Two sprawling wildfires tore through at least 4,500 acres in the San Gabriel Mountains on Monday, and only a canyon prevented the blazes from merging into one massive inferno, fire officials said.

The two brush fires broke out in Azusa and Duarte amid a blistering heat wave, forcing the evacuation of hundreds of homes and at least 750 residents. The first blaze, called the Reservoir fire, was reported about 11 a.m. at Highway 39 in the San Gabriel Mountains, said Andrew Mitchell, spokesman for the Angeles National Forest.