A river's glacial memory
The Charyn Canyon stretches over 154 kilometers, a massive gash in the semi-arid steppe east of Almaty. Its red sedimentary rock walls tower up to 300 meters high, sculpted by millions of years of erosion. But the most detailed story is not in the 12-million-year-old rocks themselves, but in the giant steps carved into their sides. These are ancient river terraces, the fossilized floodplains of the Charyn River.
There are six distinct terrace levels visible in the canyon. Each represents a complete cycle of glacial and interglacial climate. During warmer interglacial periods, the river flowed more placidly, depositing thick layers of sediment and creating a wide, flat floodplain. When the climate shifted toward an ice age, massive glaciers building up in the Tian Shan mountains locked away vast amounts of water. Then, during periods of intense melting, huge volumes of water and abrasive sediment surged down the river. This super-charged flow gave the river immense power to cut down into its own bed, a process called river incision. The old floodplain was left high and dry, becoming a terrace. This cycle repeated six times, with each new glacial pulse carving the canyon deeper and leaving a new step in the geological staircase.
A clock made of starlight
To read this history, geologists use a technique called cosmogenic nuclide dating. The Earth is constantly bombarded by cosmic rays from space. When these high-energy particles strike atoms in minerals like quartz on the Earth's surface, they can create rare isotopes, such as Beryllium-10. The longer a rock surface is exposed, the more of these cosmogenic nuclides accumulate. By measuring their concentration, scientists can calculate how long a surface—like an abandoned river terrace—has been exposed to the sky.
This method allows researchers to assign specific ages to each terrace level, linking them directly to the Pleistocene glacial cycles. The analysis shows when the terraces formed, but also the rate at which the river carved the canyon. Studies of river incision in similar geological settings reveal rates that can vary from 41 to over 300 meters per million years, depending on the specific climatic and tectonic conditions. The terraces of Charyn Canyon record of climate change, in stone and dated by starlight.