I think Ryan's mention of the 1.5" tall foamcore was meant so that if we wanted, we could make a "negative" surface area, like a small dip in the ground, or a stream of some sorts... I personally am not gonna use foam core either, I have some foam that's roughly 1.5" and plan on using that as my base... that way I can carve into if needed and build it up too.
Anyway... I'm liking the look of yours so far P, maybe take a couple "chunks" out of your base so it's not too level and looks more realistic for innards of the canyon...
I like the construction foam.. if I'm correct, that's how they made geonosis arena(well, I'm sure Lucas didn't raid any construction sites), they basically had these huge blocks of foam, and carved their arena out of it.
Diagobah, I'm liking your cave so far, it looks totally different from the first pic, to the mached and covered one, good job on that... as for painting.. the best thing I could suggest it to find your base coat you want to use.. then seperate into different parts.. one for the base, then another part where you'll lighten it with for highlights.. and a third part where you darken it and water it down...
Lay your base coat... then take the watered down "darker" color.. and lay it on letting it get into the cracks, and after a min or so, wipe away the excess... then finally, go over some raised areas with the "lighter" color dry brushing, try to start out as light as you can.. you dont want too drastic of a change in color..
I'd also recommend going to a hobby store and getting some lichen or other mossy looking material.. and put it in some corners.. maybe even getting some watered down grey/brown/green mixed colors and paint them in some of the areas where water would sit.. where you'd have some mold growing/staining the cave...
Hope that helps... I should have some WIPS up of my section this week, I'm going with a hill and a path that leads to a cliff, nothing too big.. it's gonna be roughly 12" x 12" might get a little bigger with the cliff on one side.