In the High Atlas Mountains, a tiny Moroccan village called Arroumd sits on a pile of boulders whose origins, until now, were a mystery. Turns out those boulders came from a rockfall 4,500 years ago, a new study finds. Previous theories had indicated they could have been deposited by glaciers or perhaps have fallen shortly after a local glacier melted, LiveScience reports. But neither of those was the case, a team of researchers now finds.
By assessing levels of an isotope known as beryllium-10 in the rocks, they were able to pinpoint the age of the deposit—which occurred long after the melting of the glacier some 11,700 years ago, says researcher Philip Hughes. "The cliff did not collapse until 4,500 years ago. The rock avalanche was therefore not triggered by glacier retreat," he notes. The real cause may have been seismic activity, his team says; there's a tectonic fault nearby. (A similarly cool recent discovery: Scientists have pinpointed the source of Stonehenge's rocks.)