This photo is worth looking at just to get an idea of the … non-descriptness of what scientists are looking at, and for, out there on the Red Planet. The insets point out rocks of interest, notable for their length (1-4 inches) and composition (fragments/dust/clast). Clast, for all those wondering (according to Wiki): "Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock fragments. Geologists use the term clastic with reference to sedimentary rocks as well as to particles in sediment transport whether in suspension or as bed load, and in sediment deposits." So, there.