Geologists reveal the ‘master sculptor’ behind the world’s dramatic sandstone formations

Arches 'carved' from sandstone are found all over the world

It’s long been thought that the arches, columns and bridges of natural sandstone formations were created by years of erosion from wind and rain, but new research suggests that these fantastic shapes are actually the product of gravity instead.

Scientists from the Charles University of Prague explain that it’s not that erosion doesn’t play its part, but that it is the rock’s “internal stress fields” – the different areas of pressure caused by weight and gravity - that dictate the final form.

“Erosion gets material out, but doesn’t make the shape,” explains Jiri Bruthans, a hydrogeologist who led the research. Instead, he says, the wind and the rain are merely the “tools” that reveal the shape that is latent in the rock.

Dr Bruthans began his research after visiting the Stralec Quarry in the Czech Republic and observing the unusual properties of a type of sandstone known as “rock sand”. Bruthans noticed that although the miners had to use dynamite to break the rock walls apart, once they had the material itself just crumbled apart - often leaving behind the arches and bridges seen in natural sandstone formations.

The team from Prague took away blocks of rock sand from the quarry to conduct their own experiments, exposing the samples to cycles of heat, cold and salt to simulate natural erosion, and even leaving one block out in the rain for 15 months to see if it would form a natural arch.

They found that by placing weights and pressures on the material they could accurately predict how arches, alcoves and other features would form, baking cubes of the rock sand in the oven to harden it then dunking it under water to watch erosion take place.

One of the rock sand cubes and the arch it formed after being squeezed by weights and submerged underwater. Image credit: Marek Janac

"You can control it completely," Dr Bruthans told the BBC. "You select the pillar direction, by choosing the points where you apply the compression. It's just the stress which controls the shape - nothing else."

In an article written to accompany the research in Nature Geoscience, Professor Chris Paola of the University of Minnesota described the discovery as "a lovely and elegant formative mechanism for a lovely and elegant kind of landform".

"These natural sculptures have delighted countless visitors, some of whom must have paused to wonder where they come from," said Professor Paola. "Here is an answer."

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