The California earthquake of April 18, 1906 : report of the State Earthquake Investigation Commission, in two volumes and atlas.
Geology of the Coast System of Mountains
Definitions
In common with many other mountainous tracts the world over, the Coast System has limits which are difficult of precise definition. The criteria which serve to discriminate one tract from another are various and have different values in different cases. Any attempt at precise definition must be more or less arbitrary. An outline of the extent and subdivisions of the system will, however, be presented in summary fashion.
On the north the Coast System extends to the northern end of Humboldt County, and in that county and in southern Trinity County the last typical ridge is South Fork Mountain. This is a remarkably linear ridge beginning near the coast and extending with a northwest-southeast course to the vicinity of North Yallo Bally Mountain. Beyond South Fork Mountain to the northeast lie the Klamath Mountains, a group more nearly allied in the history of its uplift and in its constituent rocks to the Sierra Nevada than to the Coast Range. On the south the Coast System is sometimes regarded as ending in Santa Barbara County; and the mountains of Southern California, thence east-southeast and south to the Mexican boundary, are regarded as a distinct system, being viewed as a northerly prolongation of the orographic axis of the peninsula of Lower California. The chief consideration favoring this distinction is the change in trend of the mountain ridges, which becomes apparent just north of the Santa Barbara Channel. Other facts favor this discrimination, such as the prevailing absence of the Franciscan formations in the mountains of southern California and the greater abundance of granitic rocks; but more especially the greater incisiveness of the structural lines, indicating, on the whole, more intense orogenic action. But these considerations are largely offset by the unmistakable continuity of the tectonic lines of the northern ranges into the mountains of southern California, and by the fact that the movements to which their larger features are due date from the close of the Tertiary. It would seem, therefore, that there is sufficient unity of character in these coastal mountains, in spite of their change of trend, to warrant their being classed as the Coast System from South Fork Mountain south to the Mexican boundary and beyond. That term may be used in a comprehensive sense, significant of the genetic and structural unity which runs thru them.
It will nevertheless be very convenient to recognize three subdivisions of the Coast System thus outlined. The first of these subdivisions extends from South Fork Mountain on the north to the Valley of the Cuyama River on the south, and may, in accordance with popular usage, be referred to simply as the Coast Ranges, the term "system" being used only when it is intended to express the more comprehensive view. The second subdivision is a broad chain extending from Santa Barbara County to the far side of the Colorado desert with a general trend of west northwest-east southeast, and including the San Rafael, Santa Ynez, Santa Susannah, Santa Monica, San Gabriel, and San Bernardino Ranges, and also, perhaps, the Chocolate Range. This chain is sometimes referred to as the Sierra Madre, tho the full application of the term in popular usage is not clear. The third subdivision embraces the mountainous country south and southeast of the valley of Southern California, the principal ranges of which are the Santa Ana and the San Jacinto. These have the northwest-southeast trend of the Coast Ranges and, in accordance with the suggestion of some of the earlier writers on Californian geology, may be referred to as the Peninsular chain.
Geological History
The Coast Ranges of California have had a long and varied geological history. Their structure is complex and the sequence of formations differs at different points. Several of the more important groups of sedimentary rocks contain, so far as known, but few fossils or none at all. Only in recent years have the topographic maps necessary for an adequate study of the stratigraphy and structure of the region become available, and then only for limited areas. Nevertheless the general outlines of the geology of the Coast Ranges are known, and in some of the localities which have been topographically mapped, a considerable body of detailed information is at hand.
The oldest sedimentary rocks of the Coast Ranges are of unknown age. They comprize impure and somewhat magnesian limestone, quartzites, and various crystalline schists. The limestones are usually in the form of coarse marble varying in color from dark gray to white and containing frequently some graphite and less commonly lime silicate. The quartzites are thoroly indurated, as a rule, sometimes to the extent of being vitreous, and usually show well-marked stratification. The schists have as yet been little studied, and no adequate observations upon their character in detail have been put on record. They are known, however, to comprize both micaceous and hornblendic varieties.
These marbles, quartzites, and crystalline schists are known only in more or less fragmentary form, associated with considerable bodies of granitic rocks which have invaded them as batholiths. The most common occurrence of the marbles, quartzites, and schists is in the form of limited belts and isolated patches embedded in the granitic rocks, or in limited areas flanking the margins of the batholiths, and showing evidence of contact metamorphism. It is evident in most cases, and is probably generally true, that the granite of the Coast Ranges is of later date than the metamorphic sedimentary rocks associated with them. While the age of these pregranitic sedimentary formations is at present unknown, the age of the granite is suggested by its seeming identity with the granite of the Sierra Nevada. The latter is a vast batholith known to be intrusive in Paleozoic and Mesozoic strata as late as the Upper Jurassic. This granite has been followed thru the Sierra Nevada to Tehachapi and Tejon Pass, where the range curves sharply around and passes into the Coast Ranges. Passing northerly thru the Coast Ranges, granite identical in character with that of the Sierra Nevada, and carrying identical inclusions of older sedimentary rocks, is traceable in more or less extensive areas from the upper reaches of the Cuyama River to Bodega Head on the coast north of the Golden Gate. It thus seems probable that the granite of the Coast Ranges, like that of the Sierra Nevada, is of late Jurassic or post-Jurassic age. The granitic rocks of the Coast Ranges, together with the pregranitic rocks into which they are irruptive, constitute a complex which is thus the probable analogue of the Bedrock Complex of the Sierra Nevada.
This Cost Range Complex was subjected to vigorous erosion and then submerged to serve as the sea floor upon which the series of rocks known as the Franciscan was deposited. This series consists for the most part of medium coarse, dark gray or greenish-gray sandstone, strongly indurated, with subordinate shales and conglomerates. Intercalated with these sandstones are important horizons of foraminiferal limestone and radiolarian chert and admixtures of volcanic rocks, chiefly basaltic in character. In the vicinity of the Bay of San Francisco, where the series is best known, it falls into seven stratigraphic divisions. These are in ascending order:
(1) A group of arkose sandstones with some conglomerates and shales reposing unconformably upon the Montara granite and with an aggregate thickness of about 800 feet.
(2) A formation of light-gray, very compact and fine-textured forminiferal limestone ranging in thickness from about 60 to possibly a few hundred feet.
(3) Sandstones aggregating 2,000 feet in thickness.
(4) A formation of radiolarian cherts from 100 to 900 feet.
(5) Sandstone, 1,000 feet.
(6) Radiolarian cherts, 500 feet.
(7) Sandstone, 1,400 feet.
In this sequence of sedimentary strata, particularly toward its upper part, there are intercalated lavas at various horizons.
After their accumulation, but before the next higher series of rocks was deposited upon them, the Franciscan strata were invaded by intrusive rocks at points so numerous and so widespread thruout the Coast Ranges that these intrusive bodies constitute one of their most characteristic associations, in contrast to the series which succeed them. The intrusive rocks are of two general types. One is a highly magnesian rock, usually a peridotite, but with facies of pyroxenite and gabbro, the peridotite being generally almost completely serpentinized. The other is a basaltic rock grading into diabase and having in many of its occurrences the peculiar structure characteristic of the spheroidal basalts. In addition to the spheroidal structure on the gross scale, it is in some eases variolitic. Associated with both of these intrusives are areas, generally of limited extent and sporadic distribution, of glaucophane and other crystalline schists, which appear, where they have been most thoroly studied, to be the result of a peculiar kind of contact metamorphism.
The stratigraphic composition of the Franciscan series indicates an interesting to-and-fro migration of the shore line of that time, probably due to a vertical oscillation of the continental margin. The basal group of sandstones, shales, and conglomerates is clearly a terrigenous deposit laid down in proximity to the margin of the continental area from which the sediments were derived. The next succeeding formation, the foraminiferal limestone, on the contrary, is nonterrigenous. Its character as nearly pure carbonate of lime, except for the flinty lenses and nodules it contains, and the abundance of foraminifera, indicates that the sea-bottom over the present position of the San Francisco Peninsula was too remote from the shore to receive an admixture of sand or clay. That is to say, the conditions which favored the deposition of the limestone were inaugurated by a withdrawal of the shore line from the position which it occupied during the deposition of the underlying sandstones. And this lateral migration of the shore was doubtless the result of a sinking of the coast.
Above the foraminiferal limestone sandstones again occur, indicating a return of the shore to about its former position, doubtless due to an uplift of the sea-bottom and coast. These sandstones are in turn followed by a nonterrigenous formation of radiolarian cherts. These are for the most part flinty rocks containing abundant remains of radiolaria, marine organisms which secrete a siliceous test instead of a calcareous one, as in the case of the foraminifera. They contain no admixture of sand, and the shaly partings which separate the layers of chert are very doubtfully referable to land waste. Here again the sea bottom must have been deprest and the shore line caused to withdraw. These radiolarian cherts are followed again by sandstones, and these by a second formation of radiolarian cherts, the former as before indicating uplift of the sea-bottom and the latter depression. The last movement in Franciscan time was uplift, indicated by the sandstones, which rest upon the second horizon of radiolarian cherts and which constitute the topmost formation of the Franciscan series.
The age of the Franciscan is not positively known. Certain general considerations, however, contribute data upon which a tentative judgment as to this question may be based. Stratigraphically, the Franciscan lies upon the eroded surface of the Coast Range granites, the correlation of which with the post-Jurassic granites of the Sierra Nevada has been suggested. If such correlation be adopted, the age of the Franciscan must be
After the accumulation of the Franciscan strata as thus characterized, and perhaps in connection with the invasion of the series by peridotitic and basaltic intrusives, the region was folded and broken, and elevated within the zone of erosion. The elevatory movement was probably quite general. The Franciscan, while subjected to general denudation, was probably nowhere stript down to the underlying basal complex before it was submerged to receive the next succeeding sedimentary strata. These comprize the Knoxville formation, consisting wholly of shales and sandstones with quite subordinate layers and lenses of limestone, all in very regular and rather thin strata, significant of deposition in a shallow basin under fluctuating conditions of transportation. The Knoxville varies in volume from a few hundred to several thousand feet and is widely distributed over the Coast Ranges. It is succeeded in the vicinity of the Bay of San Francisco, and to a less marked degree in other parts of the Coast Ranges, by a formation of coarse conglomerate known as the Oakland conglomerate. This conglomerate attains a thickness of over 1,000 feet in places and follows the Knoxville shales in apparently conformable sequence. The change in the character of the deposits from shales to coarse conglomerates, without any interruption in the continuity of sedimentation, suggests an orogenic disturbance of the margins of the basin within which the Knoxville beds were accumulating, whereby the grades of the streams were greatly accentuated and the degradation of the continental region correspondingly accelerated.
The Oakland Conglomerate, or, where that is missing, the Knoxville shale, is directly followed by a formation of thick bedded sandstones and shales known as the Chico formation. It has a thickness in places of many thousands of feet. The entire volume of strata, from the base of the Knoxville to the top of the Chico, is usually referred to as the Shasta-Chico Series, the Shasta comprizing the Knoxville and Oakland formations, together with certain other paleontological subdivisions not here particularly mentioned. The series is remarkable for its great volume. In the northern Coast Ranges to the west of the Sacramento Valley, the thickness of the sedimentary section, comprizing practically only sandstones and shales, is as much as 29,000 feet. This vast accumulation of strata clearly signifies the development of a great geosyncline, or depression of the sea-bottom in that region in which deposition kept pace with subsidence thruout this portion of Cretaceous time. The Shasto-Chico series is usually regarded as comprizing the whole of the California Cretaceous, but the considerations cited above in regard to the Franciscan indicate that the latter may perhaps be included in the lower Cretaceous section of this region.
The movements which brought the Mesozoic to a close and inaugurated the Tertiary in the Coast Range region were not those of violent orogenic deformation such as characterize this period of geological time in many other parts of the world; but were rather of the nature of a partial elevation of the region, with quite gentle deformation, resulting in a notable restriction of the basin of deposition. The earliest Eocene strata show no marked structural discordance with the Chico. It is nevertheless very probable that a notable unconformity exists, since the abundant and characteristic Cretaceous fauna disappeared and was supplanted by an almost totally distinct assemblage of life forms. The Eocene of the California Coast Ranges falls into two paleontologically distinct groups which have been classed together as the Karquines series. The lower of these
The next succeeding group of rocks, belonging to the Oligocene division of the Tertiary, has been named the San Lorenzo Formation.
Arnold, U. S. G. S. Professional Paper No. 47, p. 16.
It is known in Santa Cruz County, where it attains a thickness of 2,300 feet, made up chiefly of gray shales and fine sandstones. Its stratigraphic relations to the Tejon are not yet known, but its fauna is said by Arnold to contain many species which appear to be closely related to Tejon forms. It may thus be considered as following the Tejon conformably. It is in certain sections known to be unconformable beneath the oldest formation of the Miocene, known as the Vaqueros Sandstone, indicating that after the deposition of the San Lorenzo formation, the region of the Coast Ranges was disturbed and uplifted into the zone of erosion; and the following facts regarding the transgression of the Miocene Sea indicate that this uplift was a very extensive one. Such an uplift in time immediately preceding the Miocene is further indicative of a much closer relationship between the San Lorenzo and the Tejon than between the former and the Monterey.Miocene time in the Coast Range region was characterized by a progressive subsidence with oscillations of the coast. The Miocene sea gradually transgrest the continental margin from the southwest, and as it did so spread a formation of arkose sands and conglomerates over the greater part of the southern Coast Ranges. This was followed, as the water deepened with progressive subsidence, by a remarkable deposit of bituminous shales. These shales are usually whitish or cream-colored, tho often of a purplish or other dark tint, and may be either of a soft chalky consistency, or opaline, or hard and flinty. It is thruout an essentially siliceous formation and is largely diatomaceous in character, tho more or less admixt with volcanic pumiceous ash. In some portions the ash is a prominent constituent, and in San Luis Obispo County there is a deposit aggregating about 1,000 feet in thickness of well-stratified volcanic tuff and agglomerate. In San Mateo County there are basalts which were erupted at this period. Interstratified with these siliceous shales, thin beds of more or less ferruginous and somewhat magnesian limestones are by no means uncommon. They are, however, lenticular or non-persistent, and are of a very compact texture and usually nonfossiliferous. There are also in some places thin but persistent beds of a peculiar, very hard, fine-grained, light-colored sandstone intercalated with the shales. In the southern portion of the Coast Ranges the bituminous shales accumulated to a thickness of several thousand feet, but in the middle Coast Ranges, in the vicinity of the Bay of San Francisco, the Miocene sea was characterized by an oscillatory or to-and-fro migration of its eastern shore line, due to alternate uplift and subsidence of the coast, quite analogous to that described for the Franciscan period. This gave rise to an alternation of shallow water in which sandstones were deposited, and deep water in which siliceous ooze accumulated with but little admixture of terrigenous material. We have thus in the territory between Mount Diablo and the Bay of San Francisco an alternation of four formations of bituminous
At the close of the Miocene, the Coast Range region was disturbed by orogenic movements and uplifted into the zone of erosion. It was then deprest irregularly so as to give rise to local basins of sedimentation in which accumulated great thicknesses of Pliocene beds, particularly about the Bay of San Francisco and southward. The oldest of these Pliocene formations is the San Pablo, which lies unconformably upon the Monterey strata. This is essentially a sandstone formation with a thickness of from 1,000 to 2,000 feet. It occurs on both sides of the Coast Ranges from the vicinity of the Bay of San Francisco southward and appears to have been laid down in two basins, separated by a barrier corresponding to the general axis of the present Coast Ranges. The formation on the east side of the range is characterized by a notable admixture of dark andesitie ash, which gives the unweathered exposures of the sandstones a distinctly blue color. This formation has a fauna of over 100 species, of which more than 40 per cent are living forms. This fact, and the unconformable superposition of the formation upon the Monterey, are warrant for placing it in the Pliocene. On the west side of the Coast Ranges, the San Pablo is best known in San Luis Obispo and Santa Barbara Counties, and is there free from volcanic admixtures, tho the basal beds are very commonly characterized by the presence of asphaltum, which cements the sand together and constitutes the well-known bituminous rock of the region. This asphaltum appears to have originated in part as a seepage from the upturned bituminous shale of the Monterey along the shores of the San Pablo sea, and molluscan remains of San Pablo age are often embedded in it.
Succeeding the San Pablo, but nowhere, so far as the writer is aware, reposing directly upon it, is the Merced series. The sediments composing this series were laid down in rather acute geosynclinal troughs, resulting from orogenic deformation of the coast in middle Pliocene time. Three of these troughs are known. The most northerly is that now occupied by the Valley of the lower Eel River in Humboldt County; the second is largely occupied by the Santa Rosa Valley in Sonoma County; the third is on the Peninsula of San Francisco, extending thence south to the coast of Santa Cruz County. The Merced strata in the Valley of the lower Eel River, and the typical Merced section near San Francisco, show each a thickness of something over a mile. In Sonoma County the marine Merced beds grade eastward into fluviatile conglomerates, admixed with volcanic ashes. The maximum thickness is about 3,500 feet. The lower part of the series is here characterized by a considerable volume of white volcanic pumiceous tuffs, which thinout rapidly to the westward. These were in part laid down directly on a land surface burying forests of huge sequoia, whole trees being now completely silicified.
For a description of the Merced beds of Sonoma County and the underlying pumiceous tuff, a paper by V. C. Osmont, Bull. Dept. Geol., Univ. Cal., vol. 4, No. 3, should be consulted.
On the coast of Santa Cruz County, the series is represented by strata of lower stratigraphic horizons than nearer San Francisco, these lower beds having been called the Purissima formation, altho the sedimentation was continuous with that of the Merced. TheThe accumulation of the Merced series to the great thickness above indicated in middle and northern California proves local depressions of the coast of over a mile below sea level in later Pliocene time. Similar orogenic deformation was in progress at the same time on the eastern side of the barrier corresponding to the then axis of the Coast Ranges. These movements gave rise to great troughs from which the sea was excluded, but which were occupied by fresh water, and filled with sediments equal in volume to those of the marine troughs to the west of the barrier. The greater part of these fresh-water beds are comprized in the Orindan formation, which may be the equivalent of the Cache Lake beds of the Clear Lake district
Described by G. F. Becker, U. S. G. S. Monograph XIII, pp. 219-221, 238-242.
and of the Paso Robles in the southern Coast Ranges. They have an extensive distribution on the eastern side of the Coast Ranges, and in the vicinity of the Bay of San Francisco there intervenes between the base of the Orindan and the San Pablo a formation of white pumiceous tuff entirely similar to that at the base of the Merced series in Sonoma County, but containing here fresh-water fossils. This tuff attains a maximum thickness of about 1,000 feet and is known as the Pinole tuff. Thruout the Orindan there are occasional intercalated strata of volcanic tuff of moderate thickness. The Orindan lacustrine period was brought to a close in the region of the middle Coast Ranges by volcanic eruptions which resulted in extensive flows of lava and showers of ashes. Upon these lavas lake basins were later established and some hundreds of feet of fresh-water deposits (Siestan formation) accumulated in them, which were in turn buried by other lavas.The accumulation of the Merced marine beds and the corresponding lacustrine and volcanic rocks was brought to a close by an acute and widespread deformation regarded as part of the general mountain-making movements which ushered in the Pleistocene in western North America. As a result of these movements, the Merced and Orindan basins were folded and faulted, and the basement upon which their contained strata had been laid down was lifted in part from a position over a mile below sea-level to one far above sea-level. The Pliocene formations were brought within the zone of active erosion and the evolution of the present geomorphic features of the Coast Ranges was inaugurated. When the degradation of the folded Orindan strata was well advanced, a lake basin was established across the edges of these strata and in it accumulated the various fresh-water beds and volcanic lavas and tuffs comprizing the Campan series. At a time within the Pleistocene when the geomorphic evolution of the coast had been well advanced to its present condition, the coastal belt was deprest 1,000 to 2,000 feet lower than it is at present, and then uplifted in stages marked by marine terraces along many parts of the coast. Since this there have been oscillations of the region about the Bay of San Francisco, the net result of which has been a depression allowing the sea to invade the valley-lands and thus make the magnificent harbor to which San Francisco owes its existence.
In the foregoing sketch of the formations of the Coast Ranges and their historical significance, it is desired to emphasize particularly the remarkable series of subsidences and uplifts which have affected the coastal region from the beginning of the Franciscan to the present. This record of oscillation is in marked contrast to the comparative stability of the Sierra Nevada. Except for a marginal strip of its foot-hill slopes, the region of the present Sierra Nevada has not been submerged beneath the sea. During the geological ages in which the Coast Range region has been repeatedly deprest to receive marine sediments, the sum of the maximal sections of which amounts to 65,000 feet of strata, the western edge of the Sierra Nevada region has probably never been
Between these two strongly contrasted provinces lies the great valley of California, one of the very notable geomorphic features of the continent. This valley is but one of a long series of similar depressions which lie along the western border of the North American continent, between the coastal uplands and the western edge of the continental plateau. In the north it has its probable analogues in Hecate Strait, the Gulf of Georgia, Puget Sound, the Willamette Valley, the Ashland Valley, and the depression between the Sierra Nevada and the Klamath Mountains. On the south we see its analogues in the Colorado Desert, the Gulf of California, and in the valley which lies between the southern border of the central plateau of Mexico and the Sierra Madre del Sur. In the Californian region we must interpret the axial line of this depression as a tectonic hinge, upon which the mobile coastal region has swung in a vertical sense upon the edge of the interior plateau, here represented by the Sierra Nevada. Whether this tectonic hinge is a more or less flexible zone upon which movement has taken place without rupture, or whether it represents a zone of dislocation, is not clear; but that differential movement has taken place along the valley line is one of the salient facts in the geological history of California.
Structure
A detailed account of the structure of the Coast System would involve a discrimination between features referable to the different orogenic movements which have affected the region at various periods of its history. Owing to this succession of movements, new structures have been superimposed upon older structures, or upon remnants of older structures, so often that the resultant effect is extremely complicated and not only difficult to unravel but difficult to state or describe in any simple way. In this summary review of the subject, no such detailed discrimination will be attempted. The only effort will be to call attention to the salient features, which are for the most part referable to the orogenic movements of later Tertiary and post-Tertiary time.
Marginal Features. — In a consideration of the structural features of the Coast System, its marginal lines on the east and west first claim attention. The eastern slope of the Coast Ranges rises from the floor of the Great Valley much more abruptly in general than does the western slope of the Sierra Nevada from the same valley floor. Turner
Am. Geologist, vol. XIII, p. 248.
has suggested that the Great Valley east of the Coast Ranges is determined by a fault. There is some warrant for this view and it is certainly true in part. The very precipitous mountain front which rises from the valley at its southern end is without doubt a degraded fault-scrap, tho whether or not this fault or a series of similar faults can be followed along the edge of the mountains to their northern end is questionable. It is, however, safe to say that the eastern margin of the Coast Ranges represents a line of acute deformation, with the probability of that deformation having taken the form of faults in certain places. No one has yet made a sufficiently careful study of the question to make a more precise statement possible. In general, this line of acute deformation is notOn the seaward side the Coast System is usually regarded as being limited by the shore line. The precipitous coast rising to elevations of from 2,000 to 5,000 feet, extending from Cape Mendocino to Point Conception, and the popular notion that mountain ranges are confined to the land areas of the earth, are justification for this view. But in a more comprehensive view, embracing all inequalities of the earth's surface both above and below the sea-level, the western margin of the mountainous area, the familiar portions of which we call the Coast System, will have to be placed farther seaward. Off the coast of California the sea-bottom slopes down to the 3,000-foot submarine contour at a moderate angle and then plunges steeply to depths of over 12,000 feet. Beyond the foot of this steep slope the sea-bottom has very flat gradients and the 15,000-foot contour is far out to sea. From the Oregon line to Point Conception the 3,000-foot submarine contour, or the brink of the steep slope, lies off shore at a distance of from 15 to 35 miles; but at Cape Mendocino and at the Bay of Monterey this line is found much closer in. South of Point Conception this steep slope has the same general trend as to the north. That is to say, it shows no embayment in its course corresponding to that at the Santa Barbara channel and southward. This is particularly true of the course of the 6,000, 9,000, and 12,000-foot contours. The slope is by no means uniform for its entire length. From Point Arena to the latitude of the Golden Gate the grade is notably steep from the 3,000 foot to the 9,000-foot contour. This is also true off Point Conception. From the latter point southeastward the steep portion of the slope is from the 6,000-foot to the 12,000-foot contour; and the same statement holds for the slope off San Simeon Bay. In general, the steepest profile lies between the 6,000 and the 9,000-foot line.
This steep drop from the subcontinental platform to the broad floor of the Pacific must be regarded as the geomorphic expression of a rather acute deformation of the earth's crust, and those portions of the slope where the contours are crowded together, as for example between Point Arena and the latitude of the Golden Gate, off San Simeon Bay, off Point Conception, and off the platform of the Channel Islands, can scarcely be interpreted as other than fault-scarps. The slopes at the localities mentioned are quite comparable to the great fault-scarp which forms the eastern front of the Sierra Nevada. At the base of the slope off the Channel Island platform, the recent dredging operations of the Albatross brought up from a depth of 12,000 feet numerous fragments of rock similar to the bituminous shale of the Monterey series of the southern Coast Range. With this rock was found much asphaltum. This indicates that at the base of the slope there are talus accumulations of so recent a date that they have not yet been buried by oceanic sediments.
This line of acute deformation of the crust off the entire length of the coast of California can not be ignored in any consideration of the orographic features of the region. The slope referred to is doubtless devoid of those sculptural features characteristic of mountains within the zone of erosion, and which we are too apt to look upon as essential, but it constitutes nevertheless a notable mountain front rising from the floor of the Pacific. It is the natural western boundary of the mountainous tract which we call the
Having indicated the east and west boundaries of the Coast System as their dominant structural lines, we may now consider those features which pertain to the internal structure of the mountain tract. Here we must first take note of the coast line. The coastal slope of California characteristically rises abruptly from sea level to elevations of from 2,000 to 5,000 feet within a short distance from shore, from Cape Mendocino to Point Conception, with certain notable breaks in its continuity which are susceptible of special explanation. If along the shore line at the base of this abrupt slope we draw straight lines which are tangent to the headlands or chords to the minor embayments of the coast, these lines fall into two fairly constant orientations and clearly bring out the fact that the shore line has in reality a zigzag course, due apparently to the alternate control of two systems of structural lines, one of which is between N. 37° W. and N. 40° W., and the other between N. 10° W. and N. 15° W., thus intersecting at an angle of about 26°. Under this scheme of discrimination of the orientation of different portions of the coast line, the bearings of the following divisions may be thus listed:
| Localities. | Bearing of Mean Line. | Distance in Geographical Miles. |
| Cape Mendocino to Punta Gorda | N. 12° W. | 14 |
| Punta Gorda to Shelter Cove | N. 40° W. | 25 |
| Shelter Cove to Point Arena | N. 10° W. | 64 |
| Point Arena to Golden Gate, thru Tomales Bay | N. 40° W. | 90 |
| Golden Gate to Pigeon Point | N. 15° W. | 40 |
| Pigeon Point toward Santa Cruz | N. 40° W. | 21 |
| Point Pinos to Point Sur | N. 13° W. | 19 |
| Point Sur to Port Hartford | N. 37° W. | 89 |
| Port Hartford to Point Conception | N. 6° W. | 44 |
Now it is difficult to regard any considerable portion of the abrupt coastal slope of California between Cape Mendocino and Point Conception as other than a more or less degraded fault-scarp. If this view be accepted, it is clear that the trend of the coast and its geomorphic profile have been determined by two systems of faults meeting or intersecting at an angle of about 26° on their strike. Making some allowance for cliff recession, the base of both systems of scarps must lie some little distance off shore and be buried by the notable embankment of littoral sediments which conceals the true profile of the submarine rock surface.
Of the two systems of faults thus recognized as controlling the trend of the coast, one, viz. that which bears N. 37° W. to N. 40° W., conforms, as will be shown later, more or less closely with the prevailing structural lines, such as faults, folds, and belts of igneous rock found in the Coast Ranges; while the more meridional system is not a prominent feature of the Coast Ranges. It follows that since the mean trend of the California coast lies between the bearings of the two fault systems, the tectonic lines of the Coast System, if followed northwesterly, eventually emerge upon the coast. This obliquity of the
Am. Geologist, Vol. XI, Feb. 1893, p. 70.
but the probable explanation of it has not heretofore been set forth.The coastal scarp is interrupted at a number of points and in a variety of ways. The most notable and interesting interruption is that of the Bay of Monterey. This is not only an embayment of the coast, but is a depression in the Coast Ranges extending down over their submarine portion to the 12,000-foot contour below seal-level. It brings the 3,000-foot submarine contour well inside the general line of the coast. This submarine valley has been regarded by some writers as a submerged valley of subaerial erosion, but there is little warrant for this view and much that conflicts with it. The valley of the Bay of Monterey, subaerial and submarine, is a synclinal trough the axis of which is approximately normal to the trend of the coast and of the Coast Ranges as a belt. In the axis of the syncline, and probably parallel to it, is a fault seen in the canyon between Pajaro and Chittenden, which brings down the Tertiary rocks on the north side against the pre-Cretaceous granitic rocks of the Gavilan Range. Another interruption of the continuity of the coastal scarp is at the Golden Gate. Here the Coast Ranges have been locally deprest and the land valleys which were formerly drained by a trunk stream, where the Golden Gate now is, have been flooded by the waters of the ocean. The axis of this depression is, however, not well known. A third, apparent rather than real, interruption of the coastal scarp occurs at the place where the Point Reyes Peninsula projects out beyond the general line of the coast. Inside of the peninsula, however, there is a long narrow valley, the northern end of which is occupied by Tomales Bay and the southern end by Bolinas Lagoon, which separates it from the mainland proper; and to the east of this valley the coastal scarp rises with exceptional boldness.
The coastal scarp has had its profile modified in many places by wave-cut terraces formed during the uplift of the coast by stages in Pleistocene time, as previously stated. The relation of the coastal scarp to deformed basins of Merced (late Pliocene) strata indicate that it originated, in its essential features, at the period of orogenic activity which brought the Tertiary to a close. South of Point Conception the twofold system of faults which determines the configuration of the coast gives out and we enter upon a region of probably more complicated structure. The Santa Barbara channel appears to lie in a geosynclinal trough between the Santa Ynez range and the island chain from Anacapa to San Miguel. On the northeast side of San Clemente is a sharply defined fault-scarp, indicating that the island is a portion of an uplifted and tilted orographic block. The fault along which the scarp has been formed probably extends as far as the east side of Santa Barbara Island. San Clemente Island presents a magnificent series of wave-cut terraces up to an elevation of 1,500 feet. San Pedro Head is similarly uplifted and terraced, while the intervening island, Santa Catalina, shows no evidence of corresponding uplift, but has on the contrary been deprest. On the whole, the channel island platform between the edge of the subcontinental shelf and the coast presents the characters of a sunken mountainous tract, the inequalities of the surface of which are partly due to acute deformation and partly to erosional sculpture when the region was above sea-level. A more detailed interpretation of the structure of this region is rendered difficult by the absence of adequate soundings of the sea-bottom.
Granitic Rocks. — Coming now to the consideration of the more important structural features of the Coast System, in the territory between the coast and its eastern margin, it must be stated that even here our information is very scant. One of the most important features of the Coast System from a structural point of view is the occurrence of a belt of granitic rock having a very notable linear extent thruout the ranges. This granite, as has been already stated, appears, in the vicinity of Tejon Pass, both from
Bull. Dept. Geol. Univ. Cal., vol. 3, No. 1.
and in the Perris plain.To the northwest of Tejon Pass, this granite appears in a series of linearly disposed areas extending thru the ranges. It forms a notable feature of the Santa Lucia Range on the west of the Salinas Valley, and also of the Gavilan Range to the east of the same valley. The granite of the Santa Lucia Range runs out to sea at Point Pinos near Monterey, while that of the Gavilan Range extends into Santa Cruz County and appears on the coast at Point San Pedro, a few miles south of San Francisco. Farther north it is seen in the Farallon Islands, the Point Reyes Peninsula, and on Bodega Head. The Santa Lucia and the Gavilan thus expose two quite distinct lines of granitic outcrop, practically parallel, and both crossing the general trend of the Coast Ranges obliquely and reaching the coast. Indeed, the easterly limit of all the granite of the Coast Ranges crosses the entire breadth of the latter obliquely between the Tejon Pass and Bodega Head. This signifies, of course, that whatever manifestations of crustal deformation elevated these belts of granite, the lines or axes of such deformation were not coincident in direction with the mean trend of the Coast Ranges, or with the mean trend of either of the margins of the Coast Ranges. It is noteworthy, too, that all of the Coast Range granite as far south as the vicinity of Tejon Pass lies to the southwest of the Rift along which the movement occurred which generated the earthquake of April 18, 1906. It is further noteworthy that near the northern end of the granite belt at Tomales Bay and Bodega Head, the Rift actually follows the line of contact between the granite on the west and the sedimentary rocks which are faulted against it. These facts suggest that very probably the Rift is similarly situated in the more southern Coast Ranges with reference to a deeper-seated contact between granite and sedimentaries; in other words, that the eastern edge of the Coast Range batholith, whether that edge be an original feature of the batholith or a feature determined by faulting, is with some degree of probability the line which determines in part the course of the modern Rift. Southward from the vicinity of Tejon Pass, however, the Rift passes into the granitic terrane.
Folds. — The pre-Knoxville folds of the Coast Ranges are little known, owing partly to the burial of the Franciscan rocks by later deposits, and partly to the complexity of the structures where the rocks are exposed and the difficulty of discriminating the effects of the earlier and the later movements; but chiefly owing to the absence of adequate topographic maps, so necessary for such studies. The conspicuous folds of the Coast Ranges are those which have been imprest upon the Tertiary and older strata together. These are usually rather sharp and more or less symmetrical synclines and anticlines, involving usually many thousands of feet of strata. In some cases these are asymmetric and even overturned, as in the Mount Diablo region, but they are never so closely apprest as to induce general and important deformation of the internal structure of the rocks affected. The folding has been effected without flowage, except perhaps locally where soft clays or shales were involved, and there has been no development of slaty cleavage or schistosity. In general the axes of the folds have a northwest-southeast trend, but there are numerous deviations from this rule and the axes of the minor folds are usually more or less divergent, as is of course generally true. There is, however, a pronounced parallelism in the dominant synclines and anticlines, the axes of which extend for many miles. Several of these are more or less oblique to the mean trend of the Coast Range belt, and thus appear to be truncated on the coast line, or on the eastern margin of
Faults. — In the Coast Ranges there are numerous faults, but our knowledge of them is limited, owing to the small amount of geological mapping which has been done in the region. With the extension of cartographic work, many more than are now known will doubtless come to light. Of those at present known, the great majority have a general northwest-southeast strike, but there are several minor faults which trend transverse to the general strike. The faults of the Coast Ranges, as well as those of other parts of California, are indicated, as to position and extent, on Map No. 1. A summary reference to them is all that will be here attempted.
The most northerly fault of the Coast Ranges is one which Mr. O. H. Hershey calls Redwood Mountain fault. It is an overthrust, according to Mr. Hershey, heading to the northeast and having a throw of probably over 5,000 feet. It trends southeast along the southwest flank of South Fork Mountain for scores of miles, and doubtless determines the very straight trend of this great ridge. Parallel to it, on the southwest side of Redwood Creek, near Acorn, there is another fault having a throw of at least 1,000 feet, according to Mr. Hershey. Its extent is unknown. The precipitous southwest front of Mount St. Helena has been shown by Osmont
Bull. Dept. Geol., Univ. Cal., vol. 4, No. 3, p. 78.
to be a degraded fault-scrap; and the downthrow on the southwest side of the fault is estimated by him to be not less than 2,500 feet. The western edge of the Sacramento Valley, from Benicia to Cordelia, is probably determined by a fault with an easterly downthrow.In the Mount Diablo region, there is a pronounced overthrust fold which causes Miocene strata to rest upon Pliocene strata with a dip of 30° to 45° to the northeast. Louderback's work on the structure of Mount Diablo has shown that this over-tipt fold passes into a thrust fault whereby a considerable proportion of the mountain has been shoved to the southwest.
Results not yet published.
The west side of San Ramon and Livermore Valleys is bounded for the most part by a steep mountain wall at the base of which, near Pleasanton, the Tertiary rocks are faulted down against the Franciscan. This fault extends southward thru Calaveras Valley and past Mount Hamilton. Its general course is about N. 35° W. It has an extent of at least 60 miles and may be very much longer. In the Berkeley Hills to the east of this there are many minor faults, both overthrust and normal, which will not be described in detail. In the Mount Hamilton Range, between the crest and the Santa Clara Valley, there are several faults, notably the Mission Creek, Mission Peak, Mount Hamilton, and Master's Hill faults, which have a more or less regular northwest-southeast trend; and there are several shorter faults transverse to these, and of variable strike.The valley of the Bay of San Francisco and its prolongation southward in the Santa Clara Valley is bounded on the northeast side by a range of hills which presents a very even, straight, and on the whole, but little dissected, front to the southwest. This even front extends from near Point Pinole, on San Pablo Bay, to the vicinity of Hollister, a distance of about 100 miles, forming a very striking geomorphic feature of the Coast Ranges. At Berkeley and Oakland, and southeast of the latter, there is evidence that this even front represents a somewhat degraded fault-scrap, or series of scraps, and this interpretation may with very probable truth be placed upon it for its entire extent. Near Berkeley the slope of this degraded scrap is traversed by supplementary step faults, which are not improbably characteristic of it in other places; so that in regarding the feature as a fault-scarp it is not intended to apply that term too narrowly, but to include rather the idea of a zone of acute deformation traversed by step faults. This line has a course of about N. 35° W. North of San Pablo Bay, on the geographic prolongation of the line, a similar feature, tho by no means so straight, is found on the east side of the
An interesting and important fault traverses the peninsula of San Francisco, a little south of the city. The course of this fault can not be precisely determined, as its trace at the surface is obscured by Pleistocene and recent deposits. Its approximate position is at the southwest base of San Bruno Mountain, with a strike of about N. 43° W. By this fault the Merced strata, which are well exposed on the sea-cliffs south of Lake Merced to the thickness of over a mile, are dropt down against the Franciscan rocks, the throw being estimated at not less than 7,000 feet. To the northeast of the main fault, and close to the face of the mountain, is an auxiliary fault, and between these two faults there is a block of the Franciscan which has dropt only to a limited extent, and which is of the same character as the kernbuts of the Kern River.
Bull. Dept. Geol., Univ. Cal., vol. 3, No. 15.
The bold and precipitous southwest face of San Bruno Mountain is thus a fault-scarp with two facets, one for the main fault and the other for the auxiliary, both being well exprest in the geomorphic profile of the mountain. This fault-scarp appears to be the southern prolongation of the scarp which forms the coastal steep slope to the north of the Golden Gate, and seems to converge upon the San Andreas fault, off the Golden Gate, making a very acute angle with it. It affords an excellent illustration of the general fact above alluded to, that the northwesterly members of the fault system controlling the configuration of the coast are prolongations of fault-lines within the Coast Ranges. Knowledge of the extent of this fault, altho its throw is so notable, is limited to the peninsula of San Francisco.Outside of Fort Point, at the Golden Gate, and a little south of the point, is a very well exposed fault which appears to strike southeast across the city of San Francisco. The fault is nearly vertical and has a throw of at least some hundreds of feet, whereby the serpentine on the north has been dropt against a formation of radiolarian cherts.
The most interesting fault traversing the Peninsula of San Francisco is the San Andreas fault, on which movement was renewed on April 18, 1906, causing the earthquake. The extent and course of this fault are described in detail elsewhere. To the southwest of the San Andreas fault, on the Peninsula of San Francisco, and in the Santa Cruz Mountains, are several other faults of notable extent. Of these may be mentioned the Fifield, Pilarcitos, Castle Ridge, Butano, Boulder Creek, and San Gregorio faults, all of which are important features of the structure of the region.
On the southwest side of Montara Mountain is a very precipitous seaward slope, at the base of which strata of Miocene age are tilted at rather abrupt angles against the granite. The strata of arkose sandstone at the base still rest against the original floor of deposition, but it is difficult to see how such an acute uplift could take place in a granite massif without deformation of the granite. Such deformation might take the form of plastic flow if it were sufficiently deep-seated, or it might find its expression in a zone of faults; and as there is no evidence of plastic deformation, it is concluded that the uplift of Montara Mountain was effected by faulting within the granite, the same deformation appearing as flexure in the stratified rocks which flank the mountain on this side.
Northeast of the San Andreas fault are the Belmont and Black Mountain faults, the latter a branch from the San Andreas fault. In the gap between the Santa Cruz and Gavilan Ranges is a fault followed by the canyon of Pajaro River near Chittenden, which drops the Tertiary formations on the north against the pre-Franciscan granitic rocks of
South of the Bay of Monterey, one of the dominant structural lines of the Coast Ranges is the Santa Lucia fault, at the base of the Santa Lucia Range on the border of Salinas Valley. It is traceable from the vicinity of Bradley to the Bay of Monterey and it is probably the chief factor in determining the course of the Salinas-Valley and the steep easterly front of the Santa Lucia Range. Near its southern end, the Santa Lucia fault is paralleled on the southwest by another fault which probably determined to some extent the course of the valley of San Antonio River. Farther south a fault parallels the last two, between Dove and Templeton; and to the southwest of this lies the much longer fault which passes close to San Luis Obispo, extending from near San Simeon to the drainage of the Santa Ynez.
The northeastern flank of the San Emidio Range, at the southern end of the great valley, is with little question a fault-scarp. The same may be said of the north flank of the Santa Ynez Range and the south flank of the Santa Monica Range. The San Gabriel fault, which bounds the range of that name on the south, branches from the San Andreas fault near San Bernardino and follows the base of the range with an east-west trend. Beyond Pasadena it bends slightly to the north and extends thru to the coast in the vicinity of Carpenteria. Near Pasadena a branch fault leaves it, with a north-westerly strike, on the northeast side of the Verdugo Mountains. Southeast of Los Angeles, the most notable faults are the San Jacinto and Elsinore faults, both of which have very pronounced scarps. There are, however, several others. All the faults in this region have a northwest-southeast strike, and are thus in contrast to the system of faults extending from Point Conception to the Colorado Desert along the Sierra Madre, in which the dominant trend is east and west.
The foregoing summary enumeration of the more important faults at present known in the Coast System of mountains makes it clear that the San Andreas fault, upon which movement took place on April 18, 1906, is not a singular or unique feature of the structure of these mountains. It is only one of many faults, on all of which in time past there have occurred many differential movements, each productive of an earthquake. Map No. 1, upon which the above faults are represented, indicates other faults in California, Nevada and Oregon at present known to geologists.
In compiling the data for the representation of the faults of California, free use has been made of information kindly supplied by Messrs. H. W. Turner, W. Lindgren, W. C. Mendenhall, H. W. Fairbanks, J. S. Diller, F. M. Anderson, R. Arnold, J. C. Branner, G. D. Louderback, and O. H. Hershey.
Perhaps the most interesting of these, from the present point of view, is the fault at the eastern base of the Sierra Nevada, upon a portion of which the movement took place that caused the earthquake of 1872. The map may be regarded as a preliminary attempt to bring together, in cartographic form, our knowledge of the position of faults in this region. A full discussion of these features, with references to the literature bearing upon them, would be out of place here, altho their occurrence suggests seismic possibilities.Geomorphic Features
Certain of the geomorphic features of the Coast Ranges, particularly as regards their margins, have necessarily been alluded to in the discussion of the structure. It is proposed here to describe quite briefly the salient characters of the relief, in their relation to the structure.
The Coast Ranges in general, between the coast and the Great Valley and north of Santa Barbara Channel, comprize a series of ridges and intervening valleys of mature aspect.
U. S. G. S. Bulletin, 196.
Between the headwaters of Eel River and the Bay of San Francisco the interpretation of the drainage as subsequent is not so certain, altho here the general geomorphic profile is even more mature than it is on the north, a fact referable to the softer character of certain geological formations which prevail. Here we have, as before, a system of stream valleys, notably Russian River Valley, Sonoma Valley, Napa Valley, and Berryessa, and Clear Lake Valleys, which are clearly evolved by stream erosion under the control of structure. The transverse connecting link from one longitudinal valley to another, which is so characteristic of subsequent drainage, is not apparent on the maps, but its absence may be more apparent than real. The lower stretch of Russian River, from Healdsburg to the sea, has the appearance of a transverse stream tapping a longitudinal valley of very mature character, and may be the remnant of an original consequent stream. This view, however, is open to the objection that the lower stretch of Russian River near the sea has a more youthful aspect than might reasonably be expected under the hypothesis. On account of the rather immature character of the transverse outlet of Russian River, it has been suggested that it is of later date than Russian River and represents a small stream which has cut its way back from the coast and captured the waters of the river, which formerly went to the Bay of San Francisco, the capture being
Somewhat similar features occur on the east side of the Coast Ranges. Cache Creek and Putah Creek, draining longitudinal valleys within the Coast Ranges, both emerge upon the Great Valley thru transverse gorges in the Blue Ridge, the most easterly of the Coast Ranges. These transverse gorges can scarcely be regarded as other than consequent trunks crossing a hard barrier within which, in softer formations, longitudinal or subsequent valleys have been evolved. The apparent absence of the transverse connecting links between Napa, Sonoma, and Petaluma Valleys is explained when it is recalled that while the streams draining these valleys flow directly to salt water, they nevertheless flow to a drowned valley. The trunk stream trench from which Petaluma, Sonoma, and Napa Creeks are subsequent branches lies below the waters of San Pablo Bay. In general, Santa Rosa Valley (lower part of Russian River Valley), Petaluma Valley, Sonoma Valley, and Napa Valley have been evolved by erosion along synclinal axes. This fact also tends to weaken their interpretation as due to subsequent development by headwater erosion; since, if the synclinal folds were exprest as troughs at the surface at the time of the folding, then the drainage would have been both consequent and parallel to the structure.
Coming farther south, the valley of the Bay of San Francisco and its extension in the Santa Clara Valley is a large feature in which deformation and erosion have probably played equal rôles. Its trend is strictly determined by the Haywards fault line previously described. Southward from Hollister, the valley loses its breadth and passes into the much more constricted valley of the San Benito River, draining the Coast Ranges to the east of the Gavilan Range. The Bay itself and its inland extensions afford a magnificent illustration of a drowned valley-land due to subsidence of the valley-bottoms below sea-level.
Livermore Valley, a few miles to the east of the Bay of San Francisco and separated from it by the ridge of the Berkeley Hills, is a very noteworthy feature. It is a broadly expansive alluviated valley, bounded on the west by the degraded fault-scarp which limits the Berkeley Hills to the east; on the east by the slopes of Mount Diablo; and on the south by the slopes of Mount Hamilton. On the north it is open by way of the wide and low San Ramon Valley to Suisun Bay, and the northern portion of the valley drains this way. The greater part of the waters which come to it from Mount Diablo and from Mount Hamilton, however, are carried off by Alameda Creek thru Niles Canyon, a narrow gorge which transects the bold ridge separating it from the Bay of San Francisco. Alameda Creek has a hydrographic basin of 600 square miles, and it is a remarkable fact that it finds its outlet across the strike of the range thru a bold ridge, instead of following the wide open valley leading directly to Suisun Bay with no barrier in its path. It is a fair inference that Livermore Valley is structural rather than erosional in its origin and that, anterior to the acute deformation of the region, the drainage was consequent in the path followed by Niles Canyon. The deformation involved the uplift of the Berkeley Hills and the complementary depression of the Livermore Valley tract, and this deformation proceeded at a rate which was sufficiently slow to permit the stream, by downward corrasion across the rising mass, to maintain its course. Alameda Creek in Niles Canyon is thus a remnant of the consequent drainage of the region and is antecedent to the uplift which gave rise to the Berkeley Hills.
In the Coast Ranges between the Bay of Monterey and the Santa Barbara Channel, the chief valleys are those of Salinas River and its tributary, the San Juan; the Carissa Valley, and the valleys of the Cuyama and Santa Ynez Rivers. Of these the Salinas Valley is the largest. It is a wide, terraced valley cut by the river out of rather soft Tertiary and later deposits, which appear to have been in part let down against the older rocks of the Santa Lucia Range by the Santa Lucia fault. In its lower part it lies between the Gavilan and Santa Lucia Ranges, and the trend thus established is maintained by the main stream as far as San Miguel. Beyond that the same general trend is continued up its tributary, the San Juan, and thence thru the Carissa Plains to a point close to the southern end of the Great Valley. The eastern side of the upper end of the valley, particularly the eastern side of the Carissa Plains, follows closely the line of the modern earthquake rift to be presently described; and there can be little doubt, not only that in so far as the valley is an erosional feature its erosion has been controlled by structural features, but also that deformational processes have had a considerable share in its evolution. The axis of the valley thus indicated is singularly straight and has a length of about 175 miles. Its upper part, the Carissa Plains, is an arid plain without drainage and contains a very saline lake. This plain is a surface of alluviation. The lower end of the valley opens widely on the Bay of Monterey and the fine stream terraces which flank its sides afford an excellent record of the recent uplift of the region.
The valley affords another striking illustration of the obliquity of the geomorphic as well as the structural features to the general trend of the Coast Range belt, and their constant tendency to emerge upon the coast. From the eastern margin of the valley at the south end of the Carissa Plains, one can look down upon the Great Valley, near Sunset, a few miles distant; and only a narrow ridge separates the two valleys, altho they differ greatly in altitude. From this point in its course of 175 miles, Salinas Valley crosses the entire width of the Coast Ranges. South of San Miguel, the Salinas River proper lies in a less open valley with north and south trend as far as Templeton, a distance of about 15 miles, and then opens out into a wider valley having a northwest-southeast trend for about 35 miles to the headwaters of the stream. Several of the minor tributaries of the Salinas show a marked tendency to the development of subsequent valleys. On the east side of the river, this is particularly marked on San Lorenzo Creek in Priest Valley, and on Chalome Creek in Chalome Valley. These comparatively large valleys may be referable in part, however, to deformation, inasmuch as they are on the line of the Rift. Their geomorphic history has not yet been studied. On the west side of Salinas Valley the two chief tributaries, the San Antonio and the Nacimiento, have developed well-defined subsequent valleys in the heart of the Santa Lucia Range.
In the valley of the Cuyama or Santa Maria River, the effect of a twofold structural control is apparent. The upper reaches of the river flow thru a broad valley with an alluviated bottom on the northeast side of the San Rafael Range. The general trend of the river in this part of its course is northwest-southeast, and it is separated from the Carissa Plains by a high mountain ridge with a very precipitous southwest front, which probably represents a fault-scarp. Below this expansive high valley, the stream enters a rather narrow canyon and shortly after this bends at right angles and flows southwest toward the coast, entering eventually on the broad Santa Maria Valley which is open to the sea. The contrast in the geomorphic character of the upper and lower reaches of the river, the greater age of the former, and the sudden change in the course of the stream where the two types of geomorphy meet, suggests that the high valley of the upper reaches was once connected with the Salinas drainage and that it has been captured from the latter, in comparatively recent time, by a stream cutting back from the coast at the northwest end of the San Rafael Range.
In the valley of the Santa Ynez, there is a marked departure from the northwest-southeast trend which characterizes the geomorphic features of the Coast Ranges in general, and a more striking instance than any yet cited of the obliquity of those features to the general trend of the Coast Range belt. The valley lies nearly east and west and its general slope is southward to the base of the precipitous northern face of the Santa Ynez Range. This face is, as has been indicated, a fault-scarp; and the course of the valley is thus seen to be in intimate relation to this dominant structural feature. To the west the valley opens widely to the sea, while to the east it loses its individuality in the headwater canyons of eastern Santa Barbara County and western Ventura County.
Between the Santa Ynez Valley and the upper Cuyama is the rugged and deeply dissected country culminating in the San Rafael Mountains on the northern side of the tract. This mountainous belt has a trend intermediate between the pronounced east-west trend of the Santa Ynez Range and the northwest-southeast trend of the Coast Range ridges and valleys to the north. For a portion of its length the belt is bounded on the south by the Santa Clara Valley, with a general east and west course; but across the head-waters of Santa Clara River the mountainous tract persists and finds its prolongation, with the same general trend, in the San Gabriel Range, and beyond Cajon Pass in the San Bernardino Range, both bold and lofty sierra. It may even be considered as extending, under the name of the Chocolate Mountains, to the Colorado River above Yuma. From Tejon Pass southeast to Cajon Pass, the northern side of this mountain tract presents a very abrupt front with a very straight course. At the base of the abrupt slope lies the San Andreas Rift. To the north of this, and between it and the southeast scarp of the southern Sierra Nevada, lies the Mojave Desert. To the south of the southeast end of the San Bernardino Range and west of the Chocolate Mountains lies the Colorado Desert. As has been already indicated, the south side of the San Gabriel Range is determined by a profound fault. Lying thus between two faults, the range is a magnificent example of a horst which has been thrust up between its bounding faults. It is the convergence of these two bounding faults which segregates the San Gabriel Range from the San Bernardino Range in the vicinity of Cajon Pass. The latter range is similarly bounded on the south by the same fault as that which determines the south front of the San Gabriel Range, but here it is coincident with the Rift. Between Los Angeles and Ventura lie the short ranges known as the Santa Monica and the Santa Susannah Mountains, inclosing San Fernando Valley. The Santa Monica Range is probably on the same line of orogenic uplift which finds its expression farther west in the Santa Cruz and Santa Rosa Islands.
South of the San Gabriel Range lies the fruitful valley of southern California, extending with an east-west course from the sea to San Bernardino. South of this valley, and between the Colorado Desert and a somewhat elevated coastal plain bordering the Pacific, is a mountainous tract, the ridges of which swing around into a more northwest-southeast trend, and so conform again with the prevailing trend of the ridges and valleys of the Coast Ranges north of the San Rafael Mountains. The valleys in this region are, however, less regular in their orientation than those of the northerly Coast Ranges, and the geomorphic features are less mature, if we except certain very old features which have survived from an earlier cycle of geomorphic evolution. The consequent character of the streams on the seaward slope is much more pronounced than in any part of the northern Coast Ranges, and on the whole the geomorphy of the region must be regarded as less advanced than to the northward, and more closely allied in its morphogeny with the Sierra Nevada than with that of the Coast Ranges of northern California.
The notable ranges of this region are the Santa Ana Mountains and the San Jacinto Mountains. The former present the features of a seaward sloping, tilted, orographic
Verbal communication from Dr. H. W. Fairbanks.
To the northwest the range becomes subdued in the Puente Hills, where a broad anticlinal structure replaces in part the deformation by faulting. In two notable instances, and perhaps in others, the seaward streams of the Santa Ana Mountains cut entirely thru the range and drain the valley-land beyond its northeasterly scarp. These are the Santa Ana and the Santa Margarita Rivers. They are both probably antecedent to the more acute phases of the tilting of the region and have persisted in their course during the development of the fault-scarp.The San Jacinto Mountains form an important feature of the region as a bold ridge with northwest-southeast trend lying between Perris Plain and the northern end of the Colorado Desert. Both sides of the range are precipitous and are probably determined by faults. On the southwest side there were notable ruptures of the ground in the earthquake of 1898, indicating that the fault on that side is still in active development.
Courtesy of The Bancroft Library, University of California, Berkeley, CA 94720-6000; http://bancroft.berkeley.edu/
http://content.cdlib.org/view?docId=hb1h4n989f&brand=eqf
Title: The California earthquake of April 18, 1906 : report of the State Earthquake Investigation Commission, in two volumes and atlas.
By: California. State Earthquake Investigation Commission, Lawson, Andrew C. (Andrew Cowper), 1861-1952
Date: 1908-10
Contributing Institution: The Bancroft Library, University of California, Berkeley, CA 94720-6000; http://bancroft.berkeley.edu/
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