Lake and marine environments on Earth are favorable for fossil preservation, especially where the sediments they left behind are rich in carbonates or clays. Hundreds of highland craters on Mars have horizontally layered, sedimentary rocks that may have formed in lakes. CRISM has taken many targeted observations of these rocks to measure their mineralogy and how the minerals vary between layers. Variation between layers helps us to understand the sequence of events that formed the sedimentary rocks. The Mars Orbiter Camera found that where valley networks empty into craters, commonly the craters contain fan-shaped deposits. However it was not completely clear if the fans formed by sediment deposition on dry crater floors (alluvial fans) or in crater lakes (deltas). CRISM discovered that in the fans' lowermost layers, there are concentrated deposits of clay. More clay occurs beyond the end of the fans on the crater floors, and in some cases there is also opal. On Earth, the lowermost layers of deltas are called bottom set beds, and they are made of clays that settled out of inflowing river water in quiet, deep parts of the lakes. This discovery supports the idea that many fans formed in crater lakes where, potentially, evidence for habitable environments could be preserved.