A rockslide in a deep and narrow Colorado canyon has indefinitely closed a major east-west highway, forcing drivers on a 140-mile detour.

A tractor-trailer rig was damaged, but no one was injured Monday night when the rocks tumbled onto Interstate 70 about 125 miles east of the Utah border. The Colorado Department of Transportation shut down traffic in both directions along 24 miles of highway, from Glenwood Springs in the west to Gypsum in the east.

CDOT spokeswoman Amy Ford said Tuesday officials don't know when it will reopen.

"It could be a long-time closure," she said.

A boulder the size of a small car crashed into the truck's trailer, said Ron Milhorn, news director of KMTS radio in Glenwood Springs, who came upon the scene after the slide.

The truck driver, Ray Hatch of Las Vegas, told Milhorn he saw the car in front of him disappear into a cloud of dust before his truck hit a boulder head-on. Hatch was calm and in good spirits after the crash, Milhorn told The Associated Press.

Milhorn and his wife were returning to their home west of Glenwood Springs after a weekend in Denver when traffic came to a stop because of the slide. He said his vehicle was behind 10 or 12 others about 100 yards from the debris.

Vehicles that could turn around were escorted back out of the canyon, Milhorn said.

Average daily traffic through the canyon is about 300 vehicles per hour, CDOT said.

The shortest detour adds nearly 140 miles to the trip, taking traffic north to U.S. 40 and then back south to I-70.

A helicopter took CDOT inspectors on a flight Tuesday afternoon to assess the risk of additional rock fall from the canyon walls, Ford said. She had no reports yet on what the team found.

The Colorado River carved the scenic canyon, which is popular with anglers and rafters. The chasm is so narrow that one 12-mile section of the interstate runs through three tunnels and across 40 viaducts and bridges. It took 12 years to complete.