http://www.one-ring.co.uk/phpBB2/kb.php?mode=article&k=589


Victorinox fruit knife (for cutting and carving the foam) 
http://www.polyfilla.com.au/
Polyfilla – Fine Crack variety 
Rock with lots of bumps 
1/2 inch thick "blue" or "pink" insulation foam 
1/8 inch thick hardboard1" drywall screws 
white glue Aleene's Tacky Glue
drywall spackle 
sharp modeling knife 
screwdrivers 
paint acrylics
posterboard 
spacklin paste
crystal clear or acrylic sealer

Now that we have it all drawn out, take your x-acto and SCORE the foam core along all the lines. Make sure you don't cut all the way through the sheet!!
After you have all your lines cut, run your pencil or pen along all the cuts you've made. This should bend all the edges of your cuts in
to give you a great textured effect. You can run you pencil along these lines a couple extra times to get a pronounced look. Something like this. Sorry it's hard to see the texture here 



Adding texture to the Mountainside 


Once all the carving has been completed, Polyfilla was used to cover some sections of the mountain, particularly where any straight lines were visible. The process was repeated where necessary to improve the appearance, and then a full coat of Fine Crack Polyfilla (basically a thick paint-like liquid) was applied over all the exposed rock to give it more texture. More Polyfilla was applied to any point where straight lines were visible, and extra coats of Fine Crack Polyfilla were applied as and where necessary. More texture could be added using a small amount of fine sand mixed with the filler. Be careful not to cover up any of the detail already carved into the polyfoam. The image below is of an early stage in the texturing process; this took days as the filler must set completely before applying more over the earlier layer(s). 


I use DecoArt acrylics for large terrain pieces as they are much cheaper than the Games Workshop or similar equivalents. The base coat is Black mixed with varying percentages of Burnt Umber. It is very important to fill in all the nooks and crannies in the rocks, as well as on the tower. 

Once the base coat was thoroughly dried, I started with a heavy drybrush and stipple of less Black, varying amounts of Burnt Umber, Tan and White. I created some stratification, as would be seen in sedimentary rocks, just through use of colour. 

Another drybrushing, with paler mixes of the colours above was used to bring out the texture and detail of the rock.