I live in Mill Valley, California, a small suburb tucked away in the Coast Range fourteen miles north of San Francisco. Out of the valley and just behind downtown rises Mount Tamalpias. The wooded mountain shadows the quant municipality and soft wetlands below. It is the most prominent geographical feature in the northern Bay Area and on a clear day can be seen from miles away. Mount Tamalpias is one mountain in the 400 mile California Coast Range, that starts north of San Francisco in Humboldt County and runs south to Santa Barbara.
More than a hundred years ago, my maternal great grandparents settled under Mount Tamalpias and every generation since has made their home in the shadow of this mountain. But, where my personal history with Mount Tamalpias and the greater California Coast Range ends is certainly not where its physical history ends. Approximately 250 million years ago, the North American Plate and the Pacific Plate collided sending the thinner crust of the Pacific Plate sliding under the more sturdy North American Plate. Around 150-140 million years ago, as the crust of the Pacific Plate melted, it became molten rock and pushed the American Plate upwards, forming the first peaks of the Coast Range.
Unlike the convergent boundary formed by the Nazca and South American Plates, that have formed the great Andes Mountains, the collision of the Pacific Plate and the North American changed direction, forming a transformational boundary. Meaning they began to slide north and south along the San Andreas Fault. Today, all land west of the fault slowly moves north, while land east of the fault slowly moves south. This movement creates one of the most seismically active areas in the world. Evidence suggests that a massive earthquake occurs along the San Andreas Fault approximately every one hundred years.
Under the banks of fog and deep forests of Redwood is the solid core of the Coast Range. This core is composed of granite and serpentine, a unique type of metamorphic rock. The serpentine formed when sedimentary and shale rocks, from the Pacific Plate, melted in the collision with the North American Plate. This amalgam was then heated by seawater giving it its distinctive greenish gray hue and earning it the name serpentine.
Redwood forest.
http://wallpaperweb.org/wallpaper/nature/webb-creek-and-redwoods-mount-tamalpais-state-park-california_21849.htm
These mountains and their geology produce the perfect wet coastal climate for a flora and fauna to thrive. Redwoods and manzanita are skirted by white tailed deer, bobcat and mountain lion. Canadian geese rest in the grassy meadows abutting these mountains during their migrations. The Coastal Miwok lived in harmony with this biodiversity for thousands of years. We can be grateful for he plate tectonics of the Pacific and North American plates that catalyzed the growth of such biodiversity along the California Coast. Only recently have Westerners realized the amazing place that is the northern Californian Coast and I hope we do not spoil the spectacular biodiversity that the geology here took so long too foster.
Personal photography from the summit of Mount Tamalpias facing south towards San Fransisco
References
Cannon, Marilyn. “Lilies of the Coast: The Coast Ranges.” N.p., 17 Nov. 2002. Web. 12 Sept. 2013. <http://www.sonoma.edu/users/c/canno/bio314chapter7coastranges.html>.
“Geology of North Coastal California.” Krisweb. N.p., 2011. Web. 12 Sept. 2013. <http://www.krisweb.com/hydrol/geology.htm>.
“California’s Coastal Mountains.” Http://ceres.ca.gov/ceres/calweb/coastal/mountains.html. N.p., n.d. Web. 12 Sept. 2013.
