The California earthquake of April 18, 1906 : report of the State Earthquake Investigation Commission, in two volumes and atlas.
The San Andreas Rift as a Geomorphic Feature
General
Extending thru the greater part of the Coast System of mountains from Humboldt County to the Colorado Desert, a distance of over 600 miles, is a line or narrow zone characterized by peculiar geomorphic features, referable either directly to the modern deformation of the surface of the ground or to erosion controlled by the lines upon which such deformation has taken place. This peculiar feature has been known, both to Californian geologists and to residents of the sections where its characters are most prominent, but its extent and importance were not fully appreciated until after the earthquake of April 18, 1906. It is commonly reported among the residents of the southern interior Coast Ranges, particularly in San Benito, Monterey, and San Luis Obispo Counties, that displacement of the ground occurred on this line in the earthquake of 1857 and in certain later earthquakes. The first reference in scientific literature to this feature appears to have been in the year 1893, in a paper entitled "The Post-Pliocene Diastrophism of the Coast of Southern California," by Andrew C. Lawson, which is quoted in the sequel. The next reference to this peculiar line is in the eighteenth annual report of the U. S. Geological Survey for 1896-1897, Part IV, in a paper by Schuyler on "Reservoirs for Irrigation," where, pp. 711-713, the significance of the line is fully recognized in the following words quoted in full:
This reservoir has especial interest, not only as the first one of any magnitude completed on the Mojave Desert or Antelope Valley side of the Sierra Madre in southern California, but because it lies directly in the line of what is known as "the great earthquake crack" of this region, which is marked by a series of similar basins behind a distinct ridge that appears to have been the result of the great seismic disturbance.
This remarkable line of fracture can be traced for nearly 200 miles thru San Bernardino, Los Angeles, Kern, and San Luis Obispo Counties, and deviates but slightly here and there from a direct course of about N. 60° to 65° W. There appears to have been a distinct "fault" along the line, the portion lying south of the line having sunken and that to the north of it being raised in a well-defined ridge. In many places along the great crack, ponds and springs make their appearance, and water can be had in wells at little depth anywhere on the south side of the ridge before mentioned. A tough, plastic, blue clay distinguishes the line of the break, in this portion of its course at least; and where the line crosses Little Rock Creek, the blue clay has formed a submerged dam, which has forced the underflow near the surface and created a "cienega" immediately above it. After crossing the line, the water of the creek drops quickly away into the deep gravel and sand of the wash. The same effect is noticeable at other streams, and it has been suggested as the probable cause of the very distinct rim marking the lower margin of the San Bernardino Valley artesian basin and confining its waters within well-defined limits, as this rim is nearly on a prolongation of the line that is traceable on the north side of the mountains — the break having crost the mountains thru the Cajon Pass on the line of Swartout Canyon.
In 1899 the essential features of the same line in the region north of the Golden Gate were recognized and discust by F. M. Anderson.
The Geology of the Point Reyes Peninsula, Bull. Dept. Geol., Univ. Cal., vol. 2, No. 5, p. 143 et seq. Anderson, however, supposed, as is indicated by the last paragraph of his paper, that the faulting antedates entirely the Pleistocene terrace formations.
In later years Dr. H. W. Fairbanks has traced out the line in various field trips and has given several public lectures descriptive of its features and its significance, but has published no systematic account of his studies.The fact that the earthquake of April 18, 1906, was caused by a rupture and displacement of the earth's crust along this line for a distance of about 190 miles, immediately focussed the attention of local geologists upon it. Among those engaged upon
A summary account will now be given of this rift line as a geomorphic feature.
Humboldt County
The most northerly point in California at which geomorphic features directly referable to the violent rupture of the earth's crust have been observed are those noted by Mr. F. E. Matthes in the vicinity of Petrolia in Humboldt County. Here south of Petrolia, on high bare mountain spurs between Cooskie, Randall, and Spanish Creeks, he reports the occurrence of several small ponds and ridges such as have been familiar to those engaged in the field study of the earthquake phenomena as characteristic Rift features. Similar features are also found at the base of these spurs near the shore. These are in line with similar features found by the same observer between Telegraph Hill and Shelter Cove, a few miles to the southeast. Here, particularly in Wood Gulch (plate 1), is a narrow depression with ponds, ridges, and saddles, which appears to be essentially a feature due to deformation and to have determined the course of the drainage. The course of the depression is about N. 25° W. In this depression lies the trace of the fault upon which movement took place on April 18, 1906. Its course, if followed southward to the cliffs above Shelter Cove (plates 2A, 3A, B), heads out to sea with a trend nearly parallel to the coast. Great landslides occur along the coast in proximity to this line, and are in part on the Rift. The rocks traversed by the Rift in this part of Humboldt County appear to consist wholly of shales, sandstones, and conglomerates which are probably of Cretaceous age, altho since the geology of the region has not been studied, positive statements in this regard can not be made. The region is high and rugged, with a very precipitous descent to the sea, King Peak having an elevation of 4,090 feet at a distance of about 2 miles from the shore.
Point Arena to Fort Ross
From Shelter Cove to near Point Arena, the Rift, if continuous, lies beneath the waters of the Pacific. The continuity for this stretch is of course open to question, and in another place the considerations bearing upon this point will be presented. At the mouth of Alder Creek, 4.5 miles northwest from Point Arena, the Rift enters the coast from the sea and is thence traceable continuously to a point about 2 miles southeast of Fort Ross, a distance of about 43 miles, with a nearly but not quite straight course, being slightly curved with the convexity toward the ocean. (See map No. 2.) For our knowledge
With regard to the geology of the territory traversed by the Rift from the vicinity of Point Arena to Fort Ross, Dr. H. W. Fairbanks has kindly examined the ground and supplied the following note:
Except for a strip of sandstones (Walalla beds) of upper Cretaceous age extending along the coast north and south of the mouth of the Gualala River, and a triangular area of Monterey shale and sandstone underlying the coastal terraces in the vicinity of Point Arena, the rocks of almost the entire mountainous region between the upper Russian River Valley and the coast belong to the Franciscan. There seems to be but one fault in this region, and that is on the line followed by the Rift. The Walalla beds begin upon the coast a little south of Fort Ross and, extending inland, form the ridge between the Gualala River and the ocean. The formation thins out against the ridge bounding the Gualala Valley upon the northeast. The line of junction is an irregular one, for in places the soft sandstones reach quite to the top of the ridge referred to. These beds extend along the coast to the northwest for more than 30 miles, finally terminating 7 or 8 miles south of Point Arena, where they are overlain by the Monterey sandstones and shales. The Rift does not follow the contact between the Walalla and Franciscan formations and the vertical displacement does not appear to have been very great, as in only one place was it enough to bring up the underlying Franciscan rocks upon one of its walls. The Rift, for something more than a mile after emerging from the ocean southeast of Fort Ross, lies in the Franciscan formation, and the latter is greatly
― 28 ―crusht and broken along it. Back of Fort Ross, the surface rocks traversed by the Rift belong to the Walalla formation, and from this point for a number of miles to the northwest no other formation appears.At the point where the road from Stewarts to Geyserville crosses the Gualala River, faulting and erosion have exposed the underlying Franciscan formation. This appears upon the northeast side, showing that the opposite side, that toward the ocean, has dropt. The Franciscan occupies but a narrow strip and is replaced for some distance up the ridge upon the northeast, by Walalla sandstones. These relations are shown in the cross-section sketch shown in fig. 1. Near the mouth of the Walalla River the formation upon the coast side of the Rift still appears to be the Walalla sandstones; the rocks upon the opposite side are buried under the alluvium of the valley. After leaving the valley of the Garcia River, the Rift lies wholly within the Franciscan formation until it disappears in the ocean. The Monterey shales with sandstones at their base form nearly the whole of the coastal terraced plain in the vicinity of Point Arena. They rest unconformably upon the Franciscan rocks and dip at a steep angle to the southwest. The Monterey formation nowhere appears to come in contact with the fault.
Bodega Head to Bolinas Bay
General Note. — From the point 2 miles south of Fort Ross where the Rift in its southeasterly course leaves the shore, it passes beneath the Pacific for a distance of 12 or 13 miles. Its observed course to the northwest of Fort Ross, if projected southeasterly with a slight curvature, would strike the shore again at Bodega Head; and here it is found on the low ground of the isthmus that connects the head with the mainland. The Rift here coincides in position with a fault described by Osmont,
Bull. Dept. Geol., Univ. Cal., vol. 4, No. 3.
whereby the Franciscan rocks to the east are dropt down against the pre-Franciscan dioritic rocks of the headland. Immediately to the east of the fault-trace is a marsh. Across the mouth of the bay formed by the headland is a sandspit and the fault-trace should cross the spit near its abutment upon the shore line, but the drifting sands preclude its finding an expression here in geomorphic forms.To the south of Bodega Head the Rift follows Tomales Bay (plate 6A) to its head near Point Reyes Station. This is a remarkably linear inlet of the ocean lying between Point Reyes Peninsula and the mainland, having a length of about 15 miles and not exceeding a mile in width. It has generally been regarded as a feature determined by a fault,
Cf. Anderson, Geology of Point Reyes Peninsula, Bull. Dept. Geol., Univ. Cal., vol. 2, No. 5.
the same as that noted by Osmont at Bodega Head, whereby the Franciscan rocks of the mainland were brought against the pre-Franciscan granitic and dioritic rocks of the peninsula. The bay is quite shallow, but both of the slopes above the shore line are rather precipitous, and the ridge crests on either side attain elevations of over 1,000 feet. On the mainland side of the bay there are some rather vaguely defined terraces, both in the form of wave-cut benches and delta embankments. On the same side of the bay there are marine deposits of late Pleistocene age, containing abundant molluscan remains which have been elevated to about 25 feet above sea-level, and which are the equivalent of similar deposits at a similar elevation on the east side of San Pablo Bay.To the south of Tomales Bay the Rift lies in a remarkable defile with abnormal and ill-adjusted longitudinal drainage, which extends thru to Bolinas Bay, a distance of about 14 miles. On the east side of the defile is the steep coastal slope of the mainland, rising to a ridge crest from 1,000 to 1,700 feet in height. The transverse gullies in this slope are shallow, and detract but little from the general effect of a fairly regular but uneven steep slope. On the west is an even steeper but more incised and rugged slope, which forms the eastern edge of the peninsular land mass. This slope culminates in crests having an altitude of about 1,500 feet. The most striking geomorphic feature of the bottom of the defile is the presence of low ridges with intervening ravines or gullies elongated parallel to the general axis of the depression. More or less hummocky surfaces, with hillocks and hollows having no regular orientation, also occur. In the hollows ponds are fairly common features. The chief drainage is to Tomales Bay by Olema Creek, which heads within 2.5 miles of Bolinas Lagoon; and the divide between this stream and the parallel one which flows to the southeast has an altitude of about 400 feet above sea-level. The southeast end of the depression is submerged beneath sea-level, and is cut off from Bolinas Bay by a sandspit. The very shoal water inside of the sandspit is known as Bolinas Lagoon. (See plate 6B.)
The rocks on the east side of the defile belong wholly to the Franciscan series. On the west side, at the north end, we have chiefly the granitic and dioritic rocks of the peninsula with limited masses of crystalline limestone into which these rocks are intrusive. Farther south the granitic rocks are overlain by the shales of the Monterey series, and these rocks form the west side of the defile for several miles. The shales have inconstant and often very high dips. Still farther south the sandstones of the Merced series lie unconformably upon the Monterey shales, and near the town of Bolinas dip uniformly at moderately low angles toward the axis of the defile. It is thus apparent that the axis of the defile crosses more or less obliquely or transversely the contact between the Monterey and the granitic rocks, and also the contact between the Merced and the Monterey. It is also a remarkable fact that altho on the east side of the defile the Franciscan rocks constitute the mountain mass to a thickness of several thousand feet, this entire series, together with the Knoxville, Chico, Martinez, and Tejon, is almost entirely absent between the Monterey and the granitic rocks on the peninsula in the immediate vicinity. This indicates clearly that in pre-Monterey time the peninsular mass had been uplifted on a fault along the present coastal scarp, so that the granite was brought against the Franciscan and denuded of its unconformable mantle of sedimentary strata before it was submerged to receive the deposits of Monterey time. It is also clear that inasmuch as there is a great volume of Monterey shales on the peninsular or seaward side of this fault line, and no trace of the same formation on the mainland to the east of the fault line, one of two things must have happened. Either the submergence which permitted the deposition of the Monterey shales was confined to the peninsula and was effected by a downthrow of that block on the same fault as that upon which it had earlier been upthrust, so that there was no sea over the territory east of the fault; or, if the regions on both sides of the fault were submerged together, then in post-Monterey time the east side of the fault was lifted into the zone of erosion and denuded of its covering of Monterey shales so thoroly that no trace of them now remains. There is no escape from one or the other of these conclusions, and each of them involves a movement on the fault with relative downthrow on the southwest side, or the reverse of that which occurred in earlier, pre-Monterey time. From this interpretation it follows that the defile extending from Tomales Bay to Bolinas Bay lies along the trace of a fault which dates from pre-Miocene time, and that upon this fault there have been large movements in opposite directions so far as the vertical component of such movements is concerned. The trace of this ancient fault is also the line of the modern Rift.
The dip of the Merced beds at Bolinas toward the Franciscan rocks of the mainland is quite analogous to the dip of the same beds toward the Franciscan of San Bruno Mountain on the San Francisco Peninsula,
Cf. A Sketch of the Geology of the San Francisco Peninsula. U. S. G. S., 15th annual report.
and has the same significance, viz., that the Merced beds have been relatively downthrown on the west against the older rocks. The fault in the Tomales-Bolinas defile has usually been regarded as identical with and a continuation of the San Bruno fault of San Francisco Peninsula, and there seems to be no good reason for changing this judgment, altho, as will appear shortly, the modern Rift to the south of the Golden Gate does not coincide with the trace of the San Bruno fault, but leaves it at a small angle and pursues a course nearly parallel, but to the southwest of it. It is noteworthy, also, that while on the Point Reyes Peninsula, particularly in the vicinity of Bolinas, there is a magnificent wave-cut terrace at an altitude of about 300 feet, with a width of 1 to 1.5 miles between the base of its sea-cliff and the brink of the present sea-cliff, no such feature is to be found on the landward side of the fault-line on the coastal scarp between Bolinas Lagoon and the Golden Gate.Characteristics of the Rift (G. K. Gilbert, pp. 30-35). — In a broad sense the structural trough in which lie the two bays is a feature of the great Rift. In a narrower sense the Rift follows the lowest line of the trough, controlling the topography of a belt averaging 0.75 mile in width. The physiographic habit of the trough is that of a depression occasioned by faulting. It is remarkably straight. One wall, the southwestern, is comparatively steep; the other is comparatively gentle. The gentler slope is an inclined plateau with incised drainage. Viewing the trough from any commanding eminence, the physiographer readily frames a working hypothesis of faulting and tilting. He sees in the southwestern wall a fault-scarp of moderate freshness, and in the northwestern wall a slope originally less steep, in which erosion has been stimulated by uplift and tilting. The general facts of the geology of the district, as worked out by Anderson,
Geology of Point Reyes Peninsula, by F. M. Anderson. Bull. Dept. Geol. Univ. Cal., vol. 2, No. 5.
agree with this theory. The axial line of the valley is recognized by him as the locus of a fault, or fault-zone, and the rocks of the southwest wall are everywhere older than those which adjoin them at the base of the opposite slope. The gentler slope is well shown by plate 7A. Plates 8B and 41B also show something of the gentler slope, and plate 7B of the bolder.In a general way the two slopes are drained by streams which descend to the axis of the valley, and are there gathered in two longitudinal trunk streams which flow severally to Tomales Bay and Bolinas Lagoon; but in a central belt following the lowest part of the trough the details of drainage are comparatively complex, and their complexity is associated with peculiarities of the relief which serve to distinguish the central belt from the bordering slopes. In the bordering slopes the subordinate ridges conform in normal manner to the drainage, having evidently been developed by the erosion of the canyons which separate them. In the axial belt the ridges are evidently independent of the drainage, often running athwart the courses which would normally be followed by the drainage. In part the ridges divert or control the drainage; in part the drainage traverses and interrupts the ridges.
The influence of the ridges on the drainage is illustrated by the accompanying diagrams. Fig. 2 shows the actual drainage system; fig. 3 the system which would be developed if there were no special conditions along the axial zone. The small ridges of the axial zone trend parallel to the axis, and their interference gives parallel courses to various streams which would otherwise unite. The influence of the drainage on the ridges is illustrated by fig. 4, which shows a small ridge resting on the side slope of a larger ridge. The drainage of the larger ridge breaks thru the smaller, making gaps. Plate 7B shows the slope of a greater ridge at the right; and at the left two bushy hills
Similar relations between ridges and drainage lines are found in regions of steeply inclined strata, each ridge being determined by the outcrop of a resistant formation, or at least all of the preceding description might apply to the topography of such a region; but other characters remain to be mentioned, and these serve for discrimination. Where a steep-sided ridge is determined by the presence of a resistant formation, the determining rock follows and usually outcrops along its crest; but in the ridges under consideration there are few rock outcrops, and such as occur are not systematically related to the crest lines. The formation of the crest is not always the same thru the whole length of the ridge, and it is not always a rock of such character as to resist erosion. Between the ridges are linear valleys, and many of these are occupied by streams, but in a number of instances they are crost by the drainage. Often they include local depressions, with ponds or small swamps, this character being so pronounced that forty-seven such ponds were seen between Papermill Creek and Bolinas Lagoon, a distance of 11
In view of these characters, and especially of the abundance of ponds, it is evident that these little valleys are not products of stream erosion; and that in so far as they are occupied by streams the streams are adventitious. Their true explanation is suggested by their relation to certain of the earthquake phenomena of April, 1906. As will presently be described in detail, the trace of the earthquake fault thru the greater part of its course in the larger valley follows the edge of one or another of these small valleys; and in places where the fault movement included vertical dislocation, such dislocation nearly always tended to increase the depth of the valley. (See plate 10B and fig. 6.) Of the numerous minor or secondary cracks developed by the earthquake in the immediate vicinity of the main fault, a considerable proportion occurred at the edges of the little valleys, following more or Iess closely the line along which the bottom meets the side; and with these cracks also there was usually a little vertical dislocation, the ground
Collectively these ridges and valleys occupy a belt from 0.5 to 1 mile in width, and constitute the local development of the Rift, using that term in its narrower sense. They make up the entire surface of the belt, except where overpowered by some vigorous creek. The individual ridges are not of great length, being 2 or 3 miles at the most, and usually much less. Some of them end by wedging out, others by dropping down until replaced in the same line of trend by valleys. Their greatest height above base, except where the adjacent valleys have been deepened by erosion, is about 150 feet. The narrower have straight, acute crests; the broader have undulating backs with more diversity of form than is shown by the associated valleys. Some are crost by curved or straight depressions, and these depressions have all the characters of the parallel valleys, including the association of earthquake cracks.
In the remainder of this report the term Rift will be applied only to the narrow belt just described. Regarding it as the surface expression of a great shear zone or compound fault, the ridges are the tops of minor earth-blocks, and the valleys are in part the tops of relatively deprest blocks and in part depressions resulting from the weathering of crusht rock. Considering the Rift as a physiographic type, I find it convenient to have a specific name for one of its elements, the small valley; and in some of the descriptions which follow I shall speak of it as a fault-sag. (See plates 7B, 8A, and 11.)
The general relation of the Rift to the greater valley is illustrated by the cross-profile in fig. 7. Along its northeastern side it everywhere lies lower than the adjacent slope of the greater valley, the produced profile of the valley slope passing above the fault-ridges as well as the fault-sags. Along its southwestern side some of the fault-ridges appear to project above the restored profile of the greater valley, while the fault-sags lie below. If I interpret the structure correctly, the great compound fault concerned in the making of the valley trough — a fault of which the vertical dislocation amounts to several thousand feet — includes a certain amount of step-faulting, which
The limits of the Rift are not definite. The boundaries drawn in fig. 2 serve to indicate the belt in which the Rift structure dominates the topography, but do not indicate the limits of the Rift structure. Within the belt the dislocations have been so recent and of such amount as to keep ahead of weathering and erosion, so that their expression has been little dimmed by the processes of aqueous sculpture. Outside the belt the evidences of recent dislocation are less striking, but nevertheless exist. The inter-stream ridges of the northeastern slope are here and there indented and creased in such a way as to indicate recent faults of small amount trending parallel to the Rift. In the vicinity
Mussel Rock to Pajaro River
From Bolinas to the vicinity of Mussel Rock, about 8 miles south of the Golden Gate, the course of the Rift is beneath the waters of the Pacific, across the bar in front of the entrance to the harbor. Near Mussel Rock it intersects the shore at a great landslide (plate 12A) in rocks of the Merced series. At Mussel Rock, the basal beds of the Merced series rest directly upon an old land surface of worn-down Mesozoic rocks, and the basal bed contains abundant cones of Pinus insignis resting upon cemented alluvium. The cone-bearing bed immediately underlies marine strata and numerous fossils occur near the base of the series at the top of the ridge. The Merced strata here have a dip of about 15° to the northeast. The contact between the Merced and the older rocks trends southeast across the peninsula; and for some miles the Rift is approximately coincident with the trace of the contact and, for some portions of this distance, exactly so. From the shore line the course of the Rift is the same as that of the steep cliffs which rise at the back of the Mussel Rock slide to an altitude of over 700 feet. From the top of these cliffs, at an elevation of about 500 feet above sea-level, the course of the Rift as far as San Andreas Lake is marked by a line of shallow longitudinal depressions, ponds, and low scarps. (See plate 12B, 13, and 14.) There are eight ponds in this stretch of about 4.5 miles. This portion of the modern Rift was recognized as such in 1893.
"The line of demarkation between the Pliocene and the Mesozoic rocks, which extends from Mussel Rock southeastward, is in part also the trace of a post-Pliocene fault. The great slide on the north side of Mussel Rock is near the land terminus of this fault-zone, where it intersects the shore line. Movement on this fault-zone is still in progress. A series of depressions or sinks, occupied by ponds, marks its course. Modern fault-scarps in the Pliocene terrane are features of the country traversed by it." The Post-Pliocene Diastrophism of the Coast of Southern California, by Andrew C. Lawson, Bull. Dept. Geol., Univ. Cal., vol. 1, No. 4, pp. 150-151.
At a point about 4 miles from the Mussel Rock slide, the longitudinal depression which marks the course of the Rift becomes much more pronounced and passes into a remarkably straight and deeply trenched valley, the greater part of which has been converted by large dams into the San Andreas and Crystal Springs Lakes, used as reservoirs by the Spring Valley Water Company as water supply for the city of San Francisco. This straight valley (see plate No. 15) has an extent of 15 miles with a steady course of S. 34° E. to a flat divide southwest of Redwood City, whereby one passes over into the end of a similar but less pronounced valley, in which are situated Woodside and Portola. The San Andreas and Crystal Springs Lakes valley is almost wholly in the Franciscan terrane and the axis of the valley is discordant with the structural lines and contact planes of its constituent formations and intrusive masses. At the upper end of San Andreas Lake, however, the southwest edge of the Merced terrane forms in part the boundary of the valley on the northeast side for a short distance. The valley as a geomorphic feature (plate 16A) dates back fairly well into the Pleistocene. It is drained
To the southeast of Crystal Springs Lake, the valley followed thus far bifurcates about 2 miles beyond the lake, on either side of a median ridge. The two branches are nearly parallel. The east branch rises to a wide and rather flat divide, with streams heading in it from both sides. The other branch, altho it is more incisive, has no well-defined stream, but has a small swamp at its lower end. It rises to a sharper divide, from which there is a descent into the narrow straight canyon of West Union Creek. It is this western depression that the Rift follows. Near Woodside the canyon of West Union Creek expands into a more open valley, with steep mountains on the southwest and lower hills on the northeast. The Rift follows this straight valley (plate 16B) to its southeastern end, and then ascends to the saddle which separates Black Mountain from the mountains to the west. From this saddle it descends to the narrow canyon of Stevens Creek. It crosses the canyon at a small angle near its upper end and parallels the creek on the southwestern side, at an elevation of about 500 feet above it. It then passes thru the saddle between Stevens Creek Canyon and Congress Springs, and keeps well up on the slopes to the west of Congress Springs behind a series of shoulders and knolls to a reservoir on a saddle thru which it passes. From this saddle southeastward the line of the Rift again lies along the southwest side of a longitudinal valley and so continues on a line independent of the present drainage to the pronounced notch in the crest line of the range at Wright Station.
In this stretch of the Rift from Crystal Springs to Wright, the coincidence of the Rift with the major geomorphic features is very striking for the first half of the distance. In the second half, if we judge by the fault-trace, it appears to be quite independent of, tho parallel to, the canyons; and its only manifest relationship to the geomorphic features is its coincidence with a series of saddles or windgaps in the transverse spurs of the mountains. Its general parallelism with, and proximity to, the crest of the range thruout the entire stretch is pronounced. In the notch at Wright, the Rift intersects the crest line and passes from the northeastern flank of the range to the southwestern.
The general features of the Rift from Wright to Chittenden are described by Mr. E. S. Larsen in the following note:
From the hills above Wright Station to the village of Burrell, a distance of about 2 miles, the Rift follows along the ridge above Los Gatos Creek, which drains to the east. The drainage of the western slope of the ridge is to the Pacific. For most of this distance the Rift is a short distance on the Los Gatos Creek side. It usually occupies a small, trough-like depression; or, where it cuts just above the heads of the small gullies, there are low, rounded knolls between the gullies. These knolls are seldom over 30 feet higher than the trough. Just southeast of Burrell, the Rift traverses the ridge and follows a gully into Burrell Creek, which it crosses. It continues in a southeasterly direction, parallel to the creek and about halfway up the ridge to the southwest of it. The elevation of the ridge is only about 400 or 500 feet above the creek bed, and the top is rounded, with a steep slope below this to the Rift, and a gentle slope below the Rift
Following the Rift to the southeast, it passes at the divide into the head of Eureka Canyon, rises on the northeast bank, and slowly gets farther away from the creek, cutting across the tributary creeks and rarely following one of the smaller gulches for a short distance. The typical section here gives a steep slope on the high hills to the northeast, then about 0.25 mile of gently sloping, rolling hills, and finally the steep slope to the creek itself. The Rift is on the gentle slope, generally at some distance from either of the changes in slope. This continues for about a distance of 2 miles on to Grizzly Flat. Here the high steep hills to the northeast are separated from the lower hills to the southwest by a flat about 500 feet across. The Rift is on this flat near its center, and usually marks the northeast boundary of a series of low knolls. It continues on the flat for about 0.5 mile to where the hills close together and leave a rather steep-walled gulch. The Rift follows up this gulch for about a mile, and then crosses into the head of another creek, which it follows down for about 3 miles, where the stream turns sharply to the north. For the upper mile the gulch is rather sharp and deep, but at Hazel Dell the hills on both sides are low and rolling, while the lower mile is again rather steep, opening at the turn to a rather flat country. At Hazel Dell and other points, the Rift occupies a small but distinct trough very near the southwest bank of the creek. From here to Chittenden, a distance of about 8 miles, it follows parallel to Pajaro Valley, well up on the hills, and cuts across the canyons at almost right angles.
The typical section up one of these ridges gives a gentle slope from the valley to an elevation of about 1,000 feet; a steep slope for about 50 feet in the opposite direction, which marks the Rift; a very gentle slope for about 1,000 feet across; and finally, the steep upper slope of the hills. Over this area the Rift is nearly always marked by a trough, which often gives rise to a small lake perched on a ridge between two steep canyons. At a few points, especially about a mile northwest of Chittenden, small streams and gullies tend to follow the Rift, and they then make a sharp turn where they leave it. At Chittenden the Rift again passes thru a pronounced notch in the crest of the range occupied by the canyon of Pajaro River (plate 17A), from the western flank of the dominant ridge of the Santa Cruz Range to the eastern flank of the Gavilan Range.
The rocks traversed by the Rift from Mussel Rock to Pajaro River are, so far as known, almost wholly confined to the Franciscan and Monterey series, the former prevailing in the northern part and the latter occurring only in the southern. At Pajaro River the Rift encounters the granitic rocks of the Gavilan Range, but these lie wholly on its western side.
Pajaro River to the North End of the Colorado Desert
By H. W. Fairbanks
The earthquake of April 18, 1906, opened and displaced the walls of the old fault along the Rift as far south as the town of San Juan in San Benito County. The fault-trace passes directly under the western span of the Southern Pacific Railroad bridge across San Juan River, as shown by the displacement of the piers at the end of the bridge, a distance of 3.5 feet. For a distance of nearly half a mile on either side of the bridge, the river has established itself in the Rift. To the northwest the steep slopes of Mount Pajaro facing the canyon do not show any regular fissure. This does not, however, indicate any discontinuity in the fault, for the surface of the whole mountain is more or less broken by auxiliary cracks, secondary fissures, and slides.
Southeast over the hills from the point where the Rift leaves the river, the characteristic features of the Rift make their appearance. It is marked by a small pond (plate 17B), springs, and a more or less continuous ridge with its steeper face toward the southwest. The fissure of the recent earthquake follows this series of features (plate 18A), and, at a point halfway between the bridge and San Juan, there is shown in a broken fence a horizontal displacement of 4 feet. A mile before reaching San Juan, granitic rocks are exposed upon the southwest side of the Rift. Shortly beyond this point the Rift leaves the hills and traverses the western edge of the valley of the San Benito River. The ridge which we have been following is now lost in the level floor of the valley, but as far as traceable its course is directly toward the low bluff upon the eastern edge of the town of San Juan. The fissure of the recent earthquake is to be seen where it crosses the road 0.5 mile northwest of San Juan, but has not been noted farther along the old Rift line. It appears to bend more easterly, and this probably connects it with the disturbances of the earth between Hollister and San Juan. Mr. Abbe, of San Juan, states that the earthquake of 1890 opened the old Rift and that the displacement of the walls, tho small, was in the same direction as in the recent earthquake.
The town of San Juan stands upon a bench of gravel which dips gently in a south-westerly direction, but upon its northeastern side presents a steep face which, near the old mission, has a height of about 50 feet. This bluff is marked thruout its length of 0.5 mile by several springs; and there can be little doubt that it owes its existence to a fault movement uplifting and tilting toward the southwest a portion of the floor of the valley, and that it thus originated in the same way as other similar features which we shall find to be characteristic of the Rift. The Rift leaves the valley southeast of San Juan and gradually rises along the eastern slope of the Gavilan Range. It intersects the head of San Juan Canyon, and has here given rise to an interesting modification of the drainage. San Juan Canyon is long and narrow and is formed by the union of several small streams which, rising upon the higher slopes of the range, pursue a normal course toward the San Benito Valley, until reaching the Rift, they turn northwest and slightly away from the fracture line, giving rise to San Juan Canyon. At the point where the Rift intersects the canyon, the narrow ridge between the canyon and the valley has been broken thru, and the whole drainage passes directly down the mountain, abandoning the canyon, which is now filling with débris fan material.
For about 10 miles southeast from the head of San Juan Canyon, the Rift follows the eastern slope of the Gavilan Range. It is marked by small valleys and gulches, by hollows and ridges upon whose sides oak trees are growing; and it is followed almost continuously by a wagon road. One of the most interesting features along this portion of the Rift is Green Valley, a broad cienega due to the filling up with gravels and silt of a valley lying close under the steeper portion of the Gavilan Range. There are two fault-lines below the valley and about 0.25 mile apart. The cienega is due to vertical
The Rift comes out upon the San Benito River 4 miles above Paicenes P.O. For several miles up the river from this point, the Rift line is masked by the recent flood plain. Above Mulberry P.O., and just before coming to the bridge across the river, a most striking and interesting feature appears. Upon the east side of the river, and separated from it by a ridge, is a narrow depression half a mile long and 75 feet deep, without any external drainage. The ridge between it and the river extends a mile northwest of the sink, and presents a steep face to the northeast. The road passes along the eastern base of the ridge and opposite the sink makes use of its even crest. The river makes a sharp bend at the bridge, and the Rift crosses to the west side. Faulting has here brought to the surface, upon the west side of the Rift, limestones associated with the crystalline schists and granitic rocks of the Gavilan Range.
In order to follow the Rift beyond the mouth of Willow Creek, we leave the San Benito River road at the mouth of the creek and follow to its head a long narrow canyon which has evidently been eroded on the line of fracture. At the head of the canyon we come out upon a bit of open rolling country which, but for a low ridge, would drain into the San Benito. This ridge has evidently been raised along the Rift, diverting a stream which would naturally be tributary to the San Benito, so that now it forms the head of Bear Creek and flows down past the Chelone peaks into the Salinas River. Several undrained hollows (plate 18B) mark the Rift as it follows the ridge between Bear Valley and San Benito River. The formation of both walls is probably of Tertiary age up to a point near San Benito P.O., where the Franciscan series constitutes the southwest side and the Tertiary the northeast. South of San Benito P.O., there is a considerable area where the surface has been much changed as a result of some one of the movements along the old Rift. A fertile valley, perhaps 0.5 mile long, appears to have been formed thru subsidence, while on the southwest is an abrupt ridge 200 feet high and fully a mile long. The ridge without doubt has been produced by faulting. Its abrupt northeastern face and long, gentle, southwesterly slope suggest in a remarkable manner the great fault blocks of the west, such as the Sierra Nevada Range. The ridge gradually sinks in a southeasterly direction, blending with Dry Lake Valley. The latter is about 2 miles across and has no external drainage. The fault-scarp already mentioned extends as a low ridge part way across the valley and is utilized by the road.
Looking southeast across the valley in the direction which the Rift pursues, a mountain is seen which seems to have been sharply cut off. Descending a narrow valley to the southeast of Dry Lake Valley, we reach the foot of a steep escarpment (plate 19A) where there are apparent two, and possibly three, lines of displacement. The middle one passes at the foot of the main cliff, which is between 400 and 500 feet high. It can not be said with certainty that the whole cliff is the result of faulting, altho it is certainly so in part. The formation in the cliff is sandstone of either Tertiary or Cretaceous age. About 5 miles northwest of Bitterwater there is an interesting valley which has been so disturbed that it has no external drainage, while thru its center passes a ridge formed along the Rift. The ridge forms a fine roadbed. Descending toward Bitterwater Valley and P.O., another ridge appears which is as even and regular as a railroad grade. Bitterwater Valley is occupied during the wet season by a marshy lake.
Southeast of Bitterwater, the Rift leaves the younger formation, and at Lewis Creek both walls are in the Franciscan rocks. For 20 or 25 miles now, the peculiar features of the Rift by which we have followed it are almost absent. The Franciscan series, including old sedimentary rocks, serpentines, and other basic igneous rocks, does not lend itself well to the preservation of such records, but appears to be greatly broken and crusht and marked by enormous landslides in the vicinity of the Rift. The Rift crosses Lewis Creek about 2 miles above its mouth and then passes up over a high ridge lying between Lewis Creek and San Lorenzo Creek. On the north side of Lewis Creek there is an enormous landslide, which has nearly blocked the valley. The slide is undoubtedly hundreds of years old. The ridge on to which the Rift passes after leaving Lewis Creek is crost by it at such a small angle that it does not reach the southern base until we get to the head of Peach Tree Valley, a distance of 20 miles. The ridge its whole length is shattered and broken, and, as before said, marked by innumerable rockslides. The rather steep slopes appear to move every wet season. The headwaters of the San Lorenzo Creek (Peach Tree Valley) have been robbed by Gaviota Creek, possibly as a result of some movement connected with the Rift. Just above where the stream has been diverted, there is another great landslide which the road crosses to reach Slack Canyon.
At the mouth of Slack Canyon, the Rift leaves the Franciscan series, and coincides again with an ancient fault in which the Miocene sandstones are thrown down upon the southwest against the older formation just referred to. Passing from Slack Canyon over a divide, we come to the headwaters of Indian Creek and Nelson Canyon. As the Rift occupies steep slopes much of this distance, it is distinguished chiefly by landslides and rapid gullying of the surface. In Nelson Canyon the Rift follows an old fault in which the Miocene formation has been thrown down upon the southwest side, and the northeast wall so raised that the granite on which the Franciscan series rests is exposed. Ascending the divide toward the head of Nelson Canyon, a long, nearly straight ridge of Miocene clays divides the drainage and appears to be due to some one of the movements along the Rift.
The Rift can be traced thru the hills at the head of the Cholame Valley by its characteristic features, as well as by bluffs which are undergoing rapid erosion. It crosses the road a mile west of Parkfield and exhibits here a regularly rounded ridge 200 feet wide and 20 feet high at the most elevated point. (Plate 19B.) That the ridge must be hundreds of years old is shown by the great oak trees that are growing upon it. One white oak is fully 8 feet thru. Large springs mark the fissure at this point, and are found along it the whole length of the Cholame Valley. According to a resident, the Rift opened along the ridge in the earthquake of 1901, the opening being distinctly traceable for several miles. Southeasterly from the point just described thru the Cholame Valley, there appears no very prominent ridge or escarpment, altho springs and cienegas, marking a gentle swell in the flat open surface of the valley, indicate the line of the Rift.
The region about Parkfield, in the upper Cholame Valley, has been subjected to more frequent and violent disturbances than almost any other portion of the entire Rift. An auxiliary fissure begins near the main Rift a little west of Parkfield, and extends in a more easterly direction along the east side of Cholame Creek. (See plate 20.) The once flat, open valley has been broken along this line, and a bluff nearly 200 feet high formed facing the Creek. This bluff, now deeply eroded, must have been formed during one of the oldest disturbances. The lowland between this bluff and Cholame Creek shows the effect of great disturbance over a considerable area. Innumerable hollows interlace and extend in all directions. They resemble nearly obliterated creek beds except that they have no outlets. Parallel with the front of the dissected bluff, but a little back from
The people living along the Rift for 150 miles southeastward from the Cholame Valley tell wonderful stories of openings made in the earth by the earthquake of 1857. The first settler in Cholame Valley was erecting his cabin at that time, and it was shaken down. The surface was changed and springs broke out where there had been none before. In 1901 a fissure opened in the road which crosses the branch fault just described. After each successive shake it is reported that the fissure opened anew, so that the road had to be repaired again in order to be passable.
Upon the western side of the Cholame Valley, near its southern end, the main Rift again exhibits an interesting bluff which cuts off the débris fans of the back-lying hills. This bluff faces northeasterly. Where the Rift crosses the creek as it passes out of the Cholame Valley, a low escarpment was formed upon the west side which must for a time have dammed the creek and given rise to a lake. From the outlet of the Cholame Valley the Rift line can be seen as it rises along the low rolling hills, and disappears over their tops. It is marked by a distinctly steeper slope facing northwesterly, showing that an uplift of 30 to 50 feet took place upon the west side. The region traversed thru the Cholame Valley southeast to the Carissa Plain and for some miles beyond, exhibits no older formation than the Miocene Tertiary, the effects of older faulting, if such has occurred here, being masked by recent deposits. Continuing the examination toward the southeast, the writer came upon the Rift at the northern end of the Carissa Plain, 4 miles northeast of Simmler P.O., and in direct line with its course where last seen. Here the width of the broken country is much greater than usual, being nearly a mile. A number of lines of displacement can be distinguished; some nearly obliterated, others comparatively fresh. This is a region of light rainfall and of gentle, grass-covered slopes, presenting just such conditions as would preserve for hundreds of years the effects of moderate displacements.
The Rift zone continues to be traceable along the western base of the Temblor Range, finally passing out on to the gently rolling surface of the eastern edge of Carissa Plain. Broken and irregular slopes, cut-off ridges, blocked ravines, and hollows which are white with alkaline deposits from standing water mark the Rift. Carissa Plain has a length of about 30 miles. About halfway the Rift begins to be marked by a low and nearly obliterated bluff upon its northeastern wall. This is at first little more than a succession of ridges or hills cut off on the side next to the level plains. These detached ridges finally become connected in a regular line of hills with a steep but deeply dissected slope toward the southwest and long gentle slopes toward the northeast. This ridge is clearly a fault block, and now separates the southeastern arm of Carissa Plain from Elkhorn Plain. It probably originated during some one of the earlier movements along the Rift; in fact, it is reasonable to suppose that it is of the same age as other important scarps which mark the Rift thruout its whole course, and which came into existence as a result of some mighty movement opening the earth for several hundred miles.
Except for one slight bend, the ridge which we have been describing follows a straight course toward the southeast for a distance of nearly 20 miles, finally blending in a much larger mountain-like elevation. This has a height of perhaps 500 feet above the sink at its southern base. Its deeply dissected front is in line with the front of the ridge already described and the two appear to have originated together. The steeper face is deeply sculptured into gullies and sharp ridges, while the back slopes off gently toward the southern end of Elkhorn Plain. Plainly visible along the steep front of the line of hills described are the lesser ridges and hollows produced during the last violent earthquake in this region, probably in 1857. (See plate 21A, B, C.)
A gentle divide separates the southern end of the Carissa Plain from a long narrow sink extending along the Rift line toward the southeast. (Plate 21D, E, F.) This sink includes an area 6 miles long and in places its drainage is fully 3 miles wide. Several deprest alkali flats, covered with water during the wet season, receive the scanty run-off of this dry region. These depressions are several hundred feet wide and are bordered upon opposite sides by quite sharp bluffs, in some places 100 feet high. The phenomena suggest the sinking of long narrow blocks between two walls. This reach of 6 miles between the ranches of Job and Emerson is one of the most interesting areas examined. The larger scarps belong to some ancient disturbance, while the last one, probably dating from 1857, is marked by features comparatively insignificant.
As we ascend the long grade from the sinks just described, to Emerson's place, near Pattiway P.O., the Rift features become smaller and less regular, altho easily followed. (See plate 23A.) At Emerson's the Rift passes thru a sag in the hills and across the head of Bitter Creek. It then rises and crosses a flat-topt hill between this creek and the west fork of Santiago Canyon; and descending to the east fork keeps along the steep mountain slope upon the south until it finally crosses the divide between San Emedio Mountain and Sawmill Mountain. Thru this section the Rift gradually bends toward the east, and in Cuddy Canyon, farther east, it has an east and west direction for a few miles.
The Rift itself is scarcely distinguishable in Bitter Creek and Santiago Canyons, owing to steep slopes and rapid erosion, as well as numerous landslides. Santiago is one of the deepest and narrowest canyons in this portion of the mountains. Its whole southern slope, that traversed by the Rift, has been more or less affected by slides producing many little basins along the edge of the flat-topt divide between the drainage into the San Joaquin Valley and Cuyama River. Huge masses of earth and rock are still moving, as shown by fresh cracks and leaning trees. In one place the edge of the divide has split away in such a manner as to produce long narrow ridges with depressions behind them, closely imitating the real Rift features. Santiago Canyon marks a great fault of earlier times. Soft Tertiary formations are faulted down thousands of feet upon the south side of the canyon, while upon the north appear the steep granitic slopes of the western spur of San Emedio Mountain.
The Rift appears upon the north side of the pass which leads from Santiago Canyon to San Emedio Canyon. Two lines of disturbance are here plainly visible. Going down the west branch of San Emedio Canyon, the Rift zone is plainly traceable, but nowhere does it form important features. Passing to the east fork of the canyon, we continue on the line of the Rift to the divide leading over to Cuddy Valley. (See plate 22A.) Beginning upon the divide, a broad rounded ridge, fully 50 feet high upon its southern side, extends down the slope in a direction a little south of east. Cutting thru the center of this ridge longitudinally is a deep V-shaped depression, as tho a movement later than that which formed the ridge had opened a fissure thru its center. On the sides of the ridge, as well as the slopes of the fissure in it, large pine trees are growing in an undisturbed condition. Continuing down the ridge, we find that in the course of a mile it gives place to an escarpment facing northerly. A valley 4 miles long and 0.5 mile wide lies below the escarpment and contains meadows and a small lake without any outlet. Springs mark the Rift line. The escarpment has been much eroded, but toward its eastern end it has a height of nearly 300 feet and is covered with a growth of pine trees among which are stumps of large dead trees in an undisturbed condition. The valley and the bluff are doubtless the product of the earliest movement in the epoch of which we are treating. The last movement left a comparatively small ridge traceable here and there along the base of the greater.
Continuing on the line of the Rift, we enter and pass for 10 miles down Cuddy Canyon.
At Gorman Station, several miles below Gorman, there is a wonderfully regular ridge forming a marsh. In this vicinity the earthquake of 1857 is reported to have done much damage, shaking down an adobe house and breaking up the road. The little lake upon the divide halfway between Gorman Station and Neenach P.O. is due to débris brought down from the hills upon the south thru which the Rift zone passes. The Rift follows a very regular and straight course, a little south of east, along the mountain slopes south of Antelope Valley. Thru the most of the distance, as far as Palmdale, it occupies a series of valleys shut off by considerable elevations from the open slopes of Antelope Valley. After traversing the northern slopes of Libre and Sawmill Mountains, the Rift crosses the head of Oak Grove Canyon, then another small canyon with branches eastward and westward along the break, and eastward of this a long canyon opening out to the fertile valleys about Lake Elizabeth.
Lake Elizabeth and Lower Lake (plate 24B) are both due to the blocking of the drainage of two valleys extending along the Rift. These valleys lie on the slope of the range toward the desert (Antelope Valley), but their outlet is southward by a narrow canyon thru the heart of the mountains lying between the desert and Santa Clara River. A low escarpment along the southern side of the valley in which Lake Elizabeth lies, and eastward, is replaced by a lofty rounded ridge which appears to be due to some one of the movements along the old fault. For several miles east of the lake (plate 25A) the distinctive and characteristic features of the Rift are not as easily made out, altho the ridge just mentioned is full of springs and exhibits a widespread landslide topography. Toward the eastern end of this ridge small hollows and a low, indistinct escarpment again appear. The ridge separates Leones Valley, a fertile and well-watered district 5 or 6 miles long, from the open Mojave desert on the north.
From Leones Valley to and beyond the point where the Rift zone crosses the Southern Pacific Railway, a constant succession of cienegas is found on the upper side; that is, on the side toward the mountains. Movements have evidently been so often repeated and so intense along the Rift as to grind up the rocks and produce an impervious clayey stratum, bringing to the surface the water percolating downward thru the gravels of the waste slopes. A mile west of Alpine Station on the Southern Pacific Railway, there begins another escarpment with its abrupt face toward the south. This extends to and across the railroad. South of the escarpment the surface has sunk so as to form a basin. (Plates 25B and 26A.) This has been artificially enlarged and used as a reservoir for irrigation about Palmdale. The main escarpment is 40 to 50 feet high in places, and where the railroad crosses it there appear to be two, and older and a younger one. From the summit of the ridge marking the Rift west of Alpine, an extensive view eastward is obtained. The long desert waste plain leading up to the foot of the mountains on the south (San Gabriel Range) exhibits a strikingly interesting feature. It is not continuous across the line of the Rift, but shows a break with the uplift upon the lower side. The amount of displacement appears to be between 200 and 300 feet.
An extended study would be necessary to determine in detail the geology of the Rift from Gorman Station eastward. Near Gorman a dike of basaltic or andesitic lava extends parallel with it for some distance. Granitic rocks often form one side, while soft Tertiary beds of a light or reddish color frequently appear in the raised ridges. Between Palmdale and Big Rock Creek, low discontinuous ridges, springs, and cienegas point out the line of the Rift, altho there are stretches of several miles at a time where either the original displacement was not great or erosion has removed its effects. Four miles west of Big Rock Creek there is one fine escarpment 0.333 mile long and 40 feet high, facing the mountains on the south. (Plate 27A.) In this section there are indications of at least two movements. (See plate 26B.) The Rift passes just below Big Rock P.O. east of Big Rock; a trail on the northern slope of the mountains and a wagon road on the southern side of the divide follow the Rift continuously to a point near the mouth of Cajon Canyon. On the north side of the mountains (San Gabriel Range) there is no important depression on the Rift between Big Rock Creek and Swartout Valley; nevertheless the comparatively recent movements have been of sufficient magnitude to produce ridges and hollows giving a continuous and easy route for the trail along the slope of the mountains.
Before reaching the divide leading over to Swartout Valley, we encounter some striking features. Near the head of Mescal Canyon a ridge has been split away from the mountain, diverting the little streams from above and making two drainages where one would normally appear. In places this ridge (plate 27B) is as sharp and as perfect as tho formed but yesterday; but the great pine trees, growing upon its top and sides (the altitude here being nearly 7,000 feet), tell us that it must be hundreds of years old. At the head of the canyon the trail leads thru a sharp V-shaped cut where the bare sliding surfaces make it appear as if movement had recently taken place in the Rift. (See plate 28A.)
Passing over a sag in the mountains to Swartout Valley, the Rift is less prominent as a topographic feature, but a line of springs marks its course. Lone Pine Canyon is remarkable for its length and straightness. The Rift passes down its whole length but it is not very prominent. Springs appear at several points, also small cienegas with a slight escarpment below them. At the mouth of Lone Pine Canyon and a little above its junction with the Cajon Canyon (plate 28B) are more interesting features. Two lines of displacement appear here, and between them a long, narrow sunken block with a small lake in its lowest portion. (See plate 29A.)
The line of disturbance now crosses Cajon Canyon, giving rise to broken and sliding cliffs; and then, passing over a spur of the San Bernardino Range, comes out at its foot before reaching Cable Canyon. From this point the Rift continues on southeasterly at or near the junction of the gravel slopes of the San Bernardino Valley and the steep mountain slopes of crystalline rocks. The uniformly straight course which the Rift exhibits in this portion of its length takes it diagonally across the mountains from the northern and desert side of the San Gabriel Range to the southern side of the San Bernardino Range.
The torrential streams emerging from the San Bernardino Range upon the gravel slopes of the broad valley at its base have cut wide flood plains in the ancient gravels which accumulated along the foot of the mountains. The remaining portions of this old slope lying between the stream plains are called mesas. Back of Devore Heights there appears a rounded ridge formed out of the mesa gravels. As we continue toward Cable Creek, springs and cienegas are found to be numerous just above it. East of Cable Creek the ridge becomes an escarpment facing the valley, and in places shows a height of about 75 feet. Viewed in profile, this escarpment breaks the uniform slope of the mesa gravels, almost reversing their slope on the upper side. On the west side of Devil Canyon there is a double escarpment in the gravels, both apparently being due to movements along the Rift. (See plate 29B.) Back of the Muscupiabe Indian reservation and north of the
East of City Creek begins a huge rounded ridge formed in the mesa gravels, and this can be traced nearly to Plunge Creek. This ridge is 150 feet wide and steeper upon its upper side, where the greatest displacement shown is about 40 feet. The structure and shape of the gravel ridge make it appear likely that faulting and folding were both concerned in its making. Above this ridge and at the highest point where it crosses the mesa, water is obtained in abundance for irrigation at a depth of 18 to 20 feet, while in the mesa below the ridge no water is found at a depth of 200 feet.
The Santa Ana River has cut out a wide stretch of the mesa gravels, and has exposed upon its eastern bank a good section of these gravels. The gravels at their upper edge do not lap over the crystalline rocks but appear faulted down against them. A 0.25 mile below the fault is the mouth of Morton Canyon, the stream issuing thru a long, narrow canyon eroded in the mesa gravels. Morton Canyon emerges from the steep mountains about 2 miles to the southeast and has taken this peculiar course thru the gravels to the Santa Ana River, instead of flowing directly down across them, as do all the other streams. The explanation of the turning to the northwest of this canyon at the point where it meets the gravels is found in the peculiar appearance of the gravel slope when viewed in profile. This, instead of rising with normal slope, becomes steeper toward the upper edge, and then descends abruptly to Morton Canyon. The movement on the Rift has broken and lifted up the gravels to such an extent that the waters of Morton Canyon were diverted and turned down to the Santa Ana River along the upper side of the ridge. Since this displacement took place, they have had time to cut the canyon in which they are now flowing. Southeast of the point where the Rift crosses Mill Creek, the peculiar topographic features which have characterized it for so many miles become very indistinct. It was at first thought that the Rift terminated in this vicinity but closer examination made it clear that such is not the case.
The southern portion of the San Bernardino Range lying between Mill Creek and the Conchilla Desert appears to have undergone great disturbance at a recent date. As a consequence, erosion has been rapid and extensive, and surface features which farther north made the Rift easy to follow have in this region been almost completely obliterated. Potato Canyon extends along the line of the Rift to the southeast of Mill Creek. Its features indicate that the history of the fault is a complex one. The canyon originated thru erosion upon the fault contact between the crystalline rocks of the San Bernardino Range and the older Pleistocene deposits along its base. Following this period of erosion was one in which gravels were again deposited and this was succeeded by the present period in which erosion is active. Potato Canyon is the last of the longitudinal depressions of any size marking the line of the Rift. Between its head and the desert to the southeast the main drainage features pay little attention to the structural conditions, because of the steep grades of the stream channels and consequent rapid erosion. Nevertheless small lateral canyons have been formed along the fault contact of the gravels with the crystalline rocks of the higher portion of the San Bernardino Range, so that from the proper viewpoint the fault line can generally be traced in the topography. The drainage of Potato Canyon is clearly influenced by the fault, for instead of there being one stream course in it, there are two — one upon each side.
A mile southeast of Oak Glen, which is at the head of Potato Canyon, there are large springs which issue upon the line of the fault. Near this point a depression appears upon a gravel ridge, where it meets the crystalline rocks. The depression is in line with the course of the fault, and may with reason be attributed to dislocations similar to those so
To the east of the San Gorgonio River, the topography as shown upon the San Gorgonio quadrangle gives little indication of the presence of an important fault-line. However, an examination of Potrero Creek shows small transverse canyons and one broad, grassy flat with springs upon the line of the fault. In Stubby Canyon and other small canyons north of Cabazon Station, the fault is finely shown. Here, as at the point where the Santa Ana River issues from the mountains, the older Pleistocene gravels have been faulted down against the crystalline rocks. Rapid erosion of both the Pleistocene deposits and the crystalline rocks has given rise to steep and precipitous slopes in this section, and upon these the fault is clearly shown. The schists and gneisses thru a width of hundreds of feet adjoining the fault have been so crusht by pressure and movement that they quickly crumble upon exposure. Dark clay marks the plane of movement which inclines to the north at an angle of about 80 degrees. Later than the period of main faulting has come an elevation of the range as a whole, giving rise to rapid erosion upon both sides of the line of fracture. Remnants of gravel mesas and mature topographic forms appear in places. A notable example of an area of old topographic features now being destroyed by the modern canyons is shown to the west of Stubby Canyon and 1,000 feet above it.
There are traces here and there of recent displacements along the Rift. These are of the nature of little sags without outlets and low ridges or escarpments not easily explainable as a product of ordinary erosion. These may have arisen as the product of landslides, but the landslides themselves are doubtless related to fault movements. The great débris fans built up along the north side of San Gorgonio Pass indicate rapid removal of a vast amount of rock material from the adjoining slopes of the San Bernardino Range consequent upon recent uplift.
Before investigating this region it was thought that the Rift, if it continued on south-easterly, would be found crossing the San Gorgonio Pass in the neighborhood of Cabazon and skirting the eastern base of the San Jacinto Range; but this proved not to be the case. Instead, it was found to turn more and more easterly and finally to extend parallel with the pass without reaching it. The course of the Rift, then, instead of being in the direction of the Salton Sink, is toward the Conchilla Desert north of Palm Spring Station.
Looking east from a point near the mouth of Stubby Canyon, the gravel mesa thru which the Whitewater River issues from the mountains, appears to be faulted upward, giving rise to a well-defined escarpment facing north toward the crystalline rocks. This northward facing escarpment accords in relative position with the traces of escarpments farther north near Oak Glen, and shows that the latest displacement has been the reverse of the earlier. The last seen of the Rift is in the sides of the Whitewater Canyon, where the gravels are faulted down against the crystallines. East of the Whitewater one enters upon the Conchilla Desert over which has been spread the wash of Mission Creek. For a distance of 6 or 8 miles, and perhaps much more, the bedrock is completely buried by the recent accumulations.
The San Bernardino Range rapidly decreases in height to the southeast of Mission Creek, but appears to be continuous with the desert range lying north of the Salton Basin. The latter range of crystalline rocks appears to be separated from the lowlands of the basin by a more or less continous line of barren yellow hills formed of soft late Tertiary rocks. Judging from a cursory examination, these yellow hills are separated from the higher mountains behind by a structural break indicated by a series of longitudinal valleys. A prolongation in a northwesterly direction of the supposed fault line indicated by these
An examination of the northerly and easterly base of San Jacinto shows conditions opposite to those characterizing the southern slope of the San Bernardino Range. Erosion is generally slow upon the slopes of San Jacinto, while the rapid erosion from the opposite side of San Gorgonio Pass has crowded the stream channels close to the base of the former range. In fact, the base of the San Jacinto Range appears to be deeply buried by the stream deposits. The desert face of San Jacinto has long been free from disturbances. Long, jagged ridges project out into the desert, while the intervening canyons, instead of furnishing material for extensive débris fans, are floored by accumulations characteristic of the desert as a whole.
Toward the southern end of that spur of the San Jacinto Mountains which projects into the Colorado Desert and is known as the Santa Rosa Mountains, the débris fans are larger and remains of gravel deposits appear high up on the sides of the mountains. The only suggestion that a fault traverses the Salton Basin in the direction of the mouth of the Colorado is the presence of mud volcanoes and several small pumiceous eruptions near the center of the basin. These are, however, so far removed from any known fractures in the crust that their evidence is of little value. Besides, it is entirely possible that the mud volcanoes may be due to chemical action in the deeply buried sediments of the Colorado delta.
It may be reasonably assumed, then, from our best knowledge, that the southern end of the great Rift is to be traced for an unknown distance along the base of the mountains bordering the Salton Basin upon the northeast, in all probability gradually dying out.
San Jacinto Fault
The San Jacinto fault (plate 30), with which there has been associated at least one severe earthquake since the region has been known, has a length of at least 75 miles. The course of the fault is northwest and southeast, and it is marked by canyons or steep mountain scarps nearly its whole length. The fault first appears upon the south in the form of a regular mountain wall inclosing the north end of Borego Valley. The latter is a western arm of the Colorado Desert lying between the Santa Rosa Mountains and the main watershed of the Peninsular Range. At the northern end of Borego Valley beds of late Tertiary age appear faulted down upon the southwest side of the mountain wall referred to. The peculiar topographic features of this fault-block ridge, and the presence of gravels along portions of its summit, make it appear of recent origin. Northwest of Borego Valley the canyons entering Coyote Creek have brought down immense quantities of rock débris, a fact which indicates recent disturbance along their headwaters. Terwilliger Valley includes a broad expanse of country of low relief upon the summit of the range between San Jacinto and Borego Valley. A portion of the valley is scarcely drained at present, having apparently undergone some subsidence next to the fault-line which forms the southern face of Mount Thomas.
In a northwesterly direction, the fault can be traced in continuous mountain scarp or canyon until within about 8 miles of the town of San Jacinto. A broad valley intervenes until we get north of the town, when a mountain wall commences again, and extends for many miles in the direction of Colton. Reports state that the San Jacinto earthquake of 1899 was most severe along the line of the fault thus traced. Great masses of rock are
The regular mountain wall facing southwest and extending northwest from San Jacinto appears to be older than that toward the southern end of the fault. Mineral springs occur near or on this line, and the marshy area at the point where the San Jacinto River ceases following this fault-scarp and turns toward the southwest suggests very strongly a subsidence.
Review of Salient Features
It will be of advantage briefly to review the salient features of the San Andreas Rift, in the light of the facts presented in the foregoing detailed description of its extent and character, and of other facts to which attention will be directed.
The San Andreas Rift has been traced with three interruptions from a point in Humboldt County, between Point Delgada and Punta Gorda, to the north end of the Colorado Desert, a distance of over 600 miles. These three interruptions are: The stretch between Shelter Cove and the mouth of Alder Creek, where for a distance of about 72 statute miles it traverses the bottom of the Pacific Ocean; the stretch from the vicinity of Fort Ross to Bodega Head, where for 13 miles it is similarly on the ocean bottom; and the stretch from Bolinas Lagoon to Mussel Rock, where it lies beneath the Gulf of the Farallones for about 19 miles. Of these interruptions only the first involves any doubt as to the continuity of the feature, and this doubt is in large measure removed by the evidence cited hereafter as to the position of the trace of the fault of April 18, 1906.
Thruout its extent the Rift presents a variable relation to the major geomorphic features of the region traversed by it. In Humboldt County it lies within the mountainous tract inland from the coast but to the seaward side of the higher land. From Shelter Cove to Alder Creek it lies to the west of a steep, terraced, coastal slope. From Alder Creek to Fort Ross, it finds its expression in a series of rectilinear, sharply incised valleys, the alinement of which converges upon the coast line to the south at a very acute angle. But near Fort Ross the Rift, without deviation of its general trend, crosses the divide to the coastal side of the ridge which separates these valleys from the ocean, and traverses the terraced coastal slope. Beyond Fort Ross it again lies to the west of a steep coastal slope. From Bodega Head to Bolinas Lagoon the Rift is a remarkably pronounced depression, lying between the main coastal slope and the rather high and precipitous easterly side of the Point Reyes Peninsula. About 0.6 of this depression is below sealevel, forming Tomales and Bodega Bays. This defile is one of the most remarkable and interesting phases of the Rift. It has been a line of repeated faulting in past geological time, and evidently separates a well-marked and probably relatively mobile crustal block from the main continental land mass.
South of Mussel Rock the Rift traverses for a few miles a rolling upland, marked by ponds and old scarps, but with no very marked contrast in relief, and then passes into the very marked and rectilinear San Andreas Valley, along the base of the northeast flank of the Santa Cruz Range. From here to the gap at Wright Station it lies along the base of the range at a distance nowhere greater than 2 miles from the crest. Passing thru the gap at Wright, it crosses from the northeast flank of the range to the southwest flank. Similarly passing thru the gap between the Santa Cruz and Gavilan Ranges at Chittenden, it is again found on the northeast flank of the latter. In effecting this last-mentioned change of position relatively to the mountain crests, a distinct deviation in the trend of the Rift is observable (see map No. 5) as if the path of the Rift accommodated itself to
The Rift as a whole, when plotted upon a general map of the state on a scale of about [Mathematical Equation], appears as a sensibly even line with marked curvature, convex toward the Pacific. This curvature is for the most part due to change in the course of the Rift between the southern end of Carissa Plain and Tejon Pass. In this segment of its course its trend changes from about S. 40° E., along the edge of Carissa Plain to S. 65° to 70° E., along the southern edge of the Mojave Desert, the change being gradual and distributed over an arc about 40 miles in length. The general curvature is also accentuated by the change in course between Point Arena and Shelter Cove, on the assumption of continuity between these points. If, however, we take the segment of the Rift between Point Arena and the south end of Carissa Plain, the curvature is very much less marked; and its path on the small scale map referred to approximates a straight line. The curvature is distinct, however, and, as in the general case, is convex to the Pacific. The chord of the arc found by stretching a line from the south end of Carissa Plain to the mouth of Alder Creek has a bearing of about N. 40° W. and a length of about 360 miles. The point on the arc most distant from this chord is on a normal to the latter thru San Jose, the distance being about 15 miles.
When the Rift is plotted on larger scale maps (see maps Nos. 2, 4, 5, 21, and 23), it becomes apparent that the course of the Rift is not a smooth uniform curve, but is characterized by several minor curvatures in opposing directions. In locating these curves, advantage is taken of the fault-trace, as far as it extends, as a sharp line within the Rift indicating its mean trend at any point on its course. These curvatures are most interesting features on a line of diastrophic movement, where that movement may be, as it was on April 18, 1906, essentially horizontal on a nearly vertical plane or zone.
The most northerly curvature susceptible of measurement is that shown by the segment of the Rift between the mouth of Alder Creek and Fort Ross. The line connecting the two ends of this segment, at the points where it intersects the shore line, is a little more than 43 miles in length, and has a bearing N. 37° W. The Rift, as located for this purpose by the fault-trace, lies wholly on the southwest side of this chord. The bearing of the fault-trace at the mouth of Alder Creek, where it converges upon the chord, is N. 30° W., and at Fort Ross its bearing is N. 40° W. The fault-trace is at its maximum distance from the chord about the middle of this segment, the distance being about 0.75 mile, and here the bearing of the Rift is sensibly the same as that of the chord, N. 37° W. Between Fort Ross and Bodega Head, where the Rift passes under the Pacific, there is probably a slight reversal of this curvature; since, if the course of the fault-trace at Fort Ross were continued, even as a straight line, it would pass to the eastward of the point where it actually intersects the neck of the headland. This slight concavity to the southwest probably extends as far as the mouth of Tomales Bay. From Bodega Head south thru Tomales Bay to Bolinas Bay, the course of the Rift as a large geomorphic feature is
Between Mussel Rock and San Andreas Dam the fault-trace has a slight concavity to the southwest. The projection of its course seaward from Mussel Rock would not meet the southward projection of the fault-trace from Bolinas. There can be little question as to the continuity of the fault-trace across the Gulf of the Farallones and its path on the bottom of the Gulf must, therefore take the form of a very flat sigmoid curve with a slight concavity to the southwest in the Bolinas moiety of the submarine segment and a corresponding convexity at the Mussel Rock end. Between San Andreas dam and Chittenden, the fault-trace indicates a pronounced curvature in the general trend of the Rift. The chord between these two points is about 55 miles in length and bears N. 44° W. The fault-trace lies wholly to the southwest of this line with convexity toward the Pacific. The point on the curve most distant from the chord is about its middle part the distance being about 2.25 miles. On this segment of the Rift there is locally a rather abrupt reversal of the curve, south of Black Mountain, which is best seen on map No. 22.
Between Chittenden and a point near Priest Valley there is another pronounced curvature in the general course of the Rift, where it passes over to the northeast flank of the Gavilan Range. Here the curvature is concave toward the southwest. The chord is 60 miles long, and bears, as before, N. 44° W. and the Rift lies wholly on the northeast side. The point on it most distant from the chord is near the middle of the segment and the distance is 2.4 miles. From Priest Valley to the south end of Carissa Plain, the Rift is nearly straight, but with minor curvatures which can not be more particularly defined owing to the absence of good maps. The general bearing for this segment is about N. 40° W.
The marked curvature between the south end of Carissa Plain and Tejon Pass has already been noted. From the latter place to the north end of the Colorado Desert beyond which the Rift has not been traced, there are numerous curvatures in the course of the Rift; but since the Rift for this segment is indicated on maps Nos. 6 to 15 on a scale of 1 or 2 miles to the inch, it will be unnecessary to do more than refer to these maps for their characterization. The general course of the Rift in this region is a flat curve concave to the south-southwest.
It thus appears that the Rift, as a whole, has a curved course convex to the Pacific and that this general curvature is characterized by a succession of slightly curved rather than straight, segments. The amount of the curvature, as it appears upon the maps is determined to some slight extent by the character of the projection. But the general conclusion above reached without quantitative expression is independent of the projection adopted for the maps.
A most interesting general feature of the Rift is in relation to the granitic rocks of the Coast Ranges. The granites of the southern Sierra Nevada pass into the Coast Ranges in the vicinity of Tejon Pass, and extend thence in a series of more or less elongated but discrete areas thru the Santa Lucia, Gavilan, and Santa Cruz Ranges, and beyond the Golden Gate to Point Reyes Peninsula and Bodega Head. From the southern end of Carissa Plain to Bodega Head, this granite lies wholly to the southwest of the Rift. At one point in the Rift, however, in Nelson Canyon, Fairbanks has found the granite exposed on the northeast side of an old fault having a downthrow on the southwest. Southward it passes into a region where granitic rocks prevail on both sides of the Rift. The Rift in the Coast Ranges thus appears to serve as a line of demarkation between two
In a discussion of the rift as a geomorphic feature, it becomes a matter of interest to determine the relative importance of diastrophism and erosion in its evolution. There can be no boubt that where the rift is coincident with pronounced longitudinal valleys, the latter, altho controlled as to orientation by the faulting along the Rift, owe their features in a large measure to erosion. This is true, for example, of the valleys of the Garcia and Gualala Rivers and the San Andreas Valley. It is not so clear, however, as regards the depression between Point Reyes Peninsula and the mainland. It has been pointed out that in past geological time there has been a recurrence of faults with large vertical displacement on this portion of the Rift, dating back to pre-Miocene time and possibly to the Cretaceous and it may be that here the depression is essentially diastrophic in origin and only modified to a minor degree by erosion; similarly with some of the valleys along the Rift, and extending from it in the Southern Coast Ranges. The Cholame Valley and the valley of Carissa Plain may be essentially diastrophic in origin, modified by erosional degradation on their sides and by aggradation of their bottoms. The depressions which constitute the Rift along the southern margin of Mojave Desert appear to be practically wholly diastrophic, altho somewhat aggraded. Where the Rift hugs the $steep northwest flank of the Santa Cruz Range as far as Wright Station, and the similarly steep southwest flank of the same range from Wright to Chittenden, it is difficult to avoid the conclusion that these steep mountain flanks are in reality degraded fault-scarps, and are,therefore the walls proper of a great asymmetric Rift valley. The same conclusion applies to the steep north flank of the San Rafael and San Gabriel Ranges, on the south side of Mojave Desert. The complete discrimination of effects of diastrophism and erosion in the larger features of the relief associated with the Rift will require many years of patient field work.
With regard to the minor feature which characterize the Rift in detail, thruout its extent the discrimination is less difficult chiefly because the diastrophic effects are of comparatively recent date and their distinctive features are not yet obliterated by the ravages of erosion. These consist chiefly of scarps, low ridges, and sinks or ponds. In many cases it is apparent that both erosion and aggradation are controlled by these minor features, and that the latter tend to become obliterated. These minor scarps, ridges, and sinks are not referable to any single earth-movement, but are referable to a recurrence of movement on the same general line. In the southern Coast Ranges the observations of Fairbanks show that one of these movements was of exceptional importance. By it displacements and disturbances of the surface were effected which dwarf all similar events in historic times. For miles at a stretch the earth, upon one side or the other of the fault line, sank, giving rise to basins and to cliffs 300 to 400 feet high. These features, in the opinion of Fairbanks, who personally examined them, were the result of one movement. This displacement probably occurred not less than 1,000 years ago. It is certainly older than the great trees growing upon the ridges and hollows formed by it. Since then it is probable that numerous displacements of less extent have taken place, but the geomorphic effects of the smaller movements have, in some considerable measure, been effaced. Since the settlement of the state, the strain in the earth's
It remains to call attention, in a word, to the alinement of the Rift with certain of the larger continental features. The Rift is known from Humboldt County to the north end of the Colorado Desert. As a line of small displacements it has not been traced farther; and in the usage of the term it has been understood as terminating at the point where it eluded field observation. But it is by no means certain that, as a larger feature, it does not extend far to the south. The Colorado Desert and its continuation in the Gulf of California are certainly diastrophic depressions, and may with much plausibility be regarded as a great Rift valley of even greater magnitude than the now famous African prototype first recognized by Suess. This great depression lies between the Peninsula of Lower California and the Mexican Plateau. All three of these features find their counterpart in southern Mexico. The Sierra Madre del Sur is the analogue of the peninsular ridge; it lies on the line of its prolongation, and is similarly constituted geologically. Inside of this range, and between it and the edge of the Mexican Plateau, is a pronounced valley system which is the analogue of the Gulf of California.
On this valley-line lies the deprest region about Salina Cruz, well known to be subject to repeated seismic disturbances. On the same general line lies Chilpancingo, the seat of the recent disastrous Mexican earthquake. Following these great structural lines southward, they take on a more and more latitudinal trend; and beyond Salina Cruz the geological structure indicates that this seismic belt crosses the state of Chiapas and Guatemala, to the Atlantic side of Central America with an east and west trend, and falls into alinement with Jamaica. It thus seems not improbable that the three great earthquakes of California, Chilpancingo, and Jamaica may be on the same seismic line which is known in California as the San Andreas Rift.
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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