Looking at a map of the U.S., one might think that California's Central Valley, on the west coast, and the Chesapeake Bay, on the east, were formed in a similar way; but they would be wrong. While the Chesapeake represents a drowned river valley, the Great Valley of the Golden State is far more complex.
When the Atlantic Ocean began to open, during the Jurassic Period, the west coast of the North American Plate was forced to collide with the Farallon Plate, which underlied what is now the eastern Pacific Ocean; at that time, the west coast of North America was at the longitude of central Utah. As pressure mounted, the Farallon Plate was forced to subduct beneath North America and, as the latter moved westward, a series of exotic terrains and volcanic island arcs were added to the Continent. Between these land masses, seaways were forced to close and, in the process, their oceanic crust was either subducted beneath the growing continent or forced to crumple onto its edge.
The area now covered by California's Great Valley was once one of these entrapped seaways. In fact, it was a deep subduction canyon; to its east, the Farallon plate was melting into the massive batholith of the Sierra Nevada. Over time, this seaway became surrounded by mountainous terrain as the Cascades, Sierra, Coastal Range and Transverse Range formed; sea deposits, volcanic debris and erosional sediments from these highlands gradually filled the sea basin and the surface of the inland sea rose and fell in concert with changes in the Earth's climate. Its connection with the Pacific, once at the latitude of Monterey, shifted northward as the Farallon subduction ended and land shifts along the San Andreas Fault began to reshape the coast.
Eventually filled to the brim with sediments, the massive estuary shrunk to its famous remnant: San Francisco Bay. Throughout the Pleistocene, extensive meltwater lakes covered most of the Valley but, today, reservoirs and water diversion have cut off most of that flow, threatening the health of the Bay and its adjacent wetlands. The Valley itself, once a seaway, has become a flat landscape of desert, irrigated croplands and cities; beneath its surface are miles of sediment.
