Sedimentary rocks in the central part of South America contain ancient mineral grains that must have come from the eastern part of the continent.

Geologist Russell Mapes says this must mean that about 145-65 million years ago, the Amazon flowed east to west.

Mr Mapes will present his findings at a geology meeting in Philadelphia.

The age of rocks on the South American continent differs between east and west.

Rocks as old as 2.5 billion years are found on the eastern side of the continent. Because of continual geological activity in the Andes, on the western side, rocks there are much younger.

Changing landscape

If the Amazon had continuously flowed eastward, as it does now, much younger mineral grains would be found in the sediments, because they would have been washed down from the Andes.

"We didn't see any. All along the basin, the ages of the mineral grains all pointed to very specific locations in central and eastern South America," said Mr Mapes, a graduate student from the University of North Carolina (UNC), US.

He explained that these sediments of eastern origin were washed down from a highland area that formed in the Cretaceous Period, when the South American and African tectonic plates broke away from each other.

That might have tilted the river's flow westward, sending sediment as old as two billion years toward the centre of the continent.

Current course

Afterwards, a relatively low ridge, called the Purus Arch, rose in the middle of the continent, running north and south. This divided the Amazon's flow, so that one half flowed eastward toward the Atlantic and the other westward toward the Andes.

Over millions of years, the Amazon has reversed its flow

After millions of years of build up, the Amazon river finally broke through these sediments and flowed past the Purus arch and into the eastern side of South America. This established the river's current course.

The new data comes from zircons, a type of mineral grain that can be dated in order to determine the age of the sediment.

Previous research has identified a reverse flow, but only in segments of the river. Mr Mapes and his colleague Drew Coleman from UNC traversed about 80% of the Amazon basin, collecting samples of zircon.

Their data supported the previous findings, and illustrate the continent-wide shift of the river's flow over millions of years.

The results will be presented at the Geological Society of America's annual meeting in Philadelphia, which runs from 22-25 October.