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USGS Geology in the Parks

Geologic story of the North Cascades button North Cascades National Park:
Breaking and Folding the Rocks of the Methow Domain

Beds of the sandstone from the Methow Ocean
A view of part of very large fold, shown here as tilted beds of the sandstone from the Methow Ocean (Harts Pass Formation) on Mount Winthrop. Viewed from the Pacific Crest Trail.

Almost all sedimentary rocks begin life as flat layers of rock deposited on the flats of a river bottom, a beach, or of the ocean floor. The very old layers of rock revealed by the Grand Canyon are still lying nearly flat. Yet the traveler in the Methow Domain will notice that most of the layers of sedimentary rock are steeply tilted. Some are even vertical. Like the rocks of the Western and Metamorphic Core Domains, the Methow rocks have also been stressed by plate collision, but unlike those other rocks they were never carried down beneath another plate by a subducting slab or completely smashed between two colliding plates. As a result, their deformation is minor by comparison, a sort of fender-bender near the main crash.

fault bend fold
Formation of a fault-bend fold in sedimentary rocks of the Methow Domain. The scale of such folds varies. You could view this in an outcrop or on mountain side or even across several mountains.

Rocks, somewhat back from the major plate collision, or adjacent to a great fault where the plates rub past each other, are commonly somewhat crumpled and compressed. If they have well-developed layering, as most sedimentary rocks do, they typically deform in a predictable pattern: as upper layers slide over lower layers along planes of weakness such as shale beds. Note that a geologist would have a difficult time recognizing such a fault because the stratigraphic order of the beds would not be changed. But, in places, such faults, which mostly parallel the layers, break up across the bedding into the overlying, harder, sandstone layers, going from the lower, weaker shale beds across the harder beds to an upper bed. Where the faults breaks across the stiffer beds, folds form as older beds are shoved over younger beds. Such folds are called fault-bend folds . The size of the fold corresponds to the thickness of the stiff layer or layers separating the weak, sliding, layers.

Most of the folds in the Methow Domain appear to be fault-bend folds. As the Hozomeen Terrane was thrust eastward over the rocks of the Methow Ocean, at the west edge of the Methow Domain, subsidiary thrust faults, parallel to the layers, formed in the ocean sedimentary beds as well. In places they stepped up from lower weak beds to higher weak beds and thus stacked the rocks of the Methow Ocean and the Pasayten Group on top of themselves. The step-ups formed across very thick stronger beds and the corresponding fault-bend folds are quite large as well. In some cases the strong layer included almost all of the rocks of the Methow ocean a mile or two thick! The resulting folds measure miles and miles across.

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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