Stop 9: Corkscrew Mountain
From the yard of a friendly and accommodating property owner on the south side of the highway along Toroda Creek, who has given us permission to (gently) use his driveway and yard for this stop, look up to the north to see Corkscrew Mountain protruding hundreds of feet above the side of the valley.
This remarkable feature is said to consist of volcanic rock moderately rich in silica, a type of rock called dacite. About a mile south of here in the Toroda graben there are several dacite lava flows that erupted into the down-faulting Toroda graben during the Eocene epoch, around 50 million years ago. Corkscrew Mountain is interpreted as originating in a volcanic “pipe,” a conduit, through which magma moved upward to erupt as lava on the earth's surface. Somehow, the magma solidified and cooled within the volcanic conduit into the pattern that has been revealed and etched out by subsequent erosion.
If you look with binoculars, you can see that between the narrow screw threads of Corkscrew Mountain, the wider slots have nearly horizontal columns. Such columns form by slow cooling of magma or lava, with the columns oriented at right angles to the cooler surfaces toward which the heat flowed. Verification of this explanation awaits further investigation.
•Northwest Exposures -- a Geologic Story of the Northwest,
by David Alt and Donald Hyndman
•Roadside Geology of Washington,
also by David Alt and Donald Hyndman, Mountain Press Publishing (Missoula, Montana). ISBN number 078-0-87842-160-2
•Note: The Roadside Geology series of books also includes other states that may be of interest.