I make this into two parts, the board are almost ready but needs one more day to dry. Will try to put up part 2 as soon as possible.
I use a 80 x 40 cm thin wooden board, of course you can take any size you seem fit. On this I have polystyrene (fine-fibred styrofoam) which I cut in to and fitted perfectly on the board. Then I take ordinary wood glue on the bottom and let it set. Don't use any other glue as it may melt the polystyrene.
I then use a knife to cut out the shapes I want to make it look like a mountain wall. Keep the stuff you cut loose as you can use it as larger rocks and add height to some parts.
Then I added some filler to make the transition between the mountain wall and the ground look more natural, I also filled the joints between the two polystyrene blocks.
Then I paint everything except the rocks with wood glue and pour ordinary sand all over. When it has dried I spray paint the rocky areas with chaos black spray. This can melt some parts of the polystyrene but is doesn't matter, it just gives more structure to the surface.
The rest and the more fun parts comes in part two.