Three people are missing on Grand Mesa in western Colorado, possibly swept away by mudslide estimated at 2 miles wide, 4 miles long and 250 feet deep Sunday evening, according to the Mesa County Sheriff’s Office.

The people were not named but were said to be residents of the Collbran and Mesa areas.

The slide is along Salt Creek Road, also known as 601/2 Road, near Vega State Park, east of Collbran, which is about 40 miles east of Grand Junction.

The person who reported the slide to Mesa County sheriff’s dispatch at about 6:15 p.m. “described hearing a noise that sounded much like a freight train that is attributed to the slide,” the sheriff’s department stated.

“This slide is unbelievably big,” Mesa County Lt. Phil Stratton said.

Deputies described the slide area as unstable Sunday night, and they estimated an entire ridge had been sliding for most of the day. Heavy rain was believed to have contributed to the mudslide.

Rescue crews with the Plateau Valley Fire Department were on the scene.

The area is remote. No structures were reported damaged, but officials expected a more accurate assessment Monday morning. No major roads were affected.

Cellphone service is not available in the area.

People are discouraged from going into the area, and a road block has been established outside the mudslide perimeter for safety. The roadblock is set up at the intersection of Colorado 330 and Salt Creek Road, outside Collbran.

Joey Bunch: 303-954-1174, jbunch@denverpost.com or twitter.com/joeybunch