Geological discoveries have revealed that, millions of years ago, the (possible) longest river in the world ran in the opposite direction.
O Amazon riverknown as the world’s largest drainage system by water volume, stretches 6,400 kilometers from the Andes Mountains in Peru to the Atlantic Ocean. But geological discoveries have revealed that, millions of years ago, the enormous river flowed… opposite direction.
When Russell Mapes, a graduate student at the University of North Carolina, was investigating the flow of sediment from the Andes to the Atlantic in 2006, an anomaly in sedimentary deposits in central South America appeared.
Old zircon gemstonesoriginating in eastern South America, were in the younger Andes in the west. As evidence, Mapes cataloged marine animal fossils in unexpected inland locations, which suggested a historic east-west flow of the river.
Later analysis linked the reversed flow to a mountainous region in northeastern South America that existed during the Cretaceous period65-145 million years ago.
The flat terrain of the Amazon made it susceptible to changes in direction, due to small slopes. Before the formation of the Andes, the river flowed east to west toward the Purus Arc in Brazil, with water west of the arc emptying into the Pacific.
As the plateau eroded, water began to flow from west to east. The growth of the Andes ended up creating a basin that filled over time, reversing the flow of the river.
Mapes proposed that the final reversal occurred in the last five to ten million years — a rapid change in geological terms.
A later one from 2014 presented a complementary theory, suggesting that the rise of the Andes increased rainfall interception, intensifying erosion and forming the Pebas wetlands. The accumulation of sediment in this region ended up altering the landscape, leading to the reversal of the flow of the Amazon, around 10 million years ago.
