The rocks damaged windshields and grazed the head of a teenage boy inside one of the cars. It happened just after 10:00 p.m. Sunday on the 52 Bypass in Darlington.
Nine cars were hit Sunday night, including a pickup truck belonging to a state representative Robert Williams.
"I thought it was very bad for our area because there were cars that were traveling on vacation from Florida and other areas of the state that were also randomly hit at this section, so we're very concerned about this," said Williams, a Democrat who represents Darlington and Florence counties.
Police searched the area for evidence, and they've spoken with people who live near the overpass to track down leads.
So far there have been no arrests.