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Home Archived Aug 4, 2016

USGS Geology in the Parks

Domains of the North Cascades

FIELD TRIP STOP 8 - North Cascade Highway (State Route 20)

Washington Pass Overlook

Granite inspires

At Washington Pass, turn north into the visitor center parking lot and walk a short distance out to the overlook for spectacular views of peaks and spires carved from granite of the Golden Horn batholith. The distinctive pinnacled ridges owe their look to joints in the rock. Joints are geologist jargon for cracks. Plain and simple. But the processes that produce cracks are many.


View south from Washington Pass overlook at peaks carved mostly from granite of Golden Horn batholith.
View south from Washington Pass overlook at peaks carved mostly from granite of Golden Horn batholith.
Granitic batholiths cool and crystallize under considerable pressure within the Earth. Upon reaching the surface, through uplift and erosion, they expand, causing many cracks to form, sometimes in a pattern reflecting the shape of the batholith. Joints in granitic rocks are commonly at right angles, and where vertical joints predominate, weathering produces serrated peaks and ridges.
Some of the more prominent notches, such as those separating Liberty Bell, Early Winter Spires, and other pinnacles, are not joints, but small faults, as indicated by broken and ground up granite along them. The blocks of rock on either side have moved at least a little, and erosion has worn away the broken rock. The same faults may be seen cutting the granite in the towers of Kangaroo Ridge and the Wine Spires on the north ridge of Silver Star Mountain, to the east.
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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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