Boulders toppling from a Sausalito hillside triggered traffic accidents near the Alexander exit early Friday morning, blocking the freeway for a time and slowing the commute.

Two of the four southbound lanes were closed for about three hours before reopening at about 7:45 a.m.

One person was slightly injured as three cars hit the debris or smacked into a concrete guardrail trying to avoid it. Tow trucks were summoned to the site, just south of the Waldo Tunnel, after the rock slide was reported shortly before 5 a.m.

"Boulders came down from a rock slide and there was a lot of debris on the roadway," Southern Marin Fire Battalion Chief Matt Barnes said. "One person was treated and transported with minor injuries."

Contact Nels Johnson via email at ij.civiccenter@gmail.com. Follow him at twitter.com/nelsjohnsonnews