For some time I have been wondering about this. It's a picture from my (paved) garden. And from time to time these kind of sand formations appear between the tiles. And I wonder what kind of force is creating them. When I am flatten them with my shoe till it's even again as it should be then after a few days it comes back again. I went through all kind of scenarios, I think I finally know, although unproven yet. What's your guess?