This image shows a part of the floor of a large, heavily modified crater in the Nilosyrtis Mensae region. The cliffs in the northern and southern parts of this image are walls of an irregular erosional pit and expose some of the infilling material. The crater itself was mostly filled in and then eroded.
An interesting aspect of the rocks at the edge of the pit is the widespread fracturing. The rocks of the upper layer are broken up along cracks called joints. On Earth, these commonly form when rocks are subjected to stresses, due to factors like cooling or the removal of weight due to erosion.
The upper layer forms the cliffs because it is relatively hard to erode, and protects the underlying material. This commonly happens when a lava flow covers weaker sedimentary rocks, but the joints at this site suggest that this resistant layer might be sedimentary. The rock appears to be breaking into broad, thin slabs; lava flows often fracture into columns or relatively uniform blocks as they cool. Written by: Colin Dundas (11 May 2008)
More info and image formats at http://hirise.lpl.arizona.edu/PSP_007701_2095
Image: NASA/JPL/University of Arizona