandstone and shales deposited in submarine fans at the edge of a continent. Overlying these continent-derived rocks are sandstone and conglomerate deposited by streams and mixed with volcanic rocks of an arc. The unmetamorphosed sedimentary and minor volcanic arc rocks rocks surrender to erosion more readily than their harder, metamorphic cousins to the west. Included in the domain is the ocean floor of slightly metamorphosed basalt, which received these sediments. Compared to the jumbled and broken beds of rocks in the domains to the west, the beds in rocks of the Methow Domain display a great deal more order. Over broad areas, sedimentary beds are stacked, one on top of the other, in the same order they were deposited. Well, not totally, because fossils and sedimentary structures indicate the rocks have been strongly folded, and in some areas faulted (see Breaking and Folding the Rocks). Nonetheless, the sequence in general represents deposits from an ocean basin that gradually filled up. The rocks of the Methow Domain were lovingly studied by Julian Barksdale, Professor of Geology at the University of Washington, from 1936 until his death in 1983. We have since learned a few more details, but the basic framework was established by Barksdale.

Visit the terranes of the Methow Domain
or continue to Annealing the Parts: Metamorphism
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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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