Sandstone structures form without cement

Gravity, not glue, allows towering sandstone pillars and arches to withstand howling wind and pouring rain, researchers propose July 20 in Nature Geoscience.

Sandstone forms when tiny sand grains bind together into a solid mass. The edges of sandstone slabs wear away when exposed to the elements, leaving behind spectacular structures such as arches, columns and alcoves that resist further erosion.

Geologist Jiří Bruthans of Charles University in Prague was touring a sandstone quarry when he noticed something odd: The workers had to use explosives to break apart the solid sandstone walls, but rocks that broke free often quickly crumbled apart. Bruthans says this behavior seemed to contradict the conventional explanation that chemical cement glues sandstone structures together. Thinking another overarching force was responsible, Bruthans decided to play in the sand.

Bruthans and his colleagues used fine sand compacted into cubes 10 centimeters on a side. The team then placed weights on top to mimic forces from stacked rocks and submerged the cubes in water to simulate natural weathering. As the side of each block eroded away, an hourglass shape formed, with fewer and fewer sand grains remaining to support the load.

After a few minutes, the downward stress was high enough that the remaining sand interlocked into a solid that was more than eight times as strong as the original cube and more resistant to erosion. Tweaking the conditions in the experiment, the researchers reproduced on a small scale all of the fantastic shapes sandstone takes in nature.

“The weight allows these formations to withstand what would be horrendous weathering processes” such as rain and wind, says geologist Alan Mayo of Brigham Young University in Provo, Utah, who worked with Bruthans on the project. “These things survive thousands of years in a harsh environment.”

In an accompanying commentary in Nature Geoscience, geologist Chris Paola of the University of Minnesota in Minneapolis applauds the mechanism’s simplicity. “Bruthans and colleagues present nothing more or less than a lovely and elegant formative mechanism for a lovely and elegant kind of landform,” he writes.

Editor’s Note: This article was updated August 4, 2014, to remove the incorrect statement the weights used in the experiment all weighed 1 kilogram.