Sonoma County's remote wildfire was 85 percent contained Monday evening after a huge group of local and regional firefighters spent a fourth day battling the blaze.

An early Monday reconnaissance showed the fire had burned 3,505 acres in the rugged, northeastern corner of the county, home to the Geysers geothermal fields and just miles from Lake County.

The blaze wasn't expected to grow further now that firefighters gained significant ground, aided by favorable weather conditions, Cal Fire Capt. Scott McLean said. The initial firefight was hampered by high winds that sent flames in erratic directions through the rugged, steep terrain, keeping fire crews at a distance, he said.

"The winds died down, the fire didn't move that much and we were able to get our engine crews, hand crews, air resources and everyone into the fire and work that fire for the last two days," McLean said.

Several area roads closed during over the last few days are expected to open at 8 a.m. Tuesday, including Cloverdale–Geysers Road at Squaw Creek Road and Healdsburg Geyser Road at Mercuryville.

Cal Fire expected to reach full containment Thursday.

Investigators were still attempting to determine what caused the fire, which was first reported just after 2 a.m. Friday, Cal Fire spokeswoman Suzie Blankenship said.

The fire destroyed one structure - a cooling tower run by Calpine Corp, the principal operator of the Geysers geothermal field.

Evacuations called for earlier in the weekend were call canceled Sunday.

Monday's firefighting effort included strengthening the containment line and mopping up hot spots.

Crews also were able to begin repairing parts of the land where the fire and firefighting efforts left areas vulnerable to erosion, McLean said. That includes efforts such as building raised berms of dirt to direct the flow of water.

"We make sure we have people assigned that have the knowledge of the environmental aspects to make sure we prevent further damage," McLean said.

As of Monday evening there were 76 fire engines, 9 bulldozers and 1,184 people on the fire. Five firefighters suffered minor injuries, McLean said.

The firefighters responded from 17 local agencies. Also helping on the fire line were inmate crews from the State Department of Correction and Rehabilitation. Other state and federal officials were helping supervise the fire.

Cal Fire officials again warned motorists to be careful because of numerous firefighting equipment and vehicles on area roads.

You can reach Staff Writer Randi Rossmann at 521-5412 or