I have about 2' of sand above clay on one of my properties in Texas. These small brown rocks are at about 1 1/2' - 2' deep in the sand just above and into the top of clay. There is no rocks of any kind in the 1st foot of sand. Can you tell me what these are and why they are all over at this depth? They vary from 1/4" to 1" in diameter, the brown crust is a little harder than the center black, I took my knife and cut the one in my hand, it actually broke in half with my knife. The black is like hard ash.