What is done with the pebbles after they are thrown at Jamarat during the Stoning of the Devil?

First, my thoughts are with the more than 700 people who were killed and many hundreds who were injured. Whether one agrees with Islam or not, it's still a terrible thing that so many were hurt, especially since they were trying to fulfill a religious obligation.

I looked at photos of the walls that pilgrims throw stones at, and realize that if there are 2 million pilgrims, 7 stones each at each of the 3 walls, that's on the order of 40 million stones.

Maybe I should have asked this in current events rather than R&S.