The rocks that became California formed on the bottom of the Pacific Ocean. What was to become the Central Valley was a two-mile deep trench in the ocean. California was gradually constructed by the collision of segments or plates of the earth's oceanic and continental crusts and the motions of earthquakes. Throughout this time, sediment eroded from ancient mountains and shorelines to completely fill the deep trench, thereby forming the floor of the Valley.

Floods deposited sediment on the floor of the Sacramento Valley. The floods occurred starting about two to three million years ago, shortly after the rise of the Sierra Nevada Mountainst. They occurred during a ancient ice age, when the glacial ice in the mountains melted. Wooly mammoths, saber-toothed tigers, and giant sloths lived on the floor of the Valley at this time.