Oldest rocks in California are considered to be the ancient metamorphic rocks which are schist and gneiss and are ranged in age from about 1.4 billion years to 1.7 billion years.
These old metamorphic rocks are derived from the older sedimentary, metamorphic and igneous rocks and were highly subjected to heat, pressure, partial melting and recrystallization that occurred during the period of mountain building which is associated with the continental collisions of the tectonic plates which existed long ago in Earth's history. Such rocks are found in the Grand canyon of California.