At that time, volcanic lava flows and river sands and gravels were actively deposited together, hence "interbedded lavas and gravels". The volcanoes may have been similar to island arcs. The Petaluma Formation is found in outcrop from Sears Point to Santa Rosa (through Sonoma Mountain) and as far west as Cotati where it interfingers with a marine sandstone called the Wilson Grove Formation. Gravels in the Petaluma Formation did not come from rocks located in Napa, but have been sourced to mountains east of San Jose, California. This does not mean rivers flowed northward from San Jose to Sonoma; rather, strike-slip movement along the Hayward-Sonoma Valley-Carneros fault system has dislocated present-day Sonoma County north and away from the mountains in San Jose where the basin formed.