In traditional Dutch lay-outs, many people try things similar to this-- using terracing to make a "garden type." Amano uses rocks for some minimal terracing, normally between sand and soil, but for many levels of terracing, it's almost better if your wall goes completely through the substrate. I believe a lot of the older dutch-style scapres used cork walls to terrace this way. Cork is water safe but it floats usually but it can be weighted down if attached to some rocks.