The Curiosity rover's science team has come to a consensus: There was once a large amount of flowing water in Gale Crater. They can't say when, or for how long, but based on the types, shapes and sizes of rocks they've been seeing, they can say, finally, there was flowing water on Mars.
We aren't talking about a small amount of water either: The stream running from a canyon in Mount Sharpe called Peace Vallis could have been up to hip deep. The water eroded jagged rocks into smooth stones and deposited them in a fan shape across the floor of Gale Crater, Curiosity's landing site.
The team has examined rock outcroppings from three areas along the rover's path, all of which indicate that water had eroded rocks into gravel, then deposited them in layers. These layers cemented together forming what's called a conglomerate rock, similar to a rough cement.