The principal water-yielding materials in the north San Francisco Bay area valleys are unconsolidated and semiconsolidated marine and continental sediments and unwelded tuffaceous beds in volcanic rocks (fig. 108). Consolidated rocks of Cretaceous and Jurassic age that underlie the entire area have little permeability and form the boundaries of the ground-water flow system. The permeability and extent of water-yielding deposits varies considerably. In all the valleys, alluvial-fan deposits and stream-valley alluvium compose the major part of the aquifer. Locally, marine and estuarine deposits of sand beneath the Santa Rosa and the Petaluma Basins are an important source of ground water. Volcanic tuff of Pliocene age in the areas of volcanic rocks shown in figure 108 yields water to wells in the Sonoma and the Petaluma Basins.
Ground water is under unconfined, or water-table, conditions in shallow alluvial deposits and locally where it is near the land surface in other types of rocks. The ground water is confined or semiconfined in deeper parts of the alluvial deposits and nonalluvial formations. Because of their lenticular nature, water-yielding deposits in the north San Francisco Bay area valleys are generally discontinuous and isolated. Further, many of the deep deposits are displaced by faults. As a result, the valleys are a collection of variously connected and isolated aquifers. Generally, the alluvial-fan and stream-valley alluvial deposits in each basin are sufficiently continuous to be considered single aquifers; however, because of the geologic complexity of the area and the limited availability of data, the exact extent and degree of continuity of many deep aquifers is unknown.