Since the pebbles range in size from a golf ball to a sand grain, scientists have inferred that the stream's speed was around three feet per second.
"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 Rebecca Williams who works at the Planetary Science Institute in Tucson, Arizona.
"This is the first time we're actually seeing water-transported gravel on Mars," remarked William Dietrich from the University of California, Berkeley.
Admitting that nobody knows how long water moved over the rock, he hazarded a guess that the stream may have flowed for "thousands to millions of years."
Referring to the large rock into which the small stones seem to have been cemented, California Institute of Technology's John Grotzinger quipped that it looks "like someone jack-hammered up a slab of city sidewalk but it is really a tilted block of an ancient stream bed."
In his opinion, "The question about habitability goes beyond the simple observation of water on Mars."
Though Grotzinger agreed that "Certainly flowing water is a place where microorganisms could have lived", the chief scientist at Caltech added, "This particular kind of rock may or may not be a good place to preserve those components that we associate with a habitable environment."
Curiosity was launched from Cape Canaveral in Nov 2011. The 2.5 billion dollar project is NASA's first astrobiology mission since the 1970s-era Viking probes. It is designed primarily to search for evidence that the planet most similar to Earth may once have harboured the necessary building blocks for microbial life to evolve.
The rover is expected to minutely examine the Martian soil in the next two years. Curiosity has been fitted with a laser gun that can zap a rock from 23 feet (7 meters) away to create a spark whose spectral image is analysed by a special telescope to discern the mineral's chemical composition.
In fact, Gale was chosen as the site for landing because there is a three-mile-high mountain in its centre. The mountain's slopes contain different layers - clay at the bottom, sulfates on top of that, and sand and dust toward the top.