As the Pacific Ocean on the west and the ancient Great Basin Sea on the east alternately encroached on the California region, each supplied that part of the record which the other omitted. In formations of the last two periods, the Tertiary and the Quaternary, California is particularly rich.
Structurally the Sierra Nevada is a single colossal block of earth's crust lifted along its eastern edge to a height of more than 11,000 feet above the adjoining blocks, and gently tilted westward. The oldest known rocks making up these mountains are intrusions of molten rock (magma) and limestones, cherts, shales, and sandstones, all sedimentary, and nearly all changed into their metamorphic equivalents in the process of mountain building. These older sedimentary rocks were deposited in ancient seas of shifting extent and depth, which during the second half of the Paleozoic and the first two periods of the Mesozoic era, covered now one part, now another, of the Pacific Coast. Toward the close of the Jurassic period, the lands that were eventually to become the ancestral Sierra Nevada, the Cascades, and the Klamath Mountains began to emerge from the sea.
During the Cretaceous period the Sierra's whole block tilted westward. This process of tilting and folding wrenched open leaves of slates, once shales; heated mineral-bearing solutions escaped from the magma that was cooling and solidifying below and filled the slate openings with gold-bearing quartz. The Eocene epoch of the Tertiary period was comparatively quiet. The Sierra slowly underwent additional elevations and subsidences accompanied by active erosion of the surface rocks. Meanwhile the rivers were cutting their channels down the western slope and carrying the products of erosion to the inland sea. There was further release of gold from the bedrock, and the formation of rich placers. In the Oligocene epoch following, there was volcanic activity, and the Sierra gold-bearing stream channels were dammed and filled with rhyolite ash.
Volcanic activity continued during the Miocene age, and in addition to lava there were extensive mud flows and tuffs. In the Pliocene epoch the volcanoes were far less active, and in the Pleistocene the volcanic cover was removed in part by erosion. The veins and buried stream channels were cut into, and gold-bearing gravels were washed from their ancient channels and redistributed along new streams. This is the origin of so-called free gold. The Sierra had been greatly worn down in late Tertiary times, but the Pleistocene epoch of the Quaternary period was an era of re-elevation. There was much faulting, and a new period of volcanic activity began which is not quite ended today.
In the early Tertiary period the Sierra slopes were luxuriant with vegetation, but toward the end of that period the climate became much
