re: Look at it this way...
... on the sides of Gusev, you'll find a geologic table, maybe even fossils prior to Mars' demise and on the very bottom, if conditions were still right for Gusev to have had a lake, you might find remnants of rocks having been altered by water and even 'may' find some remnants of fossilised life (maybe some feet below since Mars has had many, many dust storms).
That's my take on it.