By Perkins, S.
Science News , Vol. 170, No. 19
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. …