Sandstone arches formed by gravity and stress, not erosion

Sandstone arches, alcoves and pillars are scattered across the globe, monuments to nature's strange beauty -- odd, gravity-defying shapes carved by the wind and rain. Or so we thought.
A new study suggests these spontaneous stone structures are not exactly carved by the whims of erosion. Although wind and water perform the grunt work -- blowing abrasive sheets of sand and water droplets against the rock, loosening and carrying away the weaker pieces of sediment -- it's the rock's internal stresses and structure that determine these formations' magnificent shapes.
In other words, it is the unique nature of sandstone -- not wind and rain -- that, with the help of gravity, creates the arches and spires of places like Utah's Bryce Canyon. Scientists proved as much by replicating these sandstone structures in lab settings, showing that even small blocks of otherwise crumbly material naturally form into arches.
As Alan Mayo, a hydrogeologist at Brigham Young University, recently explained in a report for Nature, erosion "undercuts the material" in a way that might predict collapse, but increased pressure along the edges of the un-eroded rock enables the remaining sand grains to lock together in a manner that's "incredibly stable."
"We should not say erosion or weathering carved the forms, as it was the stress field which give the forms the shape," said Jiri Bruthans, a hydrogeologist at Charles University in Prague who co-authored the study with Mayo. "Erosion processes are mere tools controlled by stress."
"To create perfect shapes you do not need intelligence or planning," Bruthans added. "The opposite is true for nature. Most perfect things are made by simple mechanisms."