Page:California a guide to the Golden state-WPA-1939.djvu/53

Rh There are no Paleozoic (old life) rocks in the northern Coast Range, but crystalline limestone and schist, probably of this age, are found in the Santa Cruz, Gabilan, and Santa Lucia Ranges. Of the next era, the Mesozoic, Triassic period remains are lacking, but from the Jurassic come most of that complex series of Coast Range rocks known as the Franciscan. These are sedimentary rocks of several types: conglomerate, sandstone, shale, variegated chert, and (rarely) limestone. With them is embedded a great series of volcanic and plutonic rocks of the same age.

Cretaceous rocks in the Coast Range are abundant. They make up considerable parts of the Santa Lucia, the Temblor, and Diablo Ranges, and they become even more widespread north of San Francisco. The rocks consist chiefly of shale, siltstone and sandstone, with some small streaks of coal, and—near Coalinga—shale, which is the source of the oil in overlying Tertiary beds. The Cretaceous sea covered considerable parts of what is now the north Coast Range, but the region that now comprises the Santa Lucia Range and the Salinas Valley was relatively higher than at present, and formed Salinia, a long narrow peninsula running out to the northwest. The Eocene strata are relatively uncommon except in the eastern foothills near Coalinga and in the Mount Diablo region. The rocks are similar to those of the Cretaceous. There are considerable beds of coal, but the latter is of poor quality. Salinia had become an island, and there was a similar island whose axis ran along what are now the Gabilan and Mount Hamilton Ranges northwest to Marin County.

The Oligocene formations in the Coast Range are chiefly of red sandstone; there are also certain organic shales, which seem to be the source rocks for the oil of Kettleman Hills. The seas had become less widespread. Salinia extended farther north and west, but the San Joaquin Valley still formed an arm of the sea into which drained the rivers of Mohavia a name given to the region now covered by the Mojave Desert, Death Valley, and the Owens River Valley. In the early Miocene there was much volcanic activity in the Coast Range, and this ultimately cut off the sedimentary deposits from Mohavia and prevented their reaching the sea. There followed in the late Miocene another period of widespread shallow seas and many coastal islands. Much organic siliceous shale was laid down, and this is the source of the oil in the Santa Barbara and Ventura coast region as well as elsewhere. Of Pliocene origin are calcareous and feldspathic sand-stones and thick beds of brown and blue sandy clay. As elsewhere in California, the climate became cooler. There was still a series of islands and peninsulas along the entire coast.

In the Pleistocene epoch most of the old interior seas and bays disappeared. This was a period of violent deformation of structure, with