Donna Curtiss, an accounts clerk at the city’s Utilities Service Center on Belknap Avenue, walked outside about 10:30 Thursday morning for a daily walk she takes during a 15-minute break.
She put her headphones on, turned on her iPod and started walking west along the bike path.
“All of sudden there was a big bang and crack, like thunder,” she said. “It definitely startled me.”
She looked up at the Rimrocks across the Yellowstone River and saw a huge cloud of dust. She snapped a couple of photos on her cell phone, capturing the immediate aftereffects of an occurrence normally considered quite rare — a major rockfall from the Rims flanking Billings.
It’s hard to say what’s normal lately. As The Gazette reported last week, there was a very large rockfall, apparently over the weekend of Feb. 26-27, relatively close to one that occurred Thursday. And five months earlier, a sizable home above 13th Street West was all but demolished when a gigantic piece of Rims sheared off and sent boulders crashing down the slopes.
Jay Gunderson, a geologist in the Billings office of the Montana Bureau of Mines and Geology, who hiked in for an up-close inspection of the other rockfall last week, said it was “pretty fascinating” to have two major incidents in less than two weeks.
“I really don’t know what to think about it,” he said. “I’m amazed.”
By coincidence, Gunderson had lunch Thursday during a meeting of the Montana Geological Society in the Petroleum Club at the Crowne Plaza Hotel. Some of the other geologists who had seen last week’s Gazette story asked him about the rockfall, and he pointed it out to them from their perch on the 22nd floor of the Crowne Plaza.
Now, he said, he wonders if he wasn’t looking at the one that occurred not long before lunch. The earlier one happened roughly across the river from Coulson Park, on the Rims beneath the Four Dances Natural Area.
The rockfall on Thursday took place above a mile upstream of the other one, directly across the river from the water treatment plant, which is attached to the Utilities Service Center.
Curtiss said that based on the texts she sent after taking photos of the rockfall, she got the first photo about 10:35. Coincidentally, Gazette photographer Bob Zellar was driving on Airport Road on top of the Rims when he noticed a cloud of dust on the Rims on the other side of the valley. He took a photograph about 10:35, when the dust was already considerably dispersed.
Unlike the late-February incident, when dozens of very large boulders, including one house-size rectangular block of sandstone, went tumbling down almost to the river’s edge, on Thursday three or four big boulders stopped moving not far below where the Rims meet the dirt slopes.
Curtiss said she didn’t actually see the rocks falling.
“I heard them, more than anything,” she said. “I really couldn’t see anything for all the dust.”
The incident above 13th Street West, directly north of the now-closed Rimrock Elementary, happened on Oct. 9 when a sandstone slab estimated at 1,000 tons fell from the Rims and broke into many pieces. One of them, estimated at 50 tons, smashed through the back of a house owned by Jon Lodge and Jane Deschner.
Gunderson explained last week that the Eagle sandstone formation is riddled with fractures that expand as a result of water seep followed by cycles of freezing and thawing.
David Lopez, a retired geologist with the Bureau of Mines and Geology, produced a map in 2003 showing “areas of potential rockfall in the Billings area.” He couldn’t be reached Thursday, but last week he added one bit of geology to the story of the falling rocks. He said in an e-mail that such falls “are part of valley forming or widening process; as the cliffs are undercut by erosion the cliff-forming rock becomes unstable and eventually falls.”
He also mentioned how the wet winter and the repeated freezing and thawing accelerate the process.
Gunderson said he “never would have guessed” there would two major rockfalls so close together.
“It’ll be interesting to see what else happens,” he said. “If the last one didn’t get people’s attention, this one should.”
Contact Ed Kemmick at firstname.lastname@example.org or 657-1293.