The aquifer system of the Santa Clara Valley is bounded on three sides by the relatively impermeable consolidated rocks that form the mountains surrounding the valley (fig. 112) and underlie the valley at depth. Ground water in the valley is contained primarily in coarse-grained lenticular deposits of sand and gravel that alternate with discontinuous beds of fine-grained clay and silt that have minimal permeability. The combined thickness of the coarse- and fine-grained deposits is as much as 1,000 feet in some parts of the valley. The alluvial-fan and river-channel deposits near the valley margins contain a higher percentage of coarse-grained materials than deposits near the valley axis and are thus more permeable.
Although interspersed with coarse-grained channel deposits, the cumulative thickness of clay and silt is sufficient in the central two-thirds of the valley to produce confined conditions in the subsurface from southeast of San Jose to beneath San Francisco Bay. Water below a depth of 150 to 200 feet in that area is confined or semiconfined, whereas shallower water is generally unconfined. The confined part of the aquifer system is as much as 800 feet thick, but locally it contains beds of fine-grained deposits that separate it into zones of permeable material sufficiently distinct to be recognized as individual aquifers.