Layers in Spallanzani Crater, as seen by CTX camera (on Mars Reconnaissance Orbiter). Note: this is an enlargement of the northern side of previous image.
Topographic location of Spallanzani crater
Many places on Mars show rocks arranged in layers. Rock can form layers in a variety of ways. Volcanoes, wind, or water can produce layers.
Many craters once contained lakes. Because some crater floors show deltas, water had to be present for some time. Dozens of deltas have been spotted on Mars. Deltas form when sediment is washed in from a stream entering a quiet body of water. Primitive organisms may have developed in such lakes; hence, some craters may be prime targets for the search for evidence of life on Mars.