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.