An unusual phenomenon that has been recognized around the world for more than a century has finally been resolved: what are the origins of the distinctive, stair-like ridges that form on steep hillsides?
These features, known as “terracettes,” have long puzzled scientists, and the question of how they form has attracted some of history’s greatest minds, including Charles Darwin.
The famous naturalist described these features as “Little horizontal ledges, one above another,” which he said had been “observed on steep grassy slopes in many parts of the world.”
According to Darwin, the formation of terracettes had “been attributed to animals traveling repeatedly along the slope in the same horizontal lines while grazing,” although he adds that “Professor Henslow (a most careful observer) told Sir J. Hooker that he was convinced that this was not the sole cause of their formation,” with the implication that these curious, structured features on steep hills might have geological origins, rather than those attributable solely to biology.
Now, researchers may finally have an answer to this pastoral riddle that baffled the likes of Darwin more than a century ago, settling the scientific debate over their origins.
Solving the Mystery of Terracettes
Benjamin Seleb, the lead author of a new study detailing the findings, said the mystery of terracettes first attracted his attention in 2024 after he observed them firsthand during a hike in the Alps.
“I suspected the numerous grazing cows—hard to miss with their clanking bells—had something to do with it, and was already sketching out how I would model their movement before the hike was over,” Seleb says.
“It wasn’t until I got back to my hotel and started digging into the topic that I learned about the contested origins of these terracettes,” Seleb adds. “With my curiosity piqued, I got to work building a simple model to ask whether grazing animals really could reshape mountains.”
According to Seleb and a team of collaborators from the Georgia Institute of Technology and the BioFrontiers Institute at the University of Colorado Boulder, computer modeling was employed to reshape hillsides into these distinctive “stepped” formations. The findings reveal that the most likely explanation is that repetitive pathways followed by grazing animals such as sheep and cattle produce remarkably ordered landscape patterns over time.
Simulating ‘Chaos’ to Solve a Mystery
To test their hypothesis, the research team simulated the movement of individual animals across steep slopes. For their analysis, they also introduced a few rules, which revealed that the “virtual grazers” preferred traversing hillsides in long, horizontal pathways that form gradual inclines. This makes sense, since it is a method of ascent that requires less energy, and humans have also traditionally employed similar approaches (think of mountain trails, which, rather than ascending straight uphill, follow longer sloping inclines and switchbacks where directional changes are necessary).
With each simulated footstep, soil and vegetation become compacted, producing subtle alterations to the terrain over time. As these pathways form, animals over successive future generations are thereby encouraged to follow similar routes, and with time, the familiar terraced patterns emerge.
“People have argued about terracettes for over a century,” says Professor Saad Bhamla, one of the study’s co-authors and leader of the Bhamla Lab of Extraordinary Organisms, which describes its efforts as work that “seeks to understand how organisms do amazing things.”
“Some say [terracettes] are purely geological, just slow soil creep and slumping,” Bhamla recently said. “Others say animals create them, and the standard criticism for the animal-origin camp is that the animals wander around randomly, so how could they ever carve something this regular?”
“What we show is that they can and that this regularity emerges out of that chaotic, random movement, not in spite of it, through the back-and-forth between the animals and the ground they’re chewing up with their mouths and packing down with their feet,” Bhamla adds.
Complexity from Simple Movement
Bhamla and his team say their research offers another example demonstrating how complex natural landscapes can emerge from simple, uncoordinated behavior, involving a process known as stigmergic feedback. This involves organisms that gradually reshape their shared environment over time.
The result of their work is an answer to a mystery that has persisted for many generations, and not only that, but also a pathway toward understanding the various ways animals influence landscapes more broadly.
The team’s findings were detailed in a new paper, “Moo-ving mountains: grazing agents drive terracette formation on steep hillslopes,” which appeared in Journal of the Royal Society Interface.
Micah Hanks is the Editor-in-Chief and Co-Founder of The Debrief. A longtime reporter on science, defense, and technology with a focus on space and astronomy, he can be reached at micah@thedebrief.org. Follow him on X @MicahHanks, and at micahhanks.com.