Reminds me of the sand art that some American Indian tribes produce, one example was a rendering of the 'US Seal' in many colours, quite large and stunningly done. It had been set in some kind of sealant and mounted for sale for some fiendish price. I've also read about some British artist who draws scenes in the dirt that collects on the back doors of delivery vans. Beautifully rendered, and done only with a fingertip! Can't remember his name tho'