There’s no need to ask what the appeal of Arches National Park is—it’s in the name. The gorgeous sandstone arches there seem almost impossible. How and why should the relentlessly erosive wind carve such a fantastic structure? The arches seem too vulnerable, too artificial.

And arches aren’t the only trick that sandstone has up its sleeve. Bizarre, mushroom-shaped pillars seem even more absurd, as if they were carefully placed by an incredibly patient and even more incredibly strong Zen garden enthusiast. In some places, networks of sandstone pillars even hold up ledges like a miniature Moria.

We know plenty about how this erosion takes place, and some details about why some sections of the rock erode faster than others, but the primary cause of these shapes has eluded geologists. A new study led by Jiri Bruthans of Charles University in Prague has revealed a surprisingly simple explanation.

To investigate, the researchers used blocks of extremely weak sandstone from a Czech quarry. Because there is almost no mineral cement between the sand grains, it crumbles apart pretty easily. So easily, in fact, that the blocks would simply fall apart when immersed in water.

When the researchers first placed flat weights on top of the block, however, something different happened. The sides of the block would still begin crumbling, but they would stop as soon as the block got a little thinner in the waist. Remove enough weight from on top of the block, and the remaining pillar would crumble away. Pressing the sand under weight didn’t expose its weaknesses—it made it stronger.

Depending on the characteristics of the block of sandstone, the researchers ended up with the variety of features we see in the real world—arches, thin pillars capped by top-heavy blocks, multiple pillars under a block, and even little alcoves. It depended on the pre-existing imperfections, like boundaries between layers that eroded more easily (leading to arches or alcoves) or vertical fractures (which could lead to series of pillars).

The researchers also tried to mimic different kinds of erosion, exposing the blocks to simulated rain, running water, and varying moisture and atmospheric pressure. They even subjected a harder sandstone to saltwater and freezing, as the growth of salt crystals and freezing water can wedge rock apart. These experiments yielded the same results.

Not content with all this sandbox fun, the researchers also used computer models to look at the stresses applied by the weights on top of these blocks. They saw that the shapes being produced were governed by the stress in the block. Areas where stress was low—because that portion of the block wasn’t bearing much of the weight—would crumble and disappear, while the high-stress portions remained.

But why? The researchers think it’s because stress turns the jumble of individual sand grains into a stronger, interlocking fabric. It presses them together so they can’t be knocked out of place so easily, which constitutes a negative feedback controlling erosion. A little erosion of the sides of a block, for example, reduces the cross-sectional area bearing the weight of what’s above. That means increased stress, helping to lock the sand grains together, which slows further erosion.

In a summary article accompanying the paper in Nature Geoscience, University of Minnesota researcher Chris Paola expresses delight at the finding. “Bruthans and colleagues present nothing more or less than 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. Here is an answer. The proposed feedback mechanism should be on the reading short list for anyone who enjoys the confluence of elegant science and natural beauty.”

Nature Geoscience, 2014. DOI: 10.1038/NGEO2209, 10.1038/NGEO2215 (About DOIs).