Amazon River Once Flowed Backwards

Sediment along the Amazon River indicate that the river once flowed from ancient highlands on the eastern coast of South America to the west. This contrasts the current flow of the Amazon from its headwaters in the Andes Mountains to the Atlantic ocean to the east.

The discovery was made by a graduate student of geology, Russell Mapes, and his advisor, Dr. Drew Coleman, at the University of North Carolina at Chapel Hill.

By analyzing the age of sediment along the Amazon Basin, the two researchers confirmed previous research indicating reverse flow along portions of the river but also found that at one time, the Amazon’s reverse flow was actually continent-wide.

If the Amazon had continuously flowed eastward, as it does now, Mapes and Coleman would have found much younger mineral grains in the sediments from the Andes.

“We didn’t see any,” Mapes said. “All along the basin, the ages of the mineral grains all pointed to very specific locations in central and eastern South America.

Mapes explains that these sediments of eastern origin were washed down from a highland area that formed in the Cretaceous Period, between 65 million and 145 million years ago, when the South American and African tectonic plates separated and passed each other. That highland tilted the river’s flow westward, sending sediment as old as 2 billion years toward the center of the continent.

A relatively low ridge, called the Purus Arch, which still exists, rose in the middle of the continent, running north and south, dividing the Amazon’s flow - eastward toward the Atlantic and westward toward the Andes.

Toward the end of the Cretaceous, the Andes started growing, which sent the river back toward the Purus Arch. Eventually, sediment from the mountains, which contained mineral grains younger than 500 million years old, filled in the basin between the mountains and the arch, the river breeched it and started its current flow.

“It was a surprise, just because I didn’t have any idea what to expect,” Mapes said. “I didn’t know it would work out so perfectly.”

The finding, Mapes said, helps illustrate that “the surface of the earth is very transient. Although the Amazon seems permanent and unchanging it has actually gone through three different stages of drainage since the mid-Cretaceous, a short period of time geologically speaking.”

Source: University of North Carolina at Chapel Hill

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