The Lick Fire had consumed about 5,200 acres near the Lick Observatory and Henry W. Coe State Park, The San Jose (Calif.) Mercury News reported. While the observatory wasn't threatened by the flames, at least one outbuilding at the state park burned during the night.
About 50 private cabins in the area were in danger from the flames.
Only dirt roads were available to move equipment and Cal Fire crews to the blaze.
"There is only one way in and one way out," Kevin Colburn, a spokesman for Cali-Fire, told the Mercury News.
Much of the vegetation in the area consists of oily scrub that burns quickly.
The equipment being used included 30 engines from Cal Fire, four helicopters, eight bulldozers, two water tenders and a DC-10 air tanker, the newspaper said.