Stonehenge secrets and paper cut physics

Rock Stonehenge’s Altar Stone, a 5 m long sandstone slab located in the middle of the Neolithic monument, continues to evade easy answers about its origins. The latest scientific evidence points toward northeast Scotland—though the islands of Orkney have now been ruled out a mere 2 weeks after a paper was published that appeared to imply that they were the stone’s most likely source. Since the 1920s, experts have assumed that the Altar Stone came to Stonehenge from Wales in 3000 BCE alongside the bluestones of the site’s inner circle (the big standing rocks are locally sourced). But as scientists learned more about the geology and chemistry of the stones, that assumption didn’t stand up to scrutiny. The Altar Stone “has a very peculiar chemistry”—unusually high in barium sulfate, says Nick J. G. Pearce, a geochemist at Aberystwyth University who has been studying the Altar Stone since 2009. When he