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Geology

The Salmon River Canyons meanders through volvanic rocks, metamorphosed sediments of the Seven Devils Group, and the Columbia River Basalt lava flows.  About 200 million years ago, the Seven Devils Mountains, now located just west of Riggins, were a chain of volcanic islands not unlike today’s Aleutian Islands, located in the Pacific Ocean.  Movement of crustal plates brought this series of islands to the west coast of North America where it collided with the continent a few centimeters every year.  Over time, the pressure caused the rocks to fold and push up, nearly vertical in some locations.  This resulted in metamorphism, chemical and textural changes in the rocks.  The boundary, or suture zone, separating the original North American continent from the exotic rocks of what were once Pacific Ocean islands, runs roughly north and south, just a few miles east of Riggins.  These rocks, the oldest in the Salmon River region at about 200 million years of age, contain a variety of mollusk shells in limestone beds.  If you’re out hiking and look, you can easily find these yourself. These historic Seven Devils rocks are well exposed in some of the steeper Salmon River canyons, such as Blue Canyon.

About 15 million years ago, molten basalt flowed repeatedly from large rifts, or cracks, in the earth’s surface.  Most of the rifts occurred near what is now the Columbia River in the central and southern regions of Washington state, as well as parts of what is today eastern Oregon.  Molten basalt is very fluid, thus resulting in enormous flows that covered huge areas- nearly half of Washington, large areas of northern Oregon, and northern and central Idaho – before they hardened.  The Columbia River basalt flow is one of the three largest in the world, the other two being one in southern Africa and one in India.  As the basalt cooled, columns were formed.  These columns are visible today in many places along the Salmon.  Keep your eyes peeled in the Wapshilla Rapids area for a fantastic visual explanation.  The width and orientation of the columns was determined by the way the hot lava cooled.  Wide columns indicate slow even cooling, while narrow columns signify rapid cooling.  Vertical columns formed when the lava cooled from the surface in.  Curved and horizontal columns resulted when water entered the lava through cracks and cooling proceeded from the center out.    

Other features you may notice on your Salmon River rafting or paddling trip also provide clues about the area’s geology.  Where the canyon walls are steep and confining, the rock is generally hard and resistant to the erosive action of water.  The pool and drop character of the river indicates that some layers of the Seven Devils bedrock are more resistant than others.  Blue Canyon is a perfect example.  Places where the canyon widens and the river slows, making lots of riffles and a few mild rapids, signify passing through Columbia River basalt.  Basalt is susceptible to erosion and the resistance of every layer is about the same.

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