The slide was reported around 11:30 p.m. Sunday at Big Rock Drive, according to the California Highway Patrol.
The road remained shut down in both directions during the morning commute, but one lane was eventually opened to drivers.
Authorities said that lane would be used to alternate eastbound and westbound traffic. Drivers were warned to expect heavy traffic as Caltrans crews worked to clear the area.
It's unclear when the road will be fully reopened. Rocks were still crumbling down the hillside Monday.
One woman who was part of the team that put together the Oscar-nominated film, "Io Capitano," says she witnessed the slide right in front of her when she was coming home from the awards show Sunday night.
"I was in front of an ongoing avalanche... It was quite impressive. It looked like being on a set," Valentina Castellani Quinn said.
Drivers are advised to take alternate routes.
Meanwhile, Topanga Canyon Boulevard was also closed due to a landslide over the weekend.