That part of the Domengine Formation in the deeper parts of the Sacramento Valley basin accumulated under predominantly transgressive marine conditions. Major textural varieties of this feldspathic subgraywacke unit in the Potrero Hills-Rio Vista area include clayey sandstone, sandy siltstone, and silty shale. The source of thesediments was the Sierra Nevada batholith complex and its western foothills belt of metamorphic rocks. Detrital contribution from the Coast Ranges is unsupported by conclusive evidence. Pyroclastic debris from the Sierra foothills or a western island chain comprises a third and minor source of sediments. Kaolinitic alteration of feldspar suggests a source area of moderate relief with warm and humid climate. Five lithologic subunits are recognized.