Layering in the Valles Marineris

McEwen et al. favored flood lava interpretation, like Columbia River basalts 

TES and Phobos ISM show mafic volcanic mineralogies

Malin & Edgett objected to lava interpretion because of paucity of large boulders on canyon floor, except on landslide lobes

Apparently boulders break into sizes below MOC resolution (~5 m) when tumbling down a steep slope for several km.   Lava will tend to break into sizes controlled by cooling joints (columns), typically < 5 m on Earth.   See poster by Milazzo et al., this meeting!