Curiosity’s telephoto camera snapped shots of three rocky outcrops not far from the rover’s landing site inside Gale Crater. One of them, called “Goulburn”, had been excavated by the rover’s own landing gear. The other two were natural outcrops dubbed “Link” and “Hottah”. All three, and Hottah in particular, were made of thin layers of rock that had been cemented together.
When the rover zoomed in, it saw rounded pebbles in the conglomerates and in surrounding gravel that were fairly large – up to a few centimetres in diameter. On Earth, roundness is a tell-tale sign that rocks have been transported a long way, since their angular edges got smoothed out as they tumbled. The Mars rocks are too big to have been blown by wind, so the team concluded it must have been flowing water. This dovetails with orbital images hinting that the rover landed in an alluvial fan, a feature that is formed on Earth by water flows.