It is now late Northern spring on Mars, so the Southern middle latitudes get very low-sun illumination that accentuates subtle topography.
This image shows sand dunes mixed with rock outcrops on the floor of a large crater. Some of the dunes have squiggly crests, which is unusual. It looks like this is due to the outcrops, which anchor the dune in places as they migrate.
Written by: Alfred McEwen (audio: Tre Gibbs) (5 February 2014)
More info and image formats at http://hirise.lpl.arizona.edu/ESP_034801_1300
Image: NASA/JPL/University of Arizona