Congo-Brazzaville

Grains of sand have helped scientists show that the Amazon once flowed the other way.

Today, both the Congo and the Amazon empty into the Atlantic, on opposite sides of an ocean. But when Africa and South America were still joined, the continents — and their drainage systems — looked very different.

By analysing minerals and dating tiny zircon grains in ancient sediments, researchers have reconstructed how the Amazon changed. Before the Andes rose, an ancestral river flowed west. The system later reversed, carrying sand and sediment east towards the Atlantic.

Tiny grains can reveal which way an entire continent once drained.