Geological Survey Bulletin 612 Guidebook of the Western United States: Part B

San Francisco, the chief seaport and the metropolis of the Pacific coast, is the tenth city in population in the United States and the largest and most important city west of Missouri River. The population in 1910 showed a gain of 20 per cent since 1900. The city is beautifully situated at the north end of a peninsula, with the ocean on one side and the Bay of San Francisco on the other. The bay is some 50 miles in length and has an area of more than 300 square miles. The entrance to the bay lies through the Golden Gate, a strait about 5 miles long and a mile wide at its narrowest point.

The site of the city is very hilly, and a line of high rocky elevations run like a crescent-formed background from northeast to southwest across the peninsula, culminating in the Twin Peaks, 925 feet high. Telegraph Hill, in the northeastern part of the city, is 294 feet above sea level. Here stood the semaphore which signaled the arrival of ships in the days of the gold seekers. The city has been laid out without the slightest regard to topography; consequently many of the streets are so steep as to be traversable only by cable cars and pedestrians. The waters of the bay formerly extended westward to Montgomery Street, and most of the level land in the business section of San Francisco has been made by filling.

Golden Gate Park, containing 1,014 acres, and extending westward from the city to the ocean, was a waste of barren sand dunes in 1870, but skillful planting and cultivation have transformed it into one of the most beautiful semitropical public parks in the country. At its west end is the famous Cliff House, overhanging the sea, and a short distance out from the shore are the Seal Rocks, where the great sea lions may often be seen. The Sutro Baths, near by, named after Adolph Sutro, constructor of the famous Sutro tunnel on the Comstock lode, contain one of the largest inclosed pools in the world.

San Francisco Bay is the largest and most active harbor on the Pacific coast. Besides the coastwise routes, the port maintains steamship connections with Australia, Hawaii, Mexico, Central and South America, the Philippines, China, and Japan. The direct foreign trade is chiefly with British Columbia, South America, China, and Japan. Although the export grain business has now largely shifted to the ports of Oregon and Washington, San Francisco's permanence as one of the greatest ports of the country is assured by its advantageous position, its wealth of back country, and its command of trans-Pacific and transcontinental trade routes. Three large railroad systems—the Southern Pacific (with two transcontinental lines), the Atchison, Topeka & Santa Fe, and the Western Pacific—connect it with the East. Lines of the Southern Pacific Co. connect the city with different parts of the State and with the northern transcontinental lines. The Northwestern Pacific serves Mendocino, Sonoma, and Mann counties, on the north, and several smaller lines radiate from different ports on the bay. Only one of the lines mentioned, the Coast Line of the Southern Pacific, actually enters the city. The other roads have their terminals in Oakland and other cities around the bay.

The first settlement on the present site of San Francisco dates from 1776. It consisted of a Spanish military post (presidio) and the Franciscan mission of San Francisco de Asís. In 1836 the settlement of Yerba Buena (yair'ba bway'na) was established in a little cove southeast of Telegraph Hill. The name San Francisco was, however, applied to all three settlements. The United States flag was raised over the town in 1846, and the population rapidly increased, reaching perhaps 900 in May, 1848. The news of the gold discoveries was followed by crowds of fortune seekers, so that by the end of 1848 the city had an estimated population of 20,000. From that time on San Francisco has grown rapidly. The first regular overland mail communication with the East was established by pony express in 1860, the charge for postage being $5 for half an ounce. In 1869 the completion of the Central Pacific Railway to Oakland marked the beginning of transcontinental railway communication.

The city suffered from severe earthquakes in 1839, 1865, 1868, and 1906. In respect to property loss the disaster of April 18, 1906, was one of the great catastrophes of history. The actual damage to the city by the earthquake was comparatively slight, but the water mains were broken and it was consequently impossible to check the fires which immediately broke out and which soon destroyed a large part of the city, including most of the business section. Some 500 persons lost their lives, and the estimated damage to property was between $350,000,000 and $500,000,000. Reconstruction began at once, and the city was practically rebuilt in the three years following the earthquake.

The Ocean Shore Railroad (station at Twelfth and Mission streets) and connecting automobile line afford a good opportunity to see the geology along the shore from San Francisco to Cruz. The return trip may be made by rail or stage across the Santa Cruz Mountains. For 4-1/2 miles north of Mussel Rock (11.9 miles from San Francisco) there is exposed in the bluffs along the coast a remarkable section of the Merced (Pliocene) formation, consisting of about 5,800 feet of highly inclined marine clays, shales, sandstones, conglomerates, and shell beds. In these beds have been found fossil remains of 53 species of marine animals, mostly mollusks, of which three-fourths are still represented by forms living in the ocean to-day. The San Andreas rift (the fracture along which displacement occurred in the San Francisco earthquake of 1906) passes out to sea at the mouth of a little ravine half a mile north of Mussel Rock and is crossed by the railroad. The exposures of the Merced formation along the sea cliffs were much finer before the San Francisco earthquake, which shook down some of the cliffs. From Tobin (18.1 miles) to Green Canyon 21.1 miles) the bed of the Ocean Shore Railroad is cut in bold sea cliffs high above the water and affords not only fine shore scenery but also an excellent section of rocks that probably belong to the Martinez (Eocene) formation. The contact of these rocks with a large mass of pre-Franciscan granite (quartz diorite), which forms Montara Mountain, a bold ridge that extends southeastward from this part of the coast, is crossed by the railroad between Tobin and Green Canyon. At the north end of Seal Cove, opposite Moss Beach station (24.1 miles), the bowldery and fossiliferous sea-beach beds here forming the base of the Merced (Pliocene) and resting on the granite of Montara Mountain are well exposed.

This delightful excursion may be extended down the coast to Pescadero, and the return made by stage across the range and rift zone to San Mateo; or the traveler may continue down the coast to Santa Cruz and return across the range on the Southern Pacific line either by way of the Big Trees and Los Gatos or by Pajaro and Gilroy.

The characteristic thin-bedded radiolarian chert of the Franciscan group is well exposed about Strawberry Hill, in Golden Gate Park. There are good exposures of the chert also on Hunter Point, reached most readily by the Kentucky Street cars from Third and Market streets. The principal rock of the point is serpentine. A mass of basalt in the sea cliffs on the south side presents a remarkable spheroidal and variolitic structure.

The summit of Mount Tamalpais is very easily and comfortably reached by ferry to Sausalito, electric train to Mill Valley, and a mountain railway to the hotel on the top. The ferry trip is one of the best to be had on the bay. The steamer passes close to the island of Alcatraz, on which is a disciplinary barracks. To the west may be seen the ocean through the Golden Gate. Angel Island, with its interesting glaucophane schists, serpentine, and other rocks, lies to the right as the boat approaches Sausalito. The sedimentary rocks of both islands belong to the Franciscan group and are chiefly sandstone. The trip from Sausalito to Mill Valley by the Northwestern Pacific Railroad gives the traveler opportunity to see some characteristic bay-shore scenery and particularly to note how the waters of the bay appear to have flooded what was once a land valley. Mill Valley is named from an old Spanish sawmill, the frame of which is still standing. The views obtainable from the scenic railway and from the summit of Mount Tamalpais are extensive and varied. To the south may be seen San Francisco and Mount Hamilton (4,444 feet). To the southeast is Mount Diablo (3,849 feet), through which runs the meridian and base line from which the public-land surveys of a large part of California are reckoned. Nearer at hand is the bay, with its dark-green bordering marshes through which wind serpentine tidal creeks. Close under the mountain to the north is Lake Lagunitas (an artificial reservoir), and beyond it ridge after ridge of the Coast Range. To the west is the vast Pacific.

From the summit of Tamalpais one sees clearly that San Francisco Bay is a sunken area in which hilltops have become islands and peninsulas. This area is the northern extension of the crustal block whose sinking formed Santa Clara Valley. A later sag admitted the ocean into the valley, and the Golden Gate, formerly a river gorge, became a strait.

Mount Tamalpais has really three peaks: East Peak (2,586 feet), near which the Tavern of Tamalpais is situated; Middle Peak (about 2,575 feet); and West Peak (2,604 feet). From the grassy hills 1-1/2 miles west of West Peak there is a good view of Bolinas Lagoon, though which passes the San Andreas rift, but for close views of the rift topography the visitor should walk or drive through the valley between Bolinas Lagoon and Tomales Bay, where the effects of the movement of 1906 are still in many places clearly evident.

Mount Tamalpais is composed wholly of the sediments of the Franciscan group and the igneous rocks usually associated with them, though it is chiefly sandstone. A mass of radiolarian chert occurs near the tavern, and serpentine may be seen at several places beyond West Peak. To one fond of walking and of marine views, a trip on foot to West Peak, thence down the main ridge to Muir Woods (redwoods), and back across the hills to Mill Valley may be heartily recommended. The distance is probably 8 or 9 miles. The Muir Woods, which bear the name of California's greatest nature lover, form a national monument, presented to the nation by William Kent, now a Member of Congress from the first California district, for the purpose of preserving untouched by the lumberman one area of redwoods. No fitter memorial could be dedicated to the memory of John Muir, whose writings have contributed so much to the movement for preserving in national ownership, for public enjoyment, some of our finest scenic resources.

The geologic event of greatest human interest on the Pacific coast in modern times was the San Francisco earthquake of 1906. It was produced by a sudden movement of the rocks (faulting) along opposite sides of a fracture which may be traced for many miles in the Coast Range. The fissure existed before the earthquake of 1906, and it is evident from the relations of hills and valleys along its course that it has been the scene of earlier and, for the most part, prehistoric movements. The last movement was mainly horizontal and in places amounted to about 20 feet. The San Andreas rift, as this fissure has been called, lies just west of San Francisco, and its course is marked on sheet 25 (p. 224).

The cracks in the soil that mark the line of the last displacement and the parallel ridges and valleys that show older displacements along the fault zone are well displayed in Spring Valley, 13 miles south of San Francisco, and especially near Skinner's ranch, 40 miles northwest of San Francisco.

To reach Spring Valley the visitor should take the Southern Pacific train to San Mateo (18 miles), where a conveyance may be obtained for a trip through Spring Valley along Crystal Spring and San Andreas lakes.

Skinner's ranch can be reached by the ferry to Sausalito and the Northwestern Pacific Railroad to Point Reyes station, from which the ranch is only 2 miles distant, near Olema. In this region may be seen best the earth cracks along the fault line. Near the ranch house there is striking evidence of the horizontal character of the movement that produced the earthquake. The house formerly had two trees in front of it. The fault line, which trends northwest, passes between the trees and the house, and the trees were moved 15 feet to the southeast with reference to the house. There was no perceptible vertical movement nor any change in the water line along Tomales Bay.