NASA's Curiosity rover, on Mars since August 6, has discovered gravel once carried by the waters of an ancient stream that "ran vigorously" through the area, the US space agency said Thursday.

Scientists had previously found other evidence of the one-time presence of water on the Red Planet, but this is the first time stream bed gravels have been discovered.

The rocky outcrop, called "Hottah," looks "like someone jack-hammered up a slab of city sidewalk, but it's really a tilted block of an ancient stream bed," project scientist John Grotzinger said in a statement.

Curiosity also investigated a second outcrop, called "Link."

The pictures transmitted by Curiosity show the pebbles have been cemented into layers of conglomerate rock at a site between the north rim of the Gale Crater and the base of Mount Sharp, where Curiosity is heading.

The sizes and the shapes of the rocks give an idea of the speed and the depth of the stream, NASA said.

"The shapes tell you they were transported, and the sizes tell you they couldn't be transported by wind. They were transported by water flow," said Curiosity science co-investigator Rebecca Williams.

The scientists estimate the water was moving at a brisk three feet a second and ran somewhere between ankle and hip deep.

Some of the rocks are rounded, indicating they traveled a long distance from above the rim, fed from a channel named "Peace Vallis," NASA said.

And thanks to imagery previously captured from Mars orbit, the scientists said they can see an alluvial fan of material washed down from the rim -- with many apparent channels "uphill" of "Link" and "Hottah."

The high number of channels between the rim and the newly-discovered rock bed suggests the stream wasn't a one-time occurrence, but that many streams flowed or repeated over a long time.

Curiosity is on a two-year mission to investigate whether it is possible to live on Mars and to learn whether conditions there might have been able to support life in the past.

[Image via Agence France-Presse]