Colorado College geology professor Christine Siddoway made the discovery of her career from the saddle of a road bike.
It was during a climb through North Cheyenne Cañon Park that Siddoway, 52, glimpsed alternating bands of sandstone and granite fused vertically in a rocky slope off Cheyenne Cañon Road - a composition so unusual, she said she "almost fell over."
"It was like a slap in the face," she recalled. "I didn't accept what I was seeing."
But it's Siddoway's nearly decadelong investigation into that site and several others in the Rocky Mountains that has some geologists reeling - potentially upending decades of established thought about the timing of geological processes that ultimately fractured an ancient supercontinent.
In a paper published this month in Lithosphere, a journal of the Boulder-based Geological Society of America, Siddoway and co-author George E. Gehrels of the University of Arizona concluded that the sandstone wedges she saw that day are 700 million years older than previously believed - presenting hikers, cyclists and motorists with a relic from a time long before dinosaurs, when single-cell organisms were the only life on Earth's changing topography.
The hypothesis could offer insight into how the supercontinent Rodinia began to break up into smaller land masses, according to coverage in Science and related journals.
Under normal circumstances, sandstone is formed through burial and compaction and isn't found within granite, an igneous rock formed by the cooling and solidification of lava deep beneath the Earth's surface.
When the two are paired, it's generally in the form of a layer of sandstone on top of granite - the result of sand and other materials settling over an igneous layer and hardening over time.
The Cheyenne Cañon rock formation - found within a half mile of the Starsmore Discovery Center - isn't the only such occurrence in the Pikes Peak region, and Siddoway wasn't the first geologist to grow bewildered at the sight.
Similar anomalous arrangements spurred disbelief in the 1890s when European geologists began documenting them in Turkey Creek, Crystola and Woodland Park.
Among those who documented the puzzling mixture was George Hapgood Stone, who, like Siddoway, was a geology professor at CC.
At the time of her fateful bike ride in North Cheyenne Cañon Park, Siddoway was aware of the previously documented formations, as well as some along Intemann Trail in Manitou Springs, but she understood that they were limited to those areas.
Additional examples have since been uncovered in a much wider geographic area, including mixed rock formations near Monarch Pass, Arapahoe Pass west of Nederland and sites north of Vail and near the Village of Pine, Siddoway said.
How the rocks formed has been a source of debate for more than 100 years.
The prevailing theory came in 1965, when John Harms, a now-retired Colorado geologist, hypothesized that sediment fell into the granite, from the top down, as a result of "fracturing and faulting" in the Earth's crust, Siddoway said.
Harms believed the sandstone was contemporary with Pikes Peak, which formed during the Laramide orogeny, a period of mountain making that occurred 65 million years ago.
Siddoway's findings potentially throw that explanation into question. Instead of a top-down movement, she hypothesizes that the sandstone is the result of an earthquake that split ancient granite and sent a river of sand-and-water slurry into the cracks, where it then hardened into what geologists call injectite sandstone.
The rocky, windswept landscape would have looked something like the Great Sand Dunes that ring the Sangre de Cristo Mountains near Alamosa, only devoid of vegetation and visible life, she said.
The force of the quake mixed sand with water from a river or ancient lake and vibrated it until it took the form of liquid sand - a mixture that beachcombers will recognize anywhere sand meets surf. The seismic disruption happened somewhere around 750 million years ago, she said, citing her results from studies of zircon granules embedded in the sandstone.
"Based on modern analogs, it would probably be a magnitude 7 or greater," she said of the quake.
The same forces were at play in Christ Church, New Zealand, between 2010 and 2012 when a series of earthquakes opened cracks in the Earth and flooded areas with liquefied soil.
During eight years spent studying the rocks, Siddoway was assisted by fellow geologists as well as a rotating cast of 15 students.
If true, her hypothesis means that the Ute Pass Fault - which eventually caused the uplift that created Pikes Peak - is 500 million years older than previously thought.
It's a discovery that adds to an evolving understanding of Earth's ancient history, but it's roiled Siddoway's regular hikes through North Cheyenne Cañon Park with her dogs.
Each time she spots a bit of sandstone, they head out into the woods in search of more mixed rock formations, she said.
For the dogs, it's all a walk in the park, but for Siddoway, it's about hunting down new portals to the past.
"This represents a window into the deep past of geologic time that we didn't even know existed in this part of the Rocky Mountains, much less in Colorado Springs."