Fossil Delta in Eberswalde Crater
http://www.msss.com/mars_images/moc/2005/0...alde/index.html
--- The Eberswalde delta provides the first clear, "smoking gun" evidence that some valleys on Mars experienced on going, persistent flow of a liquid with the physical properties of water over an extended period of time, as do rivers on Earth. In addition, because the delta today is lithified -- that is, hardened to form rock -- it provided the first unambiguous evidence that some martian sedimentary rocks were deposited in a liquid (presumably, water) environment.
The presence of meandering channels, a cut-off meander, and crisscrossing channels at different elevations (one above the other), provided the clear geologic evidence for these interpretations. ---
Imho: This lithified delta is the greatest and most fascinating clue to liquid water!
Forget Ares Vallis mouth, drainage channels, recently formed gullies, Gusev ...