We've had 4 inches of rain in the past 5 days. I'm sooo happy I covered the run with 3 to 4 inches of sand 2 weeks ago when it was bone dry. Mud, mud everywhere mud, but not in my run. I learned it here!
Coarse sand, the type used in making concrete, not mortar. Added benefit is that you will not need to buy grit. They will get all they need from the sand granules. It will just be limestone instead of granite. I know of no difference except the cost of course. ($1/Lb vs $15/ton.) Also good to put a pile of compost in center of run for variety and for them to burrow down in and scratch around in. Seed it good with baby crickets that can be bought and shipped to your door cheap. Great fresh protein for the chickydoos.