The agency announced the shutdown of the two-lane highway at the Pacific Coast Highway intersection shortly before 8:30 a.m.
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No injuries or vehicle damage were immediately reported. Photos released by Caltrans showed debris and massive rocks in the roadway.
It was unclear how long it would take to remove the boulders, mud and tree branches.
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Crews were also checking the north end of Highway 23 in the Westlake Village area, officials said.