GLENWOOD SPRINGS — Workers hiked up the side of a Colorado canyon Tuesday to examine another boulder threatening to fall on Interstate 70 after a rock slide forced the highway to close indefinitely.

The rock under scrutiny is about 20 feet in diameter and sits about 900 feet above the roadway, the Colorado Department of Transportation said.

After ascending the canyon wall, the crew began working its way back down a steeper section with climbing ropes to reach the boulder.

They will decide whether to pry it loose or break it up so it falls in smaller pieces, department spokeswoman Mindy Crane said.

About 20 boulders tumbled onto I-70 in Glenwood Canyon at about midnight Sunday.

No injuries or damage to vehicles were reported, but the slide left holes as large as 10 feet by 20 feet in a bridge-like elevated section of roadway.

Crews were breaking up the fallen boulders with explosives so they could be hauled away.

A 17-mile stretch of I-70 is closed in Glenwood Canyon from Glenwood Springs to Dotsero. Crane said the department still has no estimate of when it will reopen.

I-70 is a vital east-west link in Colorado, with up to 25,000 vehicles a day traveling through the canyon. The shortest detour is more than 200 miles.

Gov. Bill Ritter declared a disaster emergency for the highway, allowing the state to seek funding from the Federal Highway Administration to help pay for repairs.

Engineers are still developing an estimate of the repair costs. A similar slide in the same area in 2004 cost $1.2 million to repair, including a $700,000 emergency contract with a contractor.