In CUriosity, experts across the CU Boulder campus answer questions about humans, our planet and the universe beyond.
Lon Abbott, who studies how mountains form, explains how Boulder’s iconic Flatirons got their shape and why they’re so special even though they’re not one of a kind.
The Flatirons as seen from CU Boulder on Thursday, June 25, 2025. (Photo by Glenn J. Asakawa/University of Colorado)
At their core, the Flatirons are just a bunch of sand and gravel cemented together and shaped over millions of years.
To Boulderites, they’re so much more than sandstone.
They’re an outdoor playground and a rock-climbing mecca. They’re also so iconic, Boulder restricts the height of modern buildings so they don’t block their view.
But these rocks gracing photos and TV commercials from Boulder aren’t one of a kind, geologically speaking. You don’t even have to leave Colorado to find more of them.
“The Flatirons are one example of a type of landform called flatirons, and you see those other places in the world,” said Lon Abbott, an earth science professor at CU Boulder. “If you were to drive I-70 from Denver to Rifle, you would see some kind of similar landforms there. There are rocks there that are tilted in the other direction.”
Abbott has a caveat about these lowercase flatirons on the other side of the Continental Divide.
“They’re not quite as spectacular as ours,” he said.
Together, these tilting rocks are boundaries of the Rocky Mountains, with Boulder’s marking the eastern edge and the ones near Rifle marking the western edge.
The rock formations got their name because they resemble old-school irons that helped get wrinkles out of clothes.
They got this shape from a process that started hundreds of millions of years ago.
Three acts
Three hundred million years ago, the Flatirons wouldn’t have turned any heads. They were just part of a flat plain of sand and gravel that eroded off an old mountain range called the Ancestral Rocky Mountains. They remained that way and sat under a shallow ocean until about 65 million years ago.
In Act 2 of the Formation of the Flatirons, the uplift of the modern Rocky Mountains took these layers of sediment and tilted them about 50 degrees down to the east. If this stage of the Flatiron formation was a TV episode, it would be called the “Laramide Orogeny.” This mountain building event takes its name from the Laramie mountains in Wyoming.
“That’s the event that uplifted the Rockies,” Abbott said. “And as soon as you uplift a mountain range, you get stormier weather up there. You get snowfall that triggers erosion.”
As the Rockies eroded, Boulder and the Great Plains were buried by a lot more sand and gravel. It went all the way up to what is now the top of Flagstaff Mountain and covered the Flatirons.
Then came the finale.
Erosion excavated the Boulder Valley starting about five million years ago. The Flatirons we see today were sculpted by this event.
Abbott doesn’t remember the first time he set eyes on the Flatirons because he grew up in Boulder. But he knows he was instantly impressed. The rocks even tilted him toward his current career at CU.
“The Flatirons captivated me forever,” Abbott said. “And so when it was time for me to go to college, I really wanted to understand how those Flatirons came to look the way they do because they’re so iconic. So I went into geology and that passion has stayed with me.”
Mountain building
There are five numbered Flatirons. When they’re viewed from Boulder, the first Flatiron is the large one furthest to the right. The numbers go up as you look right to left.
Abbott said the third Flatiron is the most famous because “it’s the one that looks most similar to your great grandmother’s ironing board.”
He recommends hiking up to the Royal Arch to get up close and personal with the Flatirons. The classic hike is accessed from Chautauqua Park and ends with views of a natural arch and Boulder.
Abbott said there’s not much mystery left as to how the Flatirons formed. But scientists are continuing to try to piece together some lingering mysteries about how exactly the modern day Colorado Rockies came to be.
“Many people are trying to understand when the modern Rockies rose and what triggered it,” Abbott said. “There’s no obvious faults and folds, the things that geologists look for that would tell us that there was some mountain-building episode. So it’s much more cryptic.”
Source: https://www.colorado.edu/today/2026/08/26/curiosity-how-special-are-boulder-flatirons-really