The robot has returned pictures of classic conglomerates - rocks that are made up of gravels and sand.
Scientists on the mission team say the size and rounded shape of the pebbles in the rock indicate they had been transported and eroded in water.
Researchers think the rover has found a network of ancient streams.
The rocks, which were described in a media briefing at Nasa's Jet Propulsion Laboratory in California, were likely laid down "several billion years ago". But the actual streams themselves may have persisted on the surface for long periods, said Curiosity science co-investigator Bill Dietrich of the University of California, Berkeley.
Satellites at Mars have long captured images of channels on the planet's surface that were cut by some kind of flow, assumed to be liquid water. Curiosity's discovery at its landing site in the equatorial Gale Crater provides the first real ground truth for those observations.
By luck, the rover just happened to roll past a spectacular example of the conglomerate. A large slab, 10-15cm thick, was lifted out of the ground at an angle.
"We've named it Hottah," said rover project scientist John Grotzinger. The name refers to a lake in Canada's Northwest Territories. The team is using names from this region to catalogue objects at Gale.
"To us it just looked like somebody came along the surface of Mars with a jackhammer and lifted up the sidewalk that you might see in downtown LA at a construction site," he joked.
In this view, some of the pebbles have weathered free from the rock
Scientists are now studying the images of the pebbles in the rock. The sizes and shapes will give them clues to the speed and distance of the ancient water flow. More
Will unchecked climate change cause our planet to end up like Mars in the future? Editor
