The slide struck around midnight Sunday near the Hanging Lake Tunnel in Glenwood Canyon, a deep and narrow chasm about 110 miles west of Denver, the Colorado Department of Transportation said.
No injuries or damage to vehicles were reported.
All lanes are closed from Glenwood Springs east to the town of Dotsero. Up to 25,000 vehicles a day travel that section, CDOT spokeswoman Stacy Stegman said.
Officials haven’t determined how long the highway will be closed, Stegman said. Because of the rugged terrain, the shortest detour is more than 200 miles long, around the mountainous Flat Tops Wilderness Area.
Highway crews working with flashlights said the slide left two large holes in a westbound bridge, one of them about 10 feet by 20 feet. All lanes were blocked by boulders, some the size of a tractor-trailer rig.
Some of the boulders are so big they will have to be blown apart with explosives before they can be hauled away, Stegman said.
Crews planned to do a more thorough assessment in daylight.
A 1995 rock slide on I-70 in Glenwood Canyon killed three people.
A slide on Thanksgiving day in 2004 closed the highway and required nearly $700,000 worth of repairs. No one was hurt because the highway had previously been closed for an unrelated crash.
– Associated Press