Flow west, young river: ancient Amazon ran opposite today's route.

The forerunner of the mighty Amazon ran from east to west, a new analysis of rocks laid down by that ancient river suggests.

About one-fifth of all the fresh water that reaches the world's oceans today does so via the Amazon. That river now flows eastward from the Andes for more than 6,000 kilometers, notes Russell W. Mapes, a geologist at the University of North Carolina at Chapel Hill. But that wasn't always the case, Mapes and his colleagues reported last week in Philadelphia at a meeting of the Geological Society of America.

The evidence for the river's flow reversal lies within rocks deposited as sediment by the proto-Amazon when dinosaurs still roamed Earth. The researchers looked at several mineral samples collected near Santarem, Brazil, about 650 km from where the Amazon flows into the Atlantic Ocean, and from a site near Manaus, another 600 km or so upstream.

Although the river laid down the material in those rocks about 85 million years ago, those rocks contain crystals called zircons that solidified about 2.1 billion years ago. The region's only source of rocks of that age lies in northeastern South America, from which the zircon would have had to travel westward to reach its current location, says Mapes.

The configurations of ripples preserved in the rocks at Santarem and Manaus bolster the notion that the ancient river flowed in that direction, he adds.

About 167 million years ago, the great southern continent called Gondwana began to break apart. As part of that process, eastern South America became a highland, the researchers speculate. Because the Andes didn't yet exist, the upheaval made South America tilt toward the west, and rivers ran in that direction.

Other scientists have used the age of zircons in the sandstones of Utah to show that North America once hosted a continent-crossing river system that flowed from east to west (SN: 8/30/03, p. 131). Many of those crystals had eroded from the Appalachians when those mountains were young, says Mapes. He notes, "The results of that study inspired us to do our [Amazon] research."

"It's neat to see that zircon analysis is proving things that previously you could only suspect," says Paul K. Link, a geologist at Idaho State University in Pocatello.

The site at Santarem probably sat about 500 km from the source of the proto-Amazon, says Mapes. However, because he and his colleagues haven't yet analyzed samples obtained farther west than Manaus, they can't tell how long or how large that ancient river was.