Mudslides from an area stripped of vegetation by forest fires engulfed three vehicles and indefinitely closed the eastern entrance to Yellowstone National Park (search).

Park rangers helped free 16 people after the mud started flowing Sunday night following thunderstorms. Mud sloshed up to the hoods of cars, and some people had to be pulled out through car windows.

"I was told there was enough mud that you couldn't even read the license plates to see what state they were from," Yellowstone spokesman Al Nash said Monday.

No one was injured.

The slides, up to 10 feet deep and 90 yards long, blocked about three-quarters of a mile of the park's East Entrance Road at Silvan Pass.

There was no indication when the east entrance, seven miles east of the mudslide (search) zone, might reopen. An estimated 10,000 cubic feet of debris will have to be removed, and the slides wiped out guardrails and undercut part of the roadbed, Nash said.

Park staff were scanning the debris for unexploded artillery shells that might have been carried onto the highway. In the winter, shells are fired to trigger avalanches above Sylvan Pass to reduce the chances of larger, unexpected slides.

Drought-stoked fires charred 23,500 acres in the area last summer and closed the east entrance for about two weeks. A fire in 2001 closed the gate for 11 days.