Who built the mysterious steps on the mountainsides? Scientists have found an unexpected explanation

Now, researchers from the US have put forward an unexpected explanation. A computer model has shown that such terraces may gradually form as a result of ordinary cows, sheep and other ungulates walking up and down the same slopes day after day.

The findings have been published in the *Journal of the Royal Society Interface*.

A mystery that Darwin pondered

Terrassets are found in the Alps, the UK, New Zealand and other mountainous regions. They are long, narrow ledges running almost horizontally along steep grassy slopes.

As far back as 1881, Charles Darwin took an interest in their origin. He suggested that animals do indeed use these ‘steps’, but doubted that this alone fully explained their formation. Since then, scientists have debated whether terracettes are caused by the slow creep of soil or are formed by ungulates.

How the hypothesis was tested

The researchers created a computer model in which virtual animals moved down a slope, following just two simple rules: to search for food and to expend as little energy as possible.

On steep slopes, the most energy-efficient route is not to go up or down, but across the slope. Each step slightly compacted the soil and trampled the grass, making it easier for subsequent animals to pass through the same spot.

After thousands of such passages, clearly distinguishable steps gradually appeared on the slope — very similar to real terraces.

Why do straight lines form?

At first glance, it seems that the animals must move in a chaotic manner. However, the model showed the opposite.

Each new track slightly alters the surface, making the existing route easier to follow. As a result, a multitude of random movements gradually coalesce into stable paths, which over time turn into long terraces.

Scientists call this mechanism stigmergy — a process whereby living organisms indirectly coordinate their actions by altering their environment. Ant trails or paths trodden by people across lawns form in exactly the same way.

Does this mean the mystery has finally been solved?

No. The authors emphasise that their work does not rule out the involvement of geological processes.

However, the study has shown for the first time that even the simple behaviour of grazing animals is capable of independently creating regular stepped structures, which were previously explained mainly by soil movement.

Why is this important?

The study demonstrates how the collective behaviour of animals can alter the landscape on a massive scale.

The study also shows that complex natural forms do not always require complex causes — sometimes they arise from thousands of simple actions that gradually alter the environment.

Background

Terraces are long, horizontal ledges on steep grassy slopes. They are found in many mountainous regions around the world and have long been considered one of the classic mysteries of geomorphology.

For over a century, there have been two main hypotheses: either the terraces are caused by the slow creep of soil, or they are gradually formed by ungulates that follow the same routes year after year. New research has shown for the first time, using computer modelling, that animal activity alone may be sufficient for such structures to form.

Source

Benjamin Seleb et al. Moo-ving mountains: grazing agents drive terracette formation on steep hillslopes. Journal of the Royal Society Interface (2026).