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.