Eberswalde Crater offers the chance to explore an ancient river delta. In fact, the delta is the most convincing sign that a Martian river once flowed into a standing body of water. Stream channels in the delta have meandered over time. As the streams carved new channels, abandoned streambeds remained behind as "footprints." Today, many of these streambeds are higher than the surrounding terrain. That's because the water deposited sediments that hardened and became resistant to erosion. Orbiters above Mars have detected clay minerals in the sediments. Clays are evidence of past water activity for the Mars Science Laboratory to investigate. Clays form when water dissolves rocks into extremely fine-grained minerals. These minerals form layers, kind of like the parts of a sandwich.