Making lightweight rocks or mountains.

You first get the picture of your final setting in your head. In this case it is going to be one or two trees in the foreground with a far view mountain with sloping sides and some cliffs. There will be a lot of little pockets created in the mountain where moss and far view trees can be planted.

You then select a flat surface and cover it with newspapers. You can outline the slab with a pencil.

You then take a product called PU-FOAM (Spray polyurethane foam, commonly referred to as SPF is a spray-applied insulating foam plastic that is installed as a liquid and then expands many times its original size.)and do the first layer covering the newspaper. When the first layer has settled you add the second layer keeping in mind the depth of soil your bigger tree is going to need. When this second layer has dried out a bit you add the third layer. In my slab the third, fourth and fifth layer cover only the mountain area. Repeat until you get the desired height of the mountain area. Give the foam a light spray of water before adding the next layer.

When it is dry the backside of my mountain looked something(did not take a pic at that stage of this one) like this:

I then took a knife and carved the mountain area roughly into the shape I needed it to be and did the refinement with my Dremel tool:

Now you can clearly see the area where the bigger tree/s is going to be planted. The little pockets for the moss was carved out also.

I then turned it around and prepared the underside of the slab for the Rockset I am going to use to make a strong base. The foam was carved away leaving only a rim to keep the Rockset inside. If your bottom is not 100% level the rim can be build up by using Polyfilla. If there are any holes in the foam cover them with Polyfilla. if you don't do it the Rockset will leak through because the consistency of the Rockset mixed with water is almost like cream. I placed a few pots, covered with aluminum on the prepared surface where the drainage holes would be. The slab is about 6mm thick:

The bottom with some drainage holes pots removed:

I then let it dry and removed the rim that remained. I then turned it upright again placing the slab on a soft level area because the slab is not strengthened by the topside yet. I then did the refinement carving making sure some of the bigger pockets will also drain into the "pot" area. I then took Rockset again and painted on a few layers:

Here is the process 50% completed:

The foil is to prevent the Rockset to sticking to the rubber surface I placed it on and to catch up the Rockset running down. This rim forming in this foil area will strengthen the slab.

Here is a close up where the unpainted inside of the pockets are visible:

I repeated the process of painting the slab a few times until everything was covered. I believe the Rockset layer will be about 3mm thick.

I removed the foil and colored the mountain with oxide. I then sealed it with sealer looking like woodglue that would also strengthen the mountain. At this stage the mountain looks a little bit too dark but most of the dark areas are going to be planted with smaller trees, plants and moss. If necessary I could lighten it again using the sealant and oxide.

 The mountain:

If this were rock you would have a difficult time picking it up. This one can be carried with one hand.

Will post a pic of the planting when it is finished.

Another mountain/outcrop I made:
 Front:

 Back:

Top:

I am thinking of planting a bigger, but not too high tree on this one. The right hand side will show little moss but the left hand side will show a lot of greens.