@kntry - thank you :-)
Actually the rocks were attached in such a way that i kept tiny holes in them for rhizome plants to go in :-) the plants in there are java fern 'mini', anubias with riccardia sp. 'India' growing in from the rocks.
The scape idea you are planning is really nice and it does add a charm to the diorama layout style..will look forward to the project. Getting back - there are 2-3 ways by which you can stick the rocks togather
1) use a hot glue gun
2) use industrial grade epoxy putty or adhresive
3) use silicone
If you have a porous hardscape material like lava rocks your best and easiest bet is using hot glue gun..mind you - you have to be very generous while application :-) for any material irrespective of porosity you can either use epoxy putty or epoxy adhesive. Putty is a tad cheaper alternative and imo a better cementing material. Silicone i seldom use only for extremely small hardscape materials plus silicone takes a lot of time to adhere to hardscape. Note - if you are using either of the three methods to create a mountain..make sure after your hard scape is ready(wait for everything to dryoff first) that you fill the tank up without plants and run a power head after fill up for one week with water change every day to all most 100%. This way the chemical leaching in the water from the glue/gum gets reduced effectively and causes less issues when planted and filled with water.
There are a lot of ways by which one can separate sand to soil
1) use hardscape like rocks to separate
2) use some thin clear acrylic
If i need to keep sand separated from mixing say in a slope..i'll let you know of a trick that i do..wet the sand so that its kinda sticky..place a thin layer on the slope area where you want to place it on top apply a few drops of any 'cyano acrylite glue' this is completely harmless the moment it dries..and it dries in seconds(wear a gloves while using this..it can stick skin togather) once done re apply the above steps till the time you are satisfied with the layer coat. Let me know if i can help any further with your project will be glad to do so. :-)
Adding a picture of the same process as i described above to give you an idea how it looks like..since the sand on the slope is rock solid..any soil thayt rolls on top can be easily siphoned during water change. The inspiration was a painting from deviantart website and the tank is my execution trying to bring the art alive (litterally) :-) this is another new tank just started a month back.