Today's Image of Mars shows a mesa on the eastern floor of Aram Chaos, a 1300km (~800mi) diameter depression. (A mesa is a hill/mountain with a flat top) It is thought that a cataclysmic amount of water was released from this depression, which led to the creation of the 2000km long outflow channel, Ares Vallis.
Surrounding the center mesa you can see many isolated hills and eroding mesas. A variety of dune fields are visible in this image. The interplay between the dunes, eroded land forms, and light and dark-toned materials enables scientists to determine the relative age of each feature, as well as the composition of the interacting materials.
Clicking on this image will take you to a related, captioned image by HiRISE. This image is a subimage of that captioned image.