Why pea gravel? Because it is malleable, it fills in the voids in the hoof where healthy tissues would be and increases blood circulation by 90%, which in turns speeds up healing and new cell growth dramatically (study by Dr. Robert Bowker, MSU). It supports the coffin bone of the hoof and relieves peripheral loading by distributing weight over the whole hoof. Pea gravel makes great footing for areas horses spend a lot of time standing around in. It gives the horse a malleable area to stand or lay and gives the hooves a drying out time during the winter and wet season. Standing on pea gravel is like a good foot massage for the horse. It's like standing on foam pads for horses!
Interested in reading more about pea gravel, peripheral loading, and the effects it has on hooves? Read this: http://www.thehorseshoof.com/Art_Pad.html
If you have a horse with "flat soles" and would like to develop more concavity, start an area of 4-6" of pea gravel and see if that helps!