You can silicone in a smooth divider or silicone together large rocks or smooth decor as the land. You can make a ridge with the rocks on top as a border, kind of like an slightly elevated box, you can also use foam to create boundaries. Great stuff foam and/or styrofoam coated in silicone or grout. Lots of options.

This is my favorite site for some ideas... http://www.lizard-landscapes.com

Hope this helps