MONTPELIER, Vt. - A series of rock slides dumped boulders the size of cars across a downtown street Monday, forcing about 50 people to evacuate as debris spilled up to their doorsteps.

The first slide happened shortly before sundown, spreading stones and several tall trees across the road and bringing down power lines.

"There was a loud noise — I thought it was a plow coming through — then a flash of light, and then the big stuff started coming down," said Howard Curtis, who lives nearby.

Additional slides continued for roughly two hours. Police said no one was injured.

Rocks, trees and dirt fell some 60 feet from the cliffs that rise above the city, falling onto a block-long portion of Vermont Route 12. Officials estimated the boulders weighed from 20 to 30 tons.

Authorities evacuated about 50 residents of an apartment complex across the street from the cliffs as debris piled up to their doorsteps.

Emergency workers also evacuated several homes above where the slides began because of concern about parts of the cliff collapsing.

Stephen Gray, Montpelier's public works director, said the slide was probably caused by recent swings in the temperature combined with rain Sunday night. "We've had freeze and thaw, and then it probably froze again," he said.

Evan Shper, 16, was walking toward the center of the city when the rock slides began and dialed 911 on his cell phone.

"I stood there for about five seconds," he said. "Then we heard a huge sound. A huge part was coming down with a tree on it. I started running."