The slide hit Sunday in a remote area near the town of Collbran, about 40 miles east of Grand Junction. The Mesa County Sheriff's Department estimated it measured 4 miles long, 2 miles wide and as 250 feet deep in many places, but it said no structures or major roads were affected.
Mesa County Sheriff's dispatcher Amanda Orr said three men, all area residents, were unaccounted for. It wasn't known if they were in the area impacted by the massive slide.
A sheriff's helicopter was surveying the slide area early Monday. Authorities erected a roadblock outside Collbran, a town of about 700 people, to keep onlookers from the slide area, situated near Salt Creek road and Vega Reservoir.
The region features large mesas and is partially surrounded by the Grand Mesa National Forest.
Rescuers raced to the scene when it was reported around 6:15 p.m. Sunday, sheriff's spokeswoman Lisa McCammon said. She said the slide area was "very unstable."
The sheriff's office said that the person who reported the slide at about 6:15 p.m. "described hearing a noise that sounded much like a freight train."
The site is in a rural part of the county and there were no reports of any structures damaged or major roads affected, McCammon said.
"This slide is unbelievably big," said Mesa County Lt. Phil Stratton said.
A unified incident command has been established between Plateau Valley Fire Department and the Mesa sheriff, to handle the slide and search for the people possibly caught in it.
Authorities say heavy rains that fell over the weekend contributed to the slide.
The slide occurred about two months after a massive mudslide hit the Washington state community of Oso on March 22, killing 43 people.
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