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Siskiyou Wild Rivers: Geology

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What makes the Siskiyou Wild Rivers area so Geologically Unique?

The sharp-edged mountains and wild rivers that make up the Siskiyou Wild Rivers lie in what geologists call the Klamath Knot, due to the non-linear character of the rugged landscape and variety of geological formations. The region owes much of its biological wealth to the age and complexity of its rocks. Most of the rocks of the Klamath-Siskiyou are 200-400 million years old, and originated as offshore sediments that were repeatedly uplifted, folded, and mixed with the granites of the ancient seafloor bedrock. Intruding into this mixture are large geological masses formed under extreme pressure in the earth's interior: peridotite and serpentine. Because of the manner of their formation, these rocks are deficient in some minerals (including calcium and potassium) and are heavily laden with others (especially magnesium, iron, and nickel).


The strange mineral composition of serpentine and peridotite means that soils derived from them are very inhospitable to most plant life. The region is famous to botanists around the world for the amazing diversity of its serpentine-adapted plants. The wide array of soil types resulting from the mountains' complex history provide plenty of opportunities for other plant species to evolve and occupy new niches or develop new adaptations.

Peridotite is a dense, coarse-grained igneous rock that can be the parent rock for serpentinite. Serpentinite was formed from oceanic crust. These seafloor rocks were compressed by subduction and elongated by movement along faults.