Amazing natural "sculptures" like Delicate Arch in Utah seem like they must be the work of a skilled craftsman, or some unimaginably complex physical process. But according to Jiří Bruthans of Charles University in Prague, Czech Republic, all it takes is a bit of sandstone and some stress.

 

Rocks all around the world have taken on bizarre shapes, from fine arches to impossibly thin columns supporting vast platforms high in the air. Various explanations have been proposed. Softer rocks erode faster than harder rocks, so it has been suggested that layers of mixed rock types can erode into strange forms. The official explanation for the sandstone structures in Arches National Park in Utah cites repeated cycles of frost, which could cause clumps of sandstone to break away.

 

Now Bruthans says the sandstone itself is the key. Delicate Arch and many other gigantic arches in the US are made of the stuff. It is a "locked sand", whose angular grains can catch on each other.

 

His team placed weights onto blocks of sandstone. This created a low level of stress inside the sandstone, forcing the grains to start interlocking.

 

They then immersed the blocks in water to mimic erosion. The sides began to fall away, leaving a narrowing column of sandstone beneath the wider, weighted top. This thinner column had to take the full load, and this further interlocked the remaining sand grains, strengthening the centre of the block.

 

Natural arches also formed when the blocks were placed over small gullies: the load on the top of the block was then supported from either side and the centre of the bottom of the block, which was over thin air, naturally fell away.