The lakes in the crater were likely fed by a combination of groundwater and rivers. As the lakes filled, the rivers that brought water into the lake also brought sand, gravel and rocks into the region, building deltas out into the lake and leaving behind layers of sediment along the ancient shoreline. In the center of the lake, away from the turbulent river(s), fine sediments settled to the bottom of the lake, forming a thick layer of mud. Over time, the position of the rivers shifted, just like they do on Earth today. As the rivers shifted, they deposited sand and gravel in different places around the lake. These deposits of sandstone (from the river system) and mudstone (from a lake environment) are what the Curiosity Rover has found, giving scientists on Earth clues that there were once lakes on Mars.