After building the basic wood structure and the underground, I had to work on the topography of the upper part of the display board. I wanted to have something in between a canyon and rocky hills. I used PS insulation foam and cardboard to make the general layout as I wanted to add as little weight as possible. According to my calculation, the total weight of the display board would be around 16 kg just for the wood structure!
I bought a bag of rock slate in a gardening store (15€ for a bag of 10kg). I glued them using some roughcast bought in a DIY store (10€ for a drum of 15kg) that I spread as homogeneously as possible aver the entire board. I then put some barks in the areas where I wanted a bigger rock effect contrasting withe the slate rocks. Finnaly I added some gravel to give another layer of detail.