The Sierra Nevada, which forms the eastern side of the valley, is the eroded edge of a huge tilted block of crystalline rock that also partially defines the base of the valley sediments (fig. 74). Embedded in the granite and related plutonic rocks of the mountains are metamorphosed sedimentary and volcanic rocks of Ordovician to Late Jurassic age. The uplift that formed the Sierra Nevada probably took place between Late Jurassic and Late Cretaceous time.
The northeast corner of the basin is the southern terminus of the Cascade Range. This is an area of lava plateaus and volcanos, some of which have been active in modern times. Geologically, this area of the basin is relatively young; most of the volcanic activity was during late Tertiary to Holocene time.
The northwest corner of the Central Valley is bounded by metamorphosed volcanic rocks of Paleozoic and Mesozoic age. These rocks form a minor part of the valley boundary. The western side of the valley is bounded by the Coast Ranges, which are formed primarily of folded and faulted marine sedimentary rocks of Mesozoic age, which were uplifted during Tertiary time. Mesozoic marine sedimentary rocks and continental deposits underlie the Tehachapi Mountains that bound the valley to the south.
A huge volume of sediments, which is as thick as about 50,000 feet in the Sacramento Valley and about 32,000 feet in the Tulare Basin, fills the Central Valley. These sediments are marine and continental in origin; the marine sediments are the product of deposition during inundations by the Pacific Ocean, and the continental sediments were derived by erosion of the rocks that formed the surrounding mountains.
The ancestral Central Valley was, at least in part, inundated by the Pacific Ocean until 2 million to 3 million years ago. The location, depth, and age of marine sediments in the valley indicates that nearly the entire valley was covered by the sea during Paleocene and Eocene time (fig. 75). As sea level declined, the area covered by the ocean decreased until only the southern end of the basin was still under water in Pliocene time. During Pleistocene and Holocene time, the sea completed its retreat, and all oceanic deposition ceased. In total, the ocean left behind deposits that ranged in thickness from about 25,000 feet in the Sacramento Valley to about 20,000 feet in the San Joaquin Valley. These deposits are mostly consolidated and have minimal permeability.
From the time when the valley first began to form, sediments derived from erosion of igneous and metamorphic rocks and consolidated marine sediments in the surrounding mountains have been transported into the valley by streams. These continental sediments are as thick as 9,000 feet at the southern end of the valley and have an average thickness of about 2,400 feet (fig. 76). The continental sediments consist mostly of fluvial, basin-fill, or lake deposits of sand and gravel interbedded and admixed with clay and silt (fig. 77). Depending upon location, deposits of fine-grained materials--mostly clay and silt--make up as much as 50 percent of the thickness of the valley-fill sediments.
