“Monkey Face,” the sandstone formation along the Rimrocks above Zimmerman Trail, bit the dust Friday.

In one big explosion at 11 a.m., the massive chunk of unstable rock was blasted off the face of the Rims. “Monkey Face,” so named because it resembled a simian, had survived two blasts over the last week before finally crumbling.

Crews from GeoStabilization International, a Grand Junction, Colo., company hired to remove rocks in danger of falling, dangled from ropes to place explosives in the back side of the sandstone. The explosion was delayed for an hour because some charges were not going in.

But at precisely 11 a.m., an explosion ripped the rock, throwing up a huge cloud of dust and tossing boulders into the air. When everything settled, only the base of Monkey Face remained.

“It worked. The base looks good. It took it right off at the horizontal fracture,” said Debi Meling, an engineer with the City of Billings, who was on-site.

More good news was that the debris hit a berm that had been placed on Zimmerman Trial to protect the road.

The base of the formation is stable and will be left on the Rims, she said. A horizontal crack in the Monkey Face rock had made it unstable, she added.

The force of the explosion was significantly bigger than previous blasts. A seismograph set up to measure the vibrations recorded the blast as a magnitude .04, Meling said. Previous explosions were about .02. Structure damage usually occurs at about magnitude 2.0.

The blast felt by spectators as the rock exploded was from the sound, which reached 130 decibels, or louder than a band concert, Meling said.

Billings Police evacuated some homes in the area shortly before the explosion. More than a dozen residents gathered at the intersection of Edmund Street and Aljema Avenue to watch and record the spectacle.

Five minutes before the explosion, a warning siren went off to alert the area. One minute before the blast, a second siren sounded.

Then the rock exploded.

“It’s gone. The boom went right through me,” said Dianne Peterson, who lives at 3109 Edmund St.

Peterson had set up a lounge chair at the intersection. “I just came to watch,” she said.

Having lived for 10 years under the Rims, Peterson said she doesn’t worry about the rocks but was glad Friday’s explosion did the trick.

“Last week was a fiasco,” she said. Peterson, who had her daughter visiting at the time, had to leave her house four times because of the rock work.

“It kind of goofed up our plans on doing stuff,” Peterson said.

With Monkey Face gone, crews will be doing cleanup work next week, Meling said. The earthen berms will be removed and the city will fix the pavement and guardrails.

Zimmerman Trail, which has been closed since March 25 because of a rock slide, could reopen in about two weeks, Meling said. The trail is a heavily traveled route that connects the valley to Highway 3 atop of the Rimrocks.

Friday’s blast is part of a $718,220 rock stabilization project on Zimmerman Trail by the Montana Department of Transportation. The completion date is June 13, but there is a $9,000-a-day incentive if the work gets done early. The firm will be charged $9,000 each day the project goes beyond June 13.