Well, think about the size of these units of terrain. They represent many, many, many miles (kikometers) of space, so a canyon, or a sand dune, and so forth are far too small to be a new terrain space all to themselves. It's a bit abstracted, but I'm sure there are canyons, sand dunes, and all sorts of fun things in there already, they just aren't represented since they are too small. Remember the context of the spaces!