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As rivers chew their way across the landscape, they naturally wander — especially in their relatively flat deltas, where sediments can pile up and divert the river one way or another (SN: 4/1/14). Course changes can unfold gradually over time spans ranging from years to centuries, says Elizabeth Chamberlain, a geophysicist at Wageningen University & Research in the Netherlands. But channel-jumping triggered by an earthquake can occur in weeks or days, she says.
After a river channel shifts, the old waterway can gradually fill in with sediment. Still, evidence of the old channel typically remains, Chamberlain says. While looking at satellite images of the Ganges Delta, she and her colleagues spotted a slight depression that formed a crescent shape about 45 kilometers from the current Ganges. That depression measured up to nearly 2 kilometers wide and stretched for dozens of kilometers. At one time, that had probably been a main channel of the Ganges, the team thought.
While doing fieldwork nearby in 2018, the team decided to check out the depression in person, including taking samples for dating. Then, good luck struck.
While driving home, the researchers stumbled upon an open pit; someone had excavated the dirt for a pond they planned to fill in with water the very next day. In one flank of the pit, the researchers spotted distinctive, largely vertical bands of light-colored sand embedded within the darker horizontal layers of mud — a type of deposit known as seismites.
These features are, in essence, the frozen-in-time remnants of ancient sand volcanoes that formed when seismic…
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