Use sand that the glass blowers use. Its finer and the grain is smaller so it looks more to scale. Once you have the sand in place saturate the whole area with Woodland Senics scenery glue and let it set for about two days. you can also use Elmers glue diluted with water and just a few drops of Dawn dish washing detergent. Once its set you can go back over it with more sand if needed and just repeat the proccess. When you get it where you think its perfect seal the sand with a light mist of dullcote. You may want to give it a wash of flat black or brown or tan to give it more depth .
You could also look into florists shops for sand. And casting sand is small grain too... so see if there are any jewlers or foundarys in your area.