The layers visible in this subimage of Echus Chasma are very different from the light-toned, thinly bedded layers HiRISE has observed in deposits seen elsewhere in Valles Marineris.
The view of these layers in Echus Chasma shows they are rough, with knobs of rock sticking out through the dust and talus (loose debris) on the slope. This indicates that perhaps these layers are made of different materials than the light-toned deposits, which appear more friable in nature.
These rough layers may be exposures of lavas, or they might just be more resistant forms of sedimentary rocks. The layers are typical of those seen in chasma slopes and crater rims elsewhere on the Martian surface.
Written by: Ross A. Beyer (14 April 2010)
More info and image formats at http://hirise.lpl.arizona.edu/PSP_002472_1810
Image: NASA/JPL/University of Arizona