For ground with different rocks I used: Wood filler, fast dry glue for wood, gravel and two different gravel types special for our modelling purposes. I used wood glue for upper areas ( also to mount rails on styrofoam elevation ). All lower areas was covered wood filler ( it's great filler because it a little granular structure and after dry it looks like dry sand). When I do that I've started to laying gravel. And of course need to press that layer using fingers. After that need to wait for dry, It takes few hours ( my base spent two days near by hot radiator ). And now I've painted it with few different sand colors paints. Now waiting for oil wash and stones painting.