The roadway is closed between Meyers Grade Road and Timber Cove Road north of Fort Ross.
A supervisor with the Sonoma County Transportation and Public Works Department's roads division discovered the slide around 1:08 p.m., CHP Officer Dave Thomas said.
Caltrans spokesman Adam Priest said this afternoon that the closure will be in effect until further notice.
Northbound traffic will be directed off of the highway at Meyers Grade Road and will re-enter Highway 1 at Fort Ross Road, Priest said.
Southbound traffic will exit at Fort Ross Road and re-enter at Meyers Grade Road, Priest said.
Motorists are advised to allow extra travel time to reach their destinations, Priest said.
