Pea gravel is excellent. However, they need finer dirt to take dirt baths in. If they can't free-range, put in a kitty litter box like I have in my run (see it on my cams at www.HenCam.com) Put in sand and a cup of food-grade diatomaceous earth. With this and good management, I've never had to use chemical wormers (and I do fecal tests on the flock so I know I don't have a problem.)