Had an idea while cleaning up the house. I was throwing out some old boxes, and found some egg-box type of cardboard packaging material inside one of the boxes (it was from a laser printer toner cartridge box).
Looking at the outer side of the packaging I cut off the four main bulges
I painted them codex grey, then washed them with devlan mud
I poured static grass into a plastic container
Then I spread glue around the bottom of the 'boulders' and a little up the sides in places. Putting it on the bottom edges of scenery helps to blend the scenery objects into the terrain board they are sat on.
Then I drybrushed with codex grey
Daleks in the countryside :)
Watchers of Karna on the road, near some boulders