Thanks everyone! Raf now wants to tear down our amazon tank and make it a terrarium. Addictive!
The substrate is river pebbles, large enough to allow water movement through them. I stacked them using wood and petrified wood as barriers. Then a layer of re-moistened dry moss (for reptiles), then a layer of sphagnum peat moss, then the layer of living moss. Stream feature provides dampness to the top of the "hill".
-Diana