One of the world’s most scenic gorges has long been a mystery to geologists. Scientists believe that some canyons are carved simultaneously by the rivers that flow through them and the rapid rise of terrain on either side. But Tsangpo Gorge, a rugged, steep-walled river valley in southern Tibet (pictured), doesn’t seem to fit that pattern. To figure out how the gorge evolved, scientists took a look at the deepest sediments—some up to 800 meters—drilled from several sites along the gorge. They then analyzed when the sediments were last exposed to cosmic rays (and therefore at Earth’s surface, not buried). The data reveal that some portions of the ancient gorge began filling in about 2.5 million years ago, the researchers report online today in Science. That’s the time when a rapid, tectonically driven rise in the terrain in that region of the Himalayas naturally dammed the river that carved the gorge and triggered sediment accumulation. But it’s also several million years after the Tsangpo Gorge first formed, according to previous studies, which carves a big hole in the notion that rapidly rising terrain in that region of the Himalayas sparked the growth of the gorge. Similar studies elsewhere—especially in the western end of the Himalayas, where gorges bisect the rugged landscape—may reveal whether rising terrain truly triggers increased erosion rates.