South America's winding Amazon River flows in an easterly direction across the continent, dumping water into the Atlantic Ocean. But in eons past, it flowed from east-to-west and, for a time, in both directions at once, a new study finds.

About 100 million years ago, during the middle of the Cretaceous Period when dinosaurs still walked the Earth, the continents of South America and Africa broke apart. The fissure created a raised highland along the east coast of South America, which tilted the Amazon's flow, sending water and sediment rushing toward the center of the continent ..

The new finding helps illustrate the transient nature of Earth's surface, the researchers said.

"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," said study team member Russell Mapes.

The Amazon is the second longest river in the world, after the Nile River in Egypt. About 4,000 miles long, the Amazon is the equivalent of the distance from New York City to Rome.