XC jump question: What is the surface under waterjumps?

At my old barn (more tha 20 years ago) ours was a very coarse sand -- almost like the stuff that you might find in a fish tank or aquarium. It wasn't really being maintained anymore when I was there, so unfortunately it was a bit slick from the algae growing in it, but I would guess that this coarse material would stay in place very well while not being SO coarse that it would hurt the horses' feet.