You can use the sheets of foam used to insulate houses. Comes in blue and pink in several thicknesses from 1/2" up to 2". It's easy to cut and can be shaped with serrated bread knives and steak knives, files, rasps and the like. Different thicknesses cut, shaped and layered can give a good effect.

For the area where the waterfall is going to splash down your mountain you might want to use real stones as they will stand up to the wear from the water.

To give it the natural look you can use different colors of bathroom tile grout or mason's mortar with color additives. You should be able to get these at any home center. Mix it to the consistency of pancake batter and brush and dribble on your foam structure. Try to use colors compatible with the stones. Put your base layer on thick. Change colors for the next layer and brush on more randomly so some of the base color shows, next layer same thing. As you build up layers you can get a very realistic stone effect. Once complete you can seal it with several coats of satin finish Polyurethane. You might also want to get some sand. Dusting areas with some sand thrown on enhances the effect.

This will want to float so it will need to be securely attached with clear 100% silicone caulk.
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BigT
There is a difference between ignorance and stupidity. The ignorant can be taught, stupidity is beyond our control.
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