CLAYTON -- Firefighters had nearly surrounded a blaze that burned 375 acres of grassland near Mount Diablo and expected to fully contain the flames by Wednesday evening, CalFire said.

Firefighters worked to protect rural homes in the area near Morgan Territory and Curry Canyon roads, and no structures were seriously threatened. Grass and oak trees were the fire's main casualties. No evacuation orders were issued.

An official cause of the fire has not been determined. CalFire spokesman Daniel Berlant said reports by some witnesses about a downed power line near where the fire was first reported have not been confirmed.

The blaze, called the Curry fire, is the second major brush fire of this year's fire season in Contra Costa County and coincided with the hottest day of what has generally been a mild summer. A 70-acre wildfire along eastbound Highway 4 near San Marco Boulevard in Bay Point broke out Monday afternoon and was contained by that evening.

The combination of rugged terrain, low humidity and the absence of wind helped slow the Curry fire's advance.

Firefighters made significant headway overnight, increasing containment from 10 percent Tuesday evening to 80 percent by Wednesday morning. Berlant said the fire was 90 percent contained by 2 p.m. Wednesday.

The fire was reported at 4 p.m. Tuesday in a remote area outside Mt. Diablo State Park. More than 250 firefighting personnel from CalFire, the Contra Costa Fire District and the San Ramon Valley Fire District fought the blaze with help from about a dozen aircraft.

Airplanes were on alert all day Wednesday to drop additional fire retardant if necessary, according to a CalFire news release, which also reported that one firefighter sprained an ankle but that no one else was hurt.

California State Parks spokesman Roy Stearns said Mt. Diablo State Park has remained open.

"(The fire is) quite a ways away from roads and people," he said.