 Some 400 million years ago, rocks that would be accreted onto western North America to build California lay at the bottom of a large sea.Subsea volcanoes deposited lava and minerals (including gold) onto the sea floor; sometimes enough that islands were created.Between 400 million and 200 million years ago, geologic movement forced the sea floor and these volcanic islands and deposits eastwards, colliding with the North American plate, which was moving westwards.