The Sonoma Valley is part of the Coast Range Physiographic provence. Basement rocks that make up the valley at great depth are the Great Valley Sequence shale, sandstone and conglomerate deposited in a continental slope- to abysmal plain environment via turbidite flows. The Cretaceous Great Valley Sequence overlies and contacts the Franciscan Complex along the Coast Range Thrust. The Jurassic-Cretaceous Franciscan Complex includes crumpled, uplifted terranes that have resulted from the subduction of the former oceanic Farallon Plate under the North American continent. During late Miocene-Pliocene time (~10 to ~4 million years) the area was attended by volcanism (Late Miocene Tolay Volcanics and Late Miocene - Pliocene Sonoma Volcanics) which are interbedded with the late Miocene-Pliocene Petaluma Formation. The (~9 to 4 million year old) Petaluma Formation was a fresh-water river system flowing from east to west and through the volcanics.