It appears that Curiosity is traveling across an ancient streambed on Mars.
“From the size of gravels it carried, we can interpret the water was moving about 3 feet per second, with a depth somewhere between ankle and hip deep,” said Curiosity science co-investigator William Dietrich of the University of California, Berkeley. “Plenty of papers have been written about channels on Mars with many different hypotheses about the flows in them. This is the first time we’re actually seeing water-transported gravel on Mars. This is a transition from speculation about the size of streambed material to direct observation of it.”
This discovery also confirms the wisdom of Gale Crater as a target. Satellite data and images had suggested the crater had once been water filled. Now this suggestion appears confirmed.