The Flatirons, Boulder’s iconic backdrop of five signature slanted massive rock faces jutting from Green Mountain’s east slope, epitomize and fuel the jagged, upward energy that Boulder entreprenuers, athletes and hippies alike channel and harness.
Climbers love them, trail runners beat the crowds early in the morning to run them — they add a dramatic backdrop to our daily Boulder lives — shopping, commuting and more.
You could call them Boulder’s adventurous heart. With our adventure issue, we decided to explore how they formed.
Force of nature
“Let’s start with a time machine,” says CU’s teaching professor and geologist Lon Abbott, speaking with me one morning while en route with his son to Wyoming’s Bighorn Mountains.
Traveling back 300 million years, the Boulder area sits completely under water. A mountain range called the Ancestral Rocky Mountain Range (no topographic connection to the modern-day Rockies) sat roughly 30 miles west, and gradually, over time, would erode to form a new formation, the Fountain Formation, with deposits of sediment and debris that turn to rock.
The Ancestral Rockies, meanwhile, completely eroded down, “flat as a pancake — Kansas flat — then went to a sea floor,” says Lon, who grew up climbing, kayaking, skiing and backpacking in Colorado’s high country, and has focused his research on how high areas become high and how geomorphic processes sculpt that high terrain.
The sedimentary rock layers that created this formation do not differ much from the process which shed rocks and boulders out of Boulder Canyon to Eben G. Fine Park during the 2013 flood.
Between 100 and 90 million years ago, Boulder became a beach, and then a shallow sea, similar to the Gulf of Mexico, as the shoreline continued to rise and recede over time before once again becoming a beach along a coastline.
Boulder’s sea border looked quite different at different times in its history.
About 300 million years ago, the Denver area sat completely under an ocean and Boulder on a coastal debris plane, and the valley looked like California’s Big Sur coast, with mountains rising out of the sea. About 68 million years ago, the Boulder coastline was flat, as the modern Rocky Mountains had progressed east about as far as Utah at that time, with draining into the Boulder area from their slopes.
With Denver completely under an ocean and Boulder on a coastal debris plane, the valley looked like California’s Big Sur coast, with mountains rising out of the sea.
Rise of the modern Rockies
India’s collision with Asia raised the Himalayas. A piece of Africa broke off and drifted north, crashed into Europe and create the Alps (hi, Italy). The Pacific Ocean crashed into South America to form the Andes. The story of Boulder’s Flatirons goes back to a very standard way that mountains form: a collision of tectonic plates.
About 65 million years ago, off the coast of California, an oceanic plate converged with the North American plate.
“Oceanic material is more dense than continental material,” says Lon.
“When you have an oceanic plate crashing into a continental plate, the oceanic plate sinks deeper into the earth. Then you sort of stack up, almost like when you’re scraping snow off a sidewalk and snow builds up on shovel. The oceanic plates goes deep, hundred of miles into earth. Then, a giant pile of debris builds up and that becomes a mountain range.”
As the Pacific plate crashed into the Californian coast, the raised angle started out very steep, which happens when material piles up very closely to two plates. This crash and steep piling birthed the Sierra Nevada Mountains, which rise 14,505 feet into the air.
The oceanic plate continued moving inward, but its angle of subduction, the sideways and downward movement of the edge of one tectonic plate into the mantle beneath another plate, got shallower.
Due to the shallow angle, the resulting piling of debris moved further east, and a second range formed — a broad band of mountains running down western North America, the Rocky Mountains.
“As the plate got shallower and shallower, those mountains started ruffling up all the way,” says Lon. “The Front Range of Colorado is as far as it got; that’s why we’re at the boundary of the Rocky Mountains and the Great Plains.”
The pressure between the plates extended as far east as Boulder and no further.
What once lay flat with layers of sediment had hardened into rock and began to tilt upward in response to the pressure underneath, almost like a draw bridge opening for a ship below, ending at 7,132 feet in elevation.
Today, the triangular formations formed by sediment, and lifted and angled by the rise of the Rockies stand as our Flatirons. Looking east from their perch, it’s easy to imagine the sea that once lapped below.
- 300 million years ago: Ancestral Rocky Mountain range began to erode and flatten, forming Fountain Formation
- 100 million years ago: Boulder looked like a beach, with the Ancestral Rockies growing smaller and the sea level rising
- 90 million years ago: Sea level rose higher, and Boulder became a shallow sea like the Gulf of Mexico.
- 68 million years ago: Boulder became a coastline again.
- 65 million years ago: Rocky Mountains rose up from the sea as a result of tectonic collision, and the rocks formed by the Fountain Formation raised up and tilted, becoming what we know today as Boulder’s Flatirons.