According to local media, the fire erupted late morning on Thursday and quickly burned at least 20 acres.
CalFire crews were battling the blaze on the ground and in the air with the help of helicopters.
Officials have not ordered any evacuations, though there are homes nearby in the Brisbane, a small city on the slopes of the ridge.
The blaze coincides with a string of planned power outages of unprecedented scale that happened Wednesday by Pacific Gas and Electric Co (PG&E) to reduce wildfire risks posed by extremely windy, dry weather.
The power cut knocked out traffic signals, forced school closures and shut businesses and government offices across northern and central California, said Brian Ferguson, a spokesman for the state Office of Emergency Services.
A similar cutoff was under consideration by neighboring utility Southern California Edison for nearly 174,000 of its customers, about 50,000 of them in Los Angeles County, should severe winds hit southern California on Thursday as forecast, SoCal Edison spokeswoman Taelor Bakewell said.
Some of California's most devastating wildfires were sparked in recent years by damage to electrical transmission lines from recurring bouts of high winds that then spread the flames through tinder-dry vegetation into populated areas.