The Central Valley aquifer system is formed primarily of sand and gravel with significant amounts of silt and clay, all of which have been eroded mainly from older rocks at the boundaries of the valley. The environments in which the continental sediments were deposited varied, but most were deposited in fluvial environments; however, the deposits contain some lacustrine beds. Locally, volcanic rocks and dune deposits are part of the aquifer system. Specific geologic formations can be related to specific aquifers within the Central Valley aquifer system only with difficulty because many of the formations are lithologically similar, and cannot be distinguished easily in the subsurface.
Beds and lenses of fine-grained materials, such as silt and clay, constitute a significant percentage of the Central Valley aquifer system. In most parts of the valley, fine-grained materials compose 50 percent or more of the aquifer system. The most extensive clay bed, which is informally named the "E-clay" (fig. 77), consists primarily of the Corcoran Clay Member of the Tulare Formation and underlies much of the western San Joaquin Valley. Because beds of silt and clay do not readily transmit water under natural conditions, they act as barriers to vertical flow and cause differences in hydraulic head with depth.
Early investigators thought that the Sacramento Valley contained a single unconfined aquifer and that the San Joaquin Valley contained an upper unconfined to semiconfined aquifer separated from a lower aquifer confined by the Corcoran Clay or "E-clay" (fig. 78). However, recent investigations indicate that the Central Valley contains a single heterogeneous aquifer system that contains water under unconfined, or water-table, conditions in the upper few hundred feet; these conditions grade into confined conditions with depth. The confinement is the result of numerous overlapping lens-shaped clay beds. Geophysical well logs indicate that the "E-clay," although probably the largest single confining bed, constitutes only a small percentage of the total thickness of clay layers in the aquifer system. This indicates that the significance of the "E-clay" as a barrier to vertical flow may have been exaggerated. Further, the difference in hydraulic head directly above and below the "E-clay" is small when compared to head differences within intervals of the deep parts of the aquifer system.