This image captures one of the cleaner portions of the interior wall of the caldera at the top of Tharsis Tholus.
Tharsis Tholus is an intermediate sized shield volcano, much smaller than giants such as Olympus Mons, but still large by terrestrial standards. While relatively dust-free, this section of the caldera wall is still extensively covered by dust that is being sculpted into curious shapes by the wind and by gravity.
Written by: Laszlo Kestay (9 June 2010)
More info and image formats at http://hirise.lpl.arizona.edu/PSP_002169_1940
Image: NASA/JPL/University of Arizona