Not really that easy to answer since it all depends upon scale. Typically, the easiest way to depict this type of thing would be some sort of forced perspective: ISO or something like that. If you take a look Djekspek has dune a number of canyons as has Torstan(and I think both have done little mini tutorials on that subject also!!) as well as a number of other artists here. I would suggest looking around first, but scale and your "viewpoint" are going to be the most important factors to determine which way to go here.