Harvesting clay questions

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?

Go to this site start WSS: http://websoilsurvey.sc.egov.usda.gov/App/HomePage.htm put in location run report post it. A jar test and pic is worth a 1000 words.

Fine silt can look a lot like clay, but it is not a good binder. Instead of flat plates, it's made of roundish particles that are much larger.