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Cerro Arco, Paraguay
Nature's masonry
Cerro Arco, a singular stone arch in the wilds of Paraguay, almost looks like it was built out of boulders mortared together. And in a sense it was, though no human mason crafted it. The arch is made up of sandstone that long ago underwent 'silicification' – water rich in dissolved silica seeped into the cracks and voids within the sandstone, eventually drying into a sort of glue that strengthened the structure. Later, flowing water washed away the lower reaches of sediment where the silica didn't permeate, leaving this impressive crest of reinforced rock.
Nature's masonry
Cerro Arco, a singular stone arch in the wilds of Paraguay, almost looks like it was built out of boulders mortared together. And in a sense it was, though no human mason crafted it. The arch is made up of sandstone that long ago underwent 'silicification' – water rich in dissolved silica seeped into the cracks and voids within the sandstone, eventually drying into a sort of glue that strengthened the structure. Later, flowing water washed away the lower reaches of sediment where the silica didn't permeate, leaving this impressive crest of reinforced rock.
A geological process called 'silicification' made the formation of this distinctive stone arch possible.
© Jan-Schneckenhaus / iStock / Getty Images Plus 2026-09-11 08:45:51 :