Marcia wrote:
Both photos look as if they could have been caused by volcanic activity/lava flows.

Alex writes:
Just to be clear, Hynek et al. are postulating that the Malin and Edgett sedimentary features represent volcanic airfall deposits, not direct lava flows; the morphology of the layering is consistent with a mantling sediment laid down subaerially. Others have advanced similar models involving dust deposits in an ancient (Noachian) atmosphere.

Marcia wrote:
Kilauea (I'm sure Alex is familiar with these....

Alex writes:
Indeed, I've ruined several sets of hiking boots on the Kilauea flows. However, my most vivid memory is overflying the caldera and imaging it with infrared sensors (at night, of course ;-)

Alex R. Blackwell
University of Hawaii