"The fantastical arch shapes of sandstone formations have long been thought to be sculpted by wind and rain. But a team of researchers has now found that the shapes are inherent to the rock itself.
'Erosion gets [excess] material out, but doesn’t make the shape,' says Jiri Bruthans, a hydrogeologist at Charles University in Prague, who led the research. Rather, erosion is merely a 'tool' that works in combination with more fundamental factors embedded in the rock.
These factors are stress fields created by the weight of overlying rock. Under low stress, Bruthans says, sandstone erodes easily. But as stress mounts — as parts of a cliff or pillar are eroded away, for example — the sand grains on the surface of the remaining rock lock together and become more resistant to further erosion." - Richard A. Lovett, July 20, 2014, Sandstone arches form under their own stress, Nature International Weekly Journal of Science
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