Metamorphic rocks were another kind of rock before they were cooked. This was a mix of gravel and clay that had been subducted and raised up again into these hills by the Greenville fault North of Livermore California. Quartz and other worn stones are well cemented having been tharmally fused to the surrounding clay. a layer of leichen is erroding the surface of the matrix. Oil was discovered a few miles from here in the 1850s.