Found this in one of the corrals on 07/06/18. It’s the compressed material flung out of a horse’s foot as he moves. The first photo shows the outer surface that faces the ground. The second photo shows the inner surface that contacts the hoof. Note the rocks and other debris that were pulled out.
In one of his earlier videos, Rick Gore used the example of an overflowing water trough to keep mud in the pasture. He explains the vacuum effect of mud as well as the flinging of dried mud in this video.
Western Horse Watchers Association 