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California's Central Valley is like a great bath tub. Its present surface is remarkably flat and consists largely of material eroded from the Sierra Nevada to the east or the Coast Ranges to the west and deposited in low alluvial fans. On multiple occasions in the distant past, the valley has been filled with water, creating a large lake that left a veneer of muddy deposits. About 650,000 years ago, rising waters of the most recent lake (named Lake Clyde after a California geologist) broke through the Coast Ranges and drained into the Pacific Ocean through the modern San Francisco Bay.

The stark line of Mesozoic (green) rock formations against Cenozoic (yellowish) rocks near San Francisco Bay (the coastal area at the center-left of the image) represents the San Andreas fault.