My point was that valley = great age isn't a good geological equation as you yourself give an example of a valley formed by a flood. I'm sure you agree with this, since you gave an example of a canyone where this wasn't the case. And this was the only point I wanted to make.
This is all true. It is the characteristics of the canyon/valley that indicate the length of the process be it by catastrophic flooding (e.g. Channeled Scablands) or through millions of years of erosion (e.g Grand Canyon).
As to the Valles Marineris, we are probably looking at the result of tectonic activity. One of the theories that attempts to explain the Valles Marineris is called Collapse Theory:
"The collapse model for the creation of Valles Marineris however does provide an initiating event. In this process best described by Schultz (1991), fractures attributed to the Tharsis stress field unsteadies the area in question. Subsidence and collapse then occur from ground water/ice retreating and/or magma withdrawal. Macroscopic observations would show this starting out with the formation of pit craters, resulting from internal collapsing, and having them slowly grow, merging with other pit chains, eventually forming troughs as they are seen today. However, even Schultz makes the point that pit craters and chains are structurally and morphologically different from large troughs, implying that other processes may be at work."
Valles Marineris: The Past
Also, the geology of Mars can tell us a lot about what happens to planets when they cool. Given the very weak magnetic field on Mars it is very likely that its core has frozen solid. Mons Olympus and Valles Marineris may very well be the result of Mars freezing solid.
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