Benjamin Seleb first spotted them on a hike through the Alps in 2024. Hundreds of small, perfectly horizontal steps carved into the steep hillside — each one flat on top, then dropping down to the next. The clanking bells of nearby grazing cows echoed through the valley, and Seleb, a PhD student at Georgia Institute of Technology, found himself wondering: did these animals carve this hillside, step by step, over generations?
He wasn't the first to ask. More than 150 years ago, Charles Darwin himself puzzling over the same terracettes — those repeating step-like patterns found on hillsides worldwide. In a book published just months before his death in 1881, Darwin described "little horizontal ledges" on steep grassy slopes and noted that grazing animals clearly used them. But his old Cambridge mentor, John Henslow, wasn't convinced animals were the whole story. Was Darwin right — or was Henslow onto something?
Now, Seleb and his team at Georgia Tech, working with researchers at the BioFrontiers Institute at the University of Colorado Boulder, think they've confirmed Darwin's hunch. Using computer simulations that modeled how cows, sheep, and other hoofed animals move across slopes, the researchers found that even simple behaviors — like walking while grazing and avoiding unnecessary energy expenditure — could gradually reshape a hillside into those distinctive steps.
"Darwin had his son Francis measure a chalk slope near Lewes in southern England, where Francis counted about 30 parallel ledges running more than 100 yards down a 40-degree slope," said Saad Bhamla, Seleb's PhD supervisor. "Steep ground like that is coincidentally where our model produces its most regular terracettes. So, in a way, we spent a fair bit of simulations confirming something Darwin was already puzzling about nearly 150 years ago. I find that so cool."
The study, published in the Journal of the Royal Society Interface, suggests that thousands of hoofprints over centuries create flat paths where soil compacts, while loose dirt accumulates at the downhill edge of each path, slowly building up the step-like ridges. No fancy physics required — just animals doing what comes naturally: eating grass and taking the easiest route home.
So the next time you see a hillside patterned with those mysterious terracettes, you can picture Darwin walking the same slopes, his son counting ledges with a notebook, and a modern PhD student with clanking cows for company — all following the same ancient question toward the same satisfying answer.