BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA V o l . 39 . pp. 1087-1102 December 30.'1928

SIGNIFICANCE OF THE MATILIJA OVERTURN1

BY PAUL P. KERR AND HUBERT G. SCHENCK

(Read before the Society December SO, 1927) CONTENTS Page Introduction...... 1087 Strata at Matilija ...... 1089 Order and thickness ...... 1089 Chico form ation...... 1089 Tejon form ation...... 1089 Sespe formation ...... 1091 The Matilija overturn ...... 1091 Significance of the overturn ...... 1096 Conclusions ...... 1101

I ntroduction 2

The Matilija overturn is on the southern margin of the Santa Ynez Mountains, 30 miles east of Santa Barbara and 80 miles northwest of Los Angeles, in Yentura County, California. The name is taken from a station on the Ventura-Ojai Branch of the Southern Pacific Railroad and from Matilija Springs, on Matilija Creek, 3 miles north of the station. The area in which the principles illustrated by the overturn may be considered significant embraces the coastal district inland from the Santa Barbara Channel. It is a somewhat irregular area beginning 20 miles or more northwest of Los Angeles and extending northwestward to Point Conception (figure 1). The conclusions given in this paper are based on detailed maps prepared during parts of the years 1926, 1927, and 1928 and on several published reports on parts of the area. The structural characteristics of the region near Matilija may be

1 Manuscript received by the Secretary of the Society April 2, 1928. 2 The authors wish to express their appreciation of discussion and constructive criti­ cism offered in the preparation of this paper by Profs. C. F. Tolman, Jr., and Eliot Blackwelder, of Stanford University; Prof. Douglas W. Johnson, of Columbia University; Dr. W. S. W. Kew, Los Angeles, California, and Dr. Thomas L. Bailey, Ventura, Cali­ fornia. (1087)

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SYMBOLS Structurai axis with uncertain Location Normal anticline Normal syncline 1088 Overturned anticline

Overturned oyncline /formai dip Overturned dip Shear zone

F ouit N R U T R E V O ATILIJA M ------THE R C N HE SC AND KERR Thrust fault

Pt. Conception

CmmeL

F ig u r e 1.—Bend in the San Andreas Fault and the General Trend of parallel Structures between it and the Coast The Matilija overturn is shown in the central part of the map. INTRODUCTION 1081)

summarized as follows: first, the structural features have been formed by the movement of overthrust blocks from the inland toward the coast; second, the usual features produced by this movement are thrust-faults or overturned folds or combinations of the two; third, the direction of release of pressure is commonly the same, whether the resultant structure is a fold or a fault, the local conditions or the nature of the strata being the deciding elements. The structural feature to which particular attention is here directed extends axially from east to west and is a broad anticlinal fold of massive beds of sandstone and shale, which are overturned toward the south. The fold offers little novelty, considered as a structural type, for an asymmetrical arch of this kind is to be expected in massive and mod­ erately competent strata that have been compressed under a com­ paratively light load. The feature is worthy of attention, however, in view of the significant relation it bears to the general deformation in the southern Coast Ranges of California.

S t r a t a a t M a t i l i j a

ORDER AND THICKNESS Three sedimentary formations are exposed at Matilija. The oldest is the Chico (Cretaceous), the youngest is the Sespe (Oligocene), and be­ tween the two is the Tejon (Eocene). The total thickness of these formations is about 16,500 feet, divided about as follows: Feet Sespe (Oligocene (?)) ...... 4,000 Tejon (Eocene) ...... 7,500 Chico (Cretaceous) ...... 5.000 * CHICO FORMATION

Gray to black indurated shale, with intercalated beds of arkose sand­ stone, predominates throughout the Chico formation. Ripple-marks, mud-cracks, cross-beddijig, ellipsoidal nodules, concretions, fracture cleavage, and slickensides are common, but fossils are scarce. Those that have been collected include poorly preserved ammonites, Inoceramus, and a few species of gastropods and pelecypods, as well as numerous remains of plants and some arenaceous Foraminifera. The base of the formation is not exposed in the immediate vicinity of the Matilija over­ turn. TEJON FORMATION The rocks of Eocene age are divided into three lithologic units, each of which carries marine fossils that are regarded as characteristic of the

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Tejon formation of California. The necessity for giving these units specific names has become apparent, and two new names are therefore suggested—Matilija sandstone and Cozy Dell shale. The youngest unit of the Tejon formation is the Coldwater sandstone, already named and defined by Watts.3 The Matilija sandstone rests without apparent angular discordance on the Chico shale. The massive arkose at the base of the Matilija is more resistant than the underlying shale and forms a convenient guide for differentiating the formations. This unit, comprising beds about 2,500 feet thick, in which sandstone predominates over shale, is well exposed in the canyon at Matilija Springs,4 selected as the type locality, as well as on the top of Topatopa Bluff, on the south side of Santa Paula Ridge, and on San Cayetano Mountain. Near the springs is a bed of lignitic facies, distinguished by abundant specimens of Metacerithium, Ostrea, and other mollusks embedded in a green and purplish sandy shale, and the same bed is found farther east, at the same position in the geologic column. Other fossils collected from scattered localities include Nekewis io (Gabb), Tturritella wvasana Conrad, Meretrix hornii (Gabb), Pitanria uvasana (Conrad), Glycymeris sagittata (Gabb), Psammobia hornii (Gabb), and Spatangus tapinus Schenck. The Cozy Dell shale lies disconformably above the Matilija sandstone and below the Coldwater sandstone. The name Cozy Dell is proposed for the rhythmically bedded green micaceous shale and sandstone that is typically exposed in Cozy Dell Canyon, on the east side of Ventura River. This unit has an aggregate thickness of about 2,500 feet and carries such molluscan fossils as Amaurellina moragai Stewart, Ectino- "Mlus (Cowlitzia) canalifer (Gabb), and Ficopsis hornii Gabb. The third stratigraphic unit of the Tejon is the Coldwater sandstone, .vhich in the area mapped is characterized by white, friable arkose sand­ stone, interbedded reddish sandy shale, and massive hard ledges com­ posed of numerous shells of Ostrea idricemis Gabb. The striking red and green beds of shale and the gritty white sandstone form the most distinctive features of the member. These colored beds show remark­ able continuity, having been traced more than 40 miles along the Santa Ynez Range westward from the type locality in Coldwater Canyon, where, according to Kew,5 they have the same lithologic features. The

3 W. L. W atts: Oil and gas yielding formations of California. Bull. 11, 1896; Bull. 19, 1900, Calif. State Mining Bureau. 4 The localities mentioned in this paper will be found on the Yentura, Santa Paula, Piru, and Mount Pinos sheets of the topographic maps of the United States published by the U. S. Geological Survey. 6 W. S. W. Kew: Geology and oil resources of a part of Los Angeles and Ventura counties, California. U. S. Geol. Survey Bull. 753, 1924, p. 28.

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Coldwater sandstone is about 2,500 feet thick near Matilija, and at several places it carries such Tejon fossils as Turritella uvasana Conrad, Venericardin hornii Gabb, and Peden calkinsi Arnold. , « SESPE FORMATION The continental Sespe formation, originally defined by Watts6 under the descriptive term “Sespe brownstone,” is about 4,000 feet thick in this region. It consists of interbedded massive, unfossiliferous sand­ stone, shale, and conglomerate. Most of its constituent minerals are characteristic of granitic and Franciscan rocks.7 An interval of erosion probably occurred between the deposition of the Eocene and the Sespe formation, for the Sespe contains boulders of characteristic Eocene Lithothamnion limestone and fossiliferous Tejon Eocene sandstone. No marked angular unconformity, however, was observed between the Cold- water sandstone and the Sespe or between the Sespe and the over­ lying marine Vaqueros (lower Miocene) fossiliferous sandstone. The stratigraphic position of the Sespe formation indicates that it is of Oligocene age. T h e M a t i l i j a O v e r t u r n

The Matilija overturn, which extends in general from east to west, embraces an area of approximately 40 miles. It terminates in a normal anticline on the west and is cut by a thrust-fault on the east. Its location in relation to the adjoining structural features is shown in figure 1, a sketch map compiled from field observations made by the authors and from the reports listed in the footnote.8 Broadly considered, the folded beds consist of shale and sandstone. Where they are sufficiently thick, the beds of shale are more deformed than those of sandstone, many of which here include beds of shale. The lower bed of shale (Chico), the thickest of the shale beds, is in

8 Op. cit. 7 Vincent P. Gianella : Minerals of the Sespe formation, California, and their bearing on its origin. Bull. Am. Assoc. Pet. Geol., vol. 12, 1928, pp. 747-752. 8 Ralph Arnold : Geology and oil resources of the Summerland district. U. S. Geol. Survey Bull. 321, 1907, pi. L. Ralph Arnold and Robert Anderson : Geology and oil resources of the Santa Maria oil district. U. S. Geol. Survey Bull. 322, 1906, pi. 1. William S. W. Kew: Geology of a part of the Santa Ynez River district, Santa Barbara County, California. Univ. Calif. Publ. Bull., Dept. Geol., vol. 12, 1919, pi. 1. Geology and oil resources of a part of Los Ángeles and Ventura counties, California. U. S. Geol. Survey. Bull. 753, 1924, pi. 1. Geologic sketch of Santa Rosa Island, Santa Barbara County, Calif. Bull Geol. Soc. Am., 1927, vol. 38, fig. 1. Richard N. Nelson : Geology of the hydrographic basin of the Upper Santa Ynez River, California. Univ. Calif. Pub. Bull., Dept. Geol. Sci., 1926, vol. 15. Bailey Willis and H. O. Wood: Fault map of the State of California, issued by the Seismological Society of America, 1922.

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F ig u r e 2 .—The Matilija Sandstone Member of the Tejon (Eocene) Formation The exposure is on the canyon walls of North Fork near the intersection with M atilija Creek. Vertical bedding, ex­ treme jointing, and irregular fractures typical of a hard formation greatly deformed, characterize such exposures. The dimensions of the beds may be judged by comparison with a 10-foot stadia rod held by the geologist in the lower right- hand corner of the photograph. STRATA AT M ATILIJ A 1093 e h t f o t s e r c e h t g n o l a a t a r t s e h t f o n o i t a u n i t n o c t c e r i d a e r a , d n u o r g e r o f e . h d t a o r n i e h , t e n o e t v s o d b n a a s t e e a f j i l 0 i t 5 a ,2 2 M is f o h c s i p h o w r c t , u e o g id r l a c i t r e v dge, ddle member (Cozy Del shale) lower down. e h T . n w o d r e w o l ) e l a h s ll e D y z o C ( r e b m e m e l d id m e h t n o s t s e r , e g id r e h t f o t r a p r e p p u e h t n o , ) e n o t s d n a s a j i l i t a M ( uence of North Fork and Matilija Creeks. n o i t a m r o f n o j e T e h t f o r e b m e m r e w o l e h T . s k e e r C a j i l i t a M d n a k r o F h t r o N f o e c n e lu f n o c e h t t a s i y t i l a c o l s i h T F gure r u ig 3. vrund taa xoe aoe h Ra t Mtlj Springs. Matilija to Road the above exposed Strata Overturned —

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EB N SCII ------TE A 1,. A liN U T K K V O 11,1.1 A AT M ------THE K C N IE I C S AND KERB

Fkjttre 4.— The Matilija Overturn A. Part of the Santa Ynez Range, which extends from east to west across the field of view. Several of the peaks stand nearly 5,000 feet above sealevel. The Eocene strata of the range are overturned on the right-hand (east) side of the diagram. The illustration shows the evidence on which the interpretation of this part of the Santa Ynez Range as an overturn has been based. The cross-section on the west or left-hand side of the diagram shows the formations in their normal order, the Sespe formation resting on the Tejon and the Tejon in turn resting on the Chico. The eastern, or right-hand, cross-section, however, shows the strata in inverse order, the Cretaceous beds lying on those of Sespe age. Between the two sections is a rugged region in which almost continuous outcrops are so well exposed that the conclu­ sion reached in regard to the structure may oe considered observation rather than interpretation. B. Diagram illustrating the areal extent of the form ations and the structural axes of the region included in the block d i a g r a m . STRATA AT MATILIJA 1095

places folded, the axes of the folds lying parallel to the axis of the major folds, as they usually do in drag folds. The axial planes dip in the same direction as the plane of the major fold, the symmetry of which is con­ trolled by the beds of sandstone. These beds, although more competent than those of shale, retain flexibility to a degree that is unusual in ma­ terial so massive. Their flexibility in folding is apparently due to an inherent tendency of the beds to slip along the bedding planes and to produce extensive systems of joints. Much of the pressure in the beds of sandstone has been released by movement along bedding planes and joints. This fact is disclosed by an examination of exposures such as are shown in figure 2, which pictures an outcrop of the lowest sandstone of the Tejon formation (Matilija). The folding has left the beds in a nearly vertical position, as is clearly shown by the stratification. Each bed is in itself greatly fractured, and in addition there are many slicken- sides, both on bedding planes and along joints. The movement within and along the strata has been sufficiently extensive to produce extreme contortion of the formation as a whole, but has not produced extensive faults. The axis of the overturn is curved where the Yentura Eiver cuts through the Santa Ynez Range east of Matilija Hot Springs, at the locality shown in figure 3, which is reproduced from a photograph of a part of the south limb of the overturn taken within the area of bending. All the beds in view, including the Cozy Dell and Matilija members of the Tejon formation, are of Eocene age. In the main ridge, in the back­ ground, the contact between the shale and the sandstone is about half­ way up the slope, at the base of loosened blocks of light-gray sandstone. The relatively smooth slopes below, which show a few thin protruding beds of sandstone, are composed of eroded Cozy Dell shale. The Matilija sandstone, seen in the foreground of figure 3, strikes northeast and dips northwest at an angle of about 80 degrees. On the ridge in the back­ ground the same beds strike about west and dip north at an angle of about 50 degrees. The abrupt change in strike is parallel to the curve in the axis of the overturn. The dimensions in the figure may be judged by the fact that the road in the foreground is about 30 feet wide. Figure 4 illustrates the normal easterly trend of the axis of the fold. The twist transverse to the general trend is due to something more than the release of the normal north-south pressure that caused the over­ turning, probably in part to the release of horizontal east-west pressure. Beyond this, the significance of the bend is not clear. Corresponding curves, however, appear in several adjacent structures *>n the south, and LXXI— B dll. Geol. Soc. Am., Vol. 39, 1927

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there is a possibly similar though much broader curve in the San Andreas fault, 30 miles to the north.

S ignificance o f t h e O v e r t u r n

The general features of parts of the Santa Ynez Range have been recognized for several years, but a survey of the literature shows that the nature of the overturn has not been appreciated. It is bent along the axis in a significant manner. Corresponding curves also appear in several adjacent structures on the south, and there is a possibly similar, though much broader, curve in the San Andreas fault, 30 miles to the north. Antisell9 believed that the elevation of the Santa Ynez Range was produced by forces acting in a northwest and southwest direction, and Whitney10 recognized that the range has “the form of a great arch.” Fairbanks11 similarly noted that it was anticlinal, but Arnold appears to have been the first to recognize overturning. He writes :12 “The great anticline of the Santa Ynez Range is believed to be the west­ ward continuation of the overturned anticline which affects the rocks of the Topatopa Range north of the Ojai Valley, 15 miles east of the Summerland district.”

The relation of the overturn to other major structural features of the district becomes apparent on a map such as figure 1. An examination of this map and of the results of field studies leads to several generaliza­ tions that are applicable throughout the area covered by the Matilija overturn and more or less adjacent Structures. These generalizations may be summarized as follows: 1. The individual structural features of the district are not continu­ ous for great distances. Sixty major structural features south of the San Andreas fault are plotted on figure 1. None of these can be traced with certainty for more than 30 miles, the average length of the structural axes being about 7 miles. It is therefore believed that all the structural features of the district are of slight extent, no greater than the Matilija over-

9 Thomas Antisell: Geological Report, Pacific Railroad Reports, t o I. 7, part 2, chapter X, Santa Barbara Mountains, 1857, pp. 65-74. 10 J. D. Whitney: Geology of the Coast Banges. Chapter V, The Coast Eanges south of the Bay of Monterey. Geol. Surrey of California, Geology, vol. 1, 1865, pp. 108-166. 11H. W. Fairbanks: Geology of northern Ventura, Santa Barbara, San Benito, San Luis Obispo, and Monterey counties. Twelfth Annual Eeport, California State Mining Bureau, 1894, p. 11. 12 Ralph Arnold: ^Geology and oil resources of the Summerland district, Santa Barbara County, California. U. S. Geol. Survey Bull. 321, 1907.

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turn, for instance. Although the Santa Ynez Range is almost 100 miles long and is underlain for the greater part of that distance by beds like those in the range itself, the Matilija overturn is only one of several discontinuous anticlinal features, of varying form, in this range. 2. En echelon parallelisms of structural axes is common throughout this area. A study of the map shows that not all the structural features are straight or parallel to the usual trend. In general, however, a struc­ tural alignment is evident. The structural lines may be compared to the flow banding in a gneissoid rock, in which the general direction of the banding is indicated not by individual streaks, but by a summation of all of them. 3. The axial planes of a number of overturned folds dip north or northeast. It is neither necessary nor profitable to consider here all the over­ turned folds in the region, but several of the principal folds are suffi­ ciently illustrative. The Matilija overturn is the most prominent of these folds. North of Carpinteria, near the crest of the Santa Ynez Range, there is another overturned fold that is similar in form and stratigraphy to that at Matilija. Sulphur Mountain, near Santa Paula, and Red Mountain, north of Ventura, also exhibit overturning. There is an overturned fold in the Sespe formation on the eastern border of the Santa Barbara quadrangle. The structural features indicated are of major size in their respective districts. All have axial planes that dip to the north, and their chief features have been fully shown by detailed mapping. 4. Many of the principal fault-planes are not vertical, but dip north or northeast. Those that have this dip are shown on the map by the proper symbol. Several of the fault-planes observed dip at angles of 45 to 50 degrees' to the north. Most of the planes are warped and all show great dif­ ferences in dip, even over short distances. In the work of mapping the Ventura and Santa Paula quadrangles, several almost vertical planes were noted, but no major plane was found that dips to the south, though many dip to the north. The dip of the fault-planes generally agrees in direction with the inclination of the axial planes of the over­ turned folds, the two grading into each other at times. In fact, the Matilija overturn terminates at its east end in a thrust-fault having a northern dip. 5. The main trend of the structural axes is parallel to the trace of the San Andreas fault, a curve in the line of that fault being accom-

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panied by a corresponding curve in the strike of adjacent structural features. The San Andreas fault runs nearly southeastward through California, except near the south end of the Great Valley, where it swings almost eastward. Twenty-five miles south of this part of the fault the Coast Eanges similarly trend east and west, causing the Santa Ynez Range to meet the Pacific Ocean at an acute angle at Point Conception. The map (figure 1) shows that in the area north of Point Concep­ tion the structural lines trend northwest-southeast, the usual direction in the Coast Eanges, and that 100 miles to the south, in the vicinity of Los Angeles, they have a similar trend. Between the two extremes, however, in the central portion of the figures, the trend is east-west. 6. The character of the deformation is greatly influenced by differ­ ences in the type of material. Pew regions contain as many different types of sedimentary rock as are found in this part of the Coast Eanges of California. These types are deformed in distinctive ways. Three examples are selected for illustration: (a) Geologists in California are familiar with the closely folded, faulted, and contorted thinly laminated siliceous shales, variously re­ ferred to as Monterey, Modelo, Maricopa, Puente, and Salinas shales. The structure of such rocks corresponds to the drag type of fold in in­ competent beds. They are of minor magnitude and generally have steep dips. (b) Folds of this nature offer a marked contrast to those in massive Eocene sandstones. The Eocene strata are competent to resist deform­ ing pressure, which produces in them structures of large amplitude. As the two divergent types were produced during the same period, the conclusion is natural that the nature of the rock is an important factor in determining the character of the structure. (c) A third example is the soft, easily eroded sandstone and shale of the Pliocene formations, which generally suffer less change of atti­ tude in folding. No high mountains occur in areas underlain by Pliocene rocks, and the folds produced in them are broad and com­ paratively simple. The structure produced by the deformation of these three types of rock may thus be characterized as closely folded and local for the siliceous Miocene shales, open and extensive for the Eocene, and mod­ erate for the Pliocene. All three may be seen within a distance of five or six miles of one another, all deformed by the same general post- Pliocene diastrophism and under an equally moderate load.

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7. The direction of the release of pressure of the overthrust blocks is in general from the inland toward the coast. It has been suggested 13 that submarine forces, acting along the Cali­ fornia coast at this point, produce a series of thrust-faults that gradually curve outward with increase of their depth beneath the surface of the ocean. Detailed mapping and study of adjoining areas already mapped do not appear to support this suggestion. On the contrary, the pre­ ponderance of evidence points to a release of pressure in the opposite direction—from inland toward the coast—and that the principal fault- planes dip toward the north. Such major structures as the San Cayetano fault, mapped by Kew,14 and the Sulphur Mountain and Sisar faults, in the Ventura and Santa Paula quadrangles, furnish excellent examples of thrust-planes having a definite northward dip. Each overthrust block has moved from in­ land toward the coast, and the fault-planes lie at an angle directly opposite to one which would curve outward beneath the ocean. Over­ turned structures such as are illustrated by the Matilija overturn appear lo have been overturned by movement from inland toward the coast. Deformation of this type may be the result either of overthrust from inland or of underthrust in the opposite direction. In the vicinity of Matilija, however, forces acting in the opposite direction, from the coastal side, would have to act through about 18,000 feet of soft Pliocene and Pleistocene strata in the Ventura basin. The pressure of soft strata against a more competent and much harder mass in the .Santa Ynez Range should produce, it would seem, much greater folding and faulting in the younger rocks than that actually observed. On the other hand, there is no reason to disbelieve that deep-seated subcoastal forces have acted through older and more competent underlying rocks. Although the study of this subject more properly belongs in the field of tectonics, it seems apparent that overthrusting from inland near the surface is consistent with deep-seated movement occurring in the oppo­ site direction. Those who have proposed the hypothesis of subcoastal pressure have supposed that the movement from the coast extended to the surface, and have paid considerable attention to tentative conclusions published

13 Bailey Willis : A study of tlie Santa Barbara earthquake of June 29, 1925. Bull. Seis. Soc. Am., vol. 15, 1925, pp. 255-278 ; La Force Sismique en Californie : Livre Jubilaire Publié a l’occasion du Cinquantenaire de la fondation de la Société Géologique de Belgique, 1927. 14W. S. W. Kew: Op. cit., U. S. Geol. Surv. Bull. 753. 15 William Bowie : Barth movements in California. U. S. Coast and Geodetic Survey Special Publication No. 106, 22 pp., 1924 ; Bull. Geol. Soc. Am., vol. 35, 1924, pp. 60-62.

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in papers of the Coast and Geodetic Survey 15 in regard to earth move­ ments. Some have assumed that the published data show actual inland movement of the earth along the coast of California, on account of dis­ placements in triangulation stations, that are in possible accord with this hypothesis. The proof of earth movement offered by the retriangulation of the California coast in 1922-23 was tentative. Later work.of-the Survey, in 1923-24, confirmed, in large measure, the conclusions regarding earth movements reached in the earlier retriangulation in the coastal parts of central California between Monterey and San Francisco, but a recent resurvey of the triangulation system in the southern part of the State, where the earlier, work was incomplete, shows no displacement other than that which may be accounted for by probable instrumental error. A chart comparing the geographic positions of triangulation stations in California prior to 1900 with the positions of the same points as determined during 1923-24 has recently been prepared by the Survey and was kindly furnished the writers for inspection. The chart was computed on Santa Barbara as a base and includes triangulation stations along the coast throughout the area here considered. As the conclusion established by the chart in regard to earth movements is entirely negative, it is not here reproduced. The conclusion reached concerning southern California by comparing the survey of 1900 with that of 1923-24 is contained in a statement issued from the office of the Director, to the effect .that “all of the differences in geographic positions shown on the sketch . . . result from unavoidable accidental errors of triangula­ tion rather than from earth movements; or, to put it another way, the sketch does not prove that any earth movements have occurred at the stations shown on the sketch.” 16 As the stations shown on the sketch referred to lie between Santa Maria and Los Angeles, the triangulation covers the coastal portion of the area here considered. The evidence seems to indicate that no general movement has recently taken place in the region in southern California covered by the triangulation net­ work. In view of this fact, even greater weight must be placed on the geologic evidence, which indicates that the overthrusting has been

18 We desire to acknowledge the kindness of Dr. William Bowie and the members of the Division of Geodesy of the Coast and Geodetic Survey in furnishing informa­ tion regarding movements of triangulation stations and in carefully explaining the significance to be attached to the figures given.

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produced by a release of pressure on the upper blocks from inland toward the coast. C o n c l u s io n s

The explanation of the Matilija overturn furnishes a key to the gen­ eral principles underlying the deformation in the surrounding region. The following general conditions have been observed: 1. The variation in the character of the deformation is greatly in­ fluenced by differences in the type of the material deformed. 2. The main trend of the structural axes is parallel to the trace of the San Andreas fault, a curve in the direction of the fault being accom­ panied by a corresponding curve in strike lines of adjacent structural features. 3. The individual structural features of the district are not con­ tinuous for great distances. 4. In general, the structural axes show en echelon parallelism. 5. The axial planes of most of the overturned folds and of many of the principal faults are not vertical, but dip to the north or northeast. 6. The major release of pressure on overthrust blocks is probably from inland toward the coast.

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