I'm in the socal coastal mountains, trying to experiment with cob. I've got a large hillside with some exposed sandstone and plenty of grass gone to seed so all I need is clay. I've noticed while landscaping that about a foot beneath the dirt is rock which I can break up in water and forms something that may be clay. I can't really make it into a snake but when I wet it and scrape it with a knife I can ball the remains into clumps that resemble clay. It's fairly white/grey. Does anyone know if this will suffice or should I seek an alternative?