How Old is Paradise Valley?

Just before Highway 89 enters Yankee Jim Canyon at the southern end of Paradise Valley, a dirt road splits to the west. It follows Rock Creek for about six miles until it reaches the trailhead to the Gallatin Petrified Forest.

The Petrified Forest holds trees buried by volcanoes 50 million years ago, while Yankee Jim features some of the oldest exposed basement rock on the planet, estimated to have formed in the Archean Era, 3 billion years ago. 

That’s how geologist Rob Thomas describes the landscape as he leads busloads of rockhounds along the Yellowstone River. Then he’ll brace for someone on the bus to counter that the whole scene was created that way by God 6,000 years ago.

“There’s a very large percentage of people who think the earth is 6,000 years old, and I can lecture until the cows come home that we’ve known otherwise for hundreds of years,” Thomas said. “Or, in 30 minutes, I can take you to an outcrop where you can see everything from sedimentary deposition to exposure from erosion to where it was intruded by a squirt of magma that crystalized, and everybody who looks at it can recognize that that’s messed up. They’re left with a profound understanding that what you just looked at was dramatic change in a spot that requires more than 6,000 years of time.”

Such juxtaposition of new and old, science and religion, abounds in the Paradise Valley. And that’s why the University of Montana-Western professor leads field trips there. Right-out-the-window examples of planetary creation roll by on a bus ride between breakfast in Livingston and afternoon ice cream in Mammoth.

Thomas doesn’t make such points to bait Christians. Rather, he sees his over-subscribed geology tours as an opportunity to use the Paradise Valley as a way to be objective rather than objectionable.

“The scientific method is nothing more than gathering data, piecing together the best understanding we can using logic and reason,” he said. “Nothing in the Bible talks about a 6,000-year age of the Earth. That came from Bishop Ussher, who counted lifetimes in the Bible and determined the Earth was made in 4004 B.C. Ussher was trying to be an academic, and using the only tool he had to use, which was the Bible.” 

Catholic Bishop James Ussher of Ireland published that calculation in 1650. The Roman Catholic Church today has no teaching on evolution or “young earth theory,” and recently deceased Pope Francis publicly held that “God’s existence does not contradict the discoveries of science.”

Nevertheless, the debate goes on. According to the 2024 Religious Landscape Study by Pew Research Center, 55 percent of Montanans identify as Christians. The same census in 2007 found three-quarters of Montanans calling themselves Christian, equally split among Catholics, evangelical and mainline Protestants at about 23 percent each. Over the decades, the evangelicals have held steady while the Catholics and mainline Protestants have fallen away. Today about 2 percent of Montanans identify as Buddhist, about 1 percent each are Jewish, Muslim or Hindu, and 39 percent are religiously unaffiliated.

Even though more Montanans consider themselves “none of the above” than any specific religious denomination, articles of faith consistently push its state policies. The latest session of the Legislature saw passage of laws requiring public schools to give students time for religious instruction, and narrowly defeated a bill requiring display of the 10 Commandments in classrooms. 

To stay with the biblical theme, consider the Devil’s Slide. The orange-red vertical gash on the side of Cinnabar Mountain on the west side of the Yellowstone River can be seen for miles. As Thomas describes it in A Roadside Geology of Montana, which he coauthored, Devil’s Slide is not a landslide at all. Instead, it’s an iron-rich layer of sediment laid down 200 million years ago when the area was a Triassic tidal flat. Then, during a period of mountain-building, the flat seabed bent upward. Erosion exposed the layers, and oxidation rusted the iron-bearing minerals into their reddish color.

Or, as some Yellowstone River raft guides like to tell it, the Devil got kicked out of Heaven and left a bloody smear when he skidded down the mountainside.

“Belief in a Creator is not a question of science; I don’t have any view on that,” Thomas said. “You can have both science and faith at same time. The problem comes when people are rigid in their beliefs and want to teach that as science. Who do you choose? There’s a percentage of Native Americans in Montana who believe they’ve always been here and came from a specific spot of ground. How do Christians feel about teaching that creation story?”

Geology has its own holy writings and schismatic disputes. Thomas recalled the career of Marshall Kay, a Columbia University professor who championed geosynclinal theory. That model held that as sediment piled up in one place, its weight would cause a mountain to rise somewhere else, like pushing on a balloon. Kay also mentored geologists who mentored geologists who mentored Thomas, passing down braids of knowledge, tradition and personal loyalty that are hard to untangle even with new evidence. 

“That model died when we mapped the ocean floors and saw the world’s largest mountain ranges were under the ocean,” Thomas said. “Marshall Kay had the fortune to live long enough to see the demise of geosynclinal theory by what became known as continental drift and plate tectonics. He used to heckle people from the back of the room. He’d yell out ‘Bullshit!’ And the sad thing with that is much of the data collected by Marshall Kay was utilized as important data to understand plate tectonics.”

Whether what one sees out a bus window traveling down Paradise Valley is 6,000 or 3 million years old, Thomas prefers to yank the conversation to the practical here-and-now.

“You can see fault scarps — ruptured ground like what happened in 1959 at Quake Lake,” he said, referencing the body of water formed west of Ennis, Montana, when an earthquake caused rockslides to dam part of the Madison River. “To get the ground ruptured like that, you have to have very large earthquakes — over 6.5 magnitude — in the recent past. So recent, it cuts through the alluvial fan we see today.”

That kind of information has personal resonance for Thomas. His parents lost their California house to a landslide triggered by golf course construction that moved water into a geologically unstable zone.

“Everything they had was tied to that house,” Thomas recalled. “My mother lost her hair in clumps. They got divorced. I’ve never forgotten the lesson of that. They weren’t stupid people. They were just ignorant of the fact they were building on the site of a landslide. Homes are the biggest investment of most people’s lives. To lose them to an uninformed decision — some people never recover from that.”

Maps produced by the Bureau of Mines in Butte show much of the town of Big Sky is built on landslide zones. The Bridger Mountains rise from the Gallatin Valley along an active listric fault, which is still likely capable of generating large earthquakes.

“And people are putting their homes all over the Bridgers,” Thomas said. “We live in a state that allows you to build wherever you want without anyone telling you about those natural hazards. If you don’t want rules, buyer beware.”

For those intimidated by the Bureau of Mines, A Roadside Geology of Montana makes a much easier introduction to the Treasure State’s stony wonders. The original version of the book came out in 1972 and covered all of the Northern Rocky Mountains. Geologist authors David Alt and Donald Hyndman soon expanded it to more specific landscapes of Montana, Idaho, Wyoming and Washington.

“They represent the best-selling books we do,” said John Rimel, owner of Mountain Press which published the Roadside guidebooks. “A lot of the books we sold were to other geologists, who were specialists in one thing but didn’t know what they were looking at when they went on a family vacation. This was their CliffsNotes of what they ought to know with their families.”

The series’ popularity inspired similar versions for other states from Arizona to West Virginia. Alt died in 2015 at age 81 just as Mountain Press was preparing to revise the Roadside Geology of Montana title. Rimel invited Thomas to replace Alt, and the new edition came out in 2020.

Last year, the American Geological Society approached Mountain Press about buying its entire geological book catalog, including the Roadside series. Rimel said the professional organization’s leadership had realized it was too focused on its own membership at a time when geology was losing interest both in universities and the general public. They wanted a way to do outreach with mass appeal.

Thomas has explored numerous ways to make his field inviting and accessible, Rimel said.

“Rob has embraced social media as a means of educating folks and spreading the word about geology,” Rimel said. “It’s fascinating to see his public presentations and posts about things geologic. If there was ever a time when we needed more public science, now is that time.”

As Regents Professor of Geology at University of Montana Western, Thomas helped develop the college’s immersion scheduling format, where students study a single subject for a whole semester. That’s allowed him to devote extensive time to field experience, leading classes throughout Montana and beyond, including Greece and Nepal. And he’s been leading field trips with organizations such as Yellowstone Forever around Greater Yellowstone since 1986.

“A lot of the approach we take in education is rooted in the training of priests,” Thomas said. “That’s good for memorizing and regurgitating information, but not learning. Learning requires experience.”

Now approaching retirement, Thomas said his dedication to getting folks outside and around rocks is his best teaching tool. Rather than declaring what forces caused a 350-million-year-old limestone formation to tilt along a fault, he asks his audience to eliminate the things that don’t withstand logic and reason. What remains is useful.

 “Nothing makes me happier than being wrong, because then I’ve learned something,” he said. “I don’t learn anything from being right. Truth is a word for theologians, not scientists. I don’t like certainty. I know the world is not certain.”