What’s going on geologically in Billings’ stunning but fracturing Rimrocks is really pretty simple, a Billings geologist said Tuesday.
“Mother Nature always wants to flatten the Earth, to put it in layman’s terms,” said Jay Gunderson, a research geologist with the Montana Bureau of Mines and Geology’s Billings office. “Erosion always takes over.”
Under the sweeping cliff faces are huge fractures that Gunderson said extend “tens of meters” into the subsurface rock. After a storm, moisture creeps down and loosens the dirt, eroding the soil “that holds things in place,” he said.
The sandstone that makes up the Rims is about 80 million years old. They’re the result of the Yellowstone River cutting through rock and clay, which is “why we have the cliffs we do today,” he said.
Beneath the relatively soft sandstone face is what’s geologically referred to as the Telegraph Creek Form. It's shale, said Larry Smith, a member of the geological engineering faculty at Montana Tech in Butte. Because the shale is not well exposed, water doesn’t easily penetrate it. Water passing through the sandstone instead pools on the shale, and that pooling increases pressure on rock pores and decreases rock strength, he said.
“Water is a pretty important factor,” Smith said. “It wouldn’t surprise me that this wet winter has increased loading or decreased the strength of the rock” near the point of contact between the two rock types.
The freeze/thaw cycle, especially during the spring, helps pry the rock away from the Rims. Water freezes at night, then thaws the next day. Since ice is thicker than water, it can slowly separate soft rock.
“It is an occasional thing that happens, and it’s hard to predict,” Gunderson said. “This could continue for a while, or it could be 10, 20 or 100 years before we see it happen again. It’s kind of like predicting an earthquake.”
“It is so hard to study those things,” Smith added. “It happens infrequently but catastrophically. Setting up a monitoring system over the long term is probably the way to go.”