A Fourth of July wildfire that scorched one acre on Mount Tamalpais could have been far worse had it not occurred in the shorter grasses of a fuel break created several years ago, Marin County Fire Department officials said. The fire, not traced to holiday fireworks but to arcing power lines along Indian Fire Road near Mill Valley, was reported at 9:30 p.m. on Thursday and was contained by 7 a.m. the following morning. Eighty people from various agencies fought the blaze and crews remained on Friday, searching for any hot spots on the rugged slopes. Battalion chief Mike Giannini said that despite the strong westward winds during the night, the lower grasses and brush, cleared by a fire crew in 2006, led to a relatively easy conquest. “In this case, all the pieces kind of aligned well for us,” he said. But he warned that a similar fire could have wreaked havoc on the surrounding open space and destroyed many homes. The department urged residents to maintain adequate defensible space around all structures; tips are available at marincountyfire.org.