Sarah McCullough, a cultural studies PhD grad from UC Davis, gives some insight into how mountain biking grew out of the 1970s Northern California counter-culture.
Mountain biking, as we know it today, really did start in Marin County, just north of San Francisco. It was begun by a group of people that were more into biking for adventure. What they would do is take pre-World War II Schwinn Cruiser bikes and get a ride on a truck to the top of Mt. Tamalpais and then proceed to rocket down the mountain on dirt trails as fast as they could.
McCullough also touches on the nascent sport’s connection to The Grateful Dead, who played the Tour Del Mar, a combination bike race/folk festival held in Pescadero, California in the late-1960s.