Bull. Qeoc. Soc Am. Vol. 11, 1899, Pl. 25 Map of Santa Lucia

BULL. QEOC. SOC AM. VOL. 11, 1899, PL. 25

MAP OF SANTA LUCIA RANGE, CALIFORNIA, AND ADJACENT FEATURES

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SOME COAST MIGRATIONS, SANTA LUCIA RANGE, CALIFORNIA

BY BAILEY WILLIS

( Read, before the Society December 27, and before the Cordilleran Section December 29, 1899)

CONTENTS Page Introductory note...... 417 The present scene...... 418 Pre-Franciscan history ...... 419 Coast complex...... 419 Pre-Franciscan erosion ...... 419 Franciscan history .. ■...... 420 Franciscan stratigraphy...... 420 Franciscan geographic conditions...... 420 Nature of deformation...... 420 Post-Franciscan history...... 421 Cretaceous land...... 421 Miocene (?) stratigraphy...... 422 Geographic conditions of the Miocene (?)...... 422 Relation of the Miocene (?) and Miocene strata...... 423 Pine Mountain section; Franciscan to Pliocene...... 424 Pliocene to present...... 425 Physiography of the coastal slope...... 425 Three types of coastal slopes...... - ...... 427 Incidents in the scarp-growth...... 429 Summary...... 430 Explanation of figure 2 ...... 431

I ntroductory N o t e

The following article presents observations made during a trip from Monterey to San Luis Obispo along the intervening Coast ranges. I was accompanied by Doctor H. W. Fairbanks, in whose articles many of the facts herein presented have already been published,* and I am indebted to him not only for pointing out the most interesting and significant

»Stratigraphy of Slate Springs. Amer. Geologist, vol. xviii, 1896.

LX—Boil. Geol. Soc. Am., Vol. 11, 1899 (417)

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phenomena of the region, but also for helpful discussion of their inter pretation. Professor A. C. Lawson * has described the geology of Cartnelo bay just south of Monterey, a district which adjoins on the north that herein referred to and which includes the Santa Lucia granite, the youngest member of the Coast complex hereafter defined. My observations are in general in accord with those made by Lawson at Carmelo bay and by Fairbanks farther south.

T h e p r e s e n t S cene

Point Sur, a prominence of the California coast, lies 24 miles south of Monterey. From it the coast stretches 57 miles southeast to Piedras Blancas. Along this section extends the westernmost of the Coast ranges, locally known as the Santa Lucia range, which attains elevations of 4,000 to 5,000 feet. The mountains present a bold front to the ocean, precipitous, it is true, only near sealevel at isolated promontories, but generally steep. Ravines gash this front deeply. In general, the aspect of the mountain slope is that of mature topography. In detail, there are variations of form significant of successive episodes of uplift. Near Slate Springs, 35 miles southeast of point Sur, there are two basal conglomerates—the one, that which is being formed by the waves which now dash against the rocky shore; the other, that which was similarly formed when they broke on a shore of Mesozoic date. The present conglomerate is the beach; the older conglomerate is the base of the Franciscan or Golden Gate series, and rises in cliffs several hun dred feet above the beach, the strata dipping 70 to 80 degrees to the southwest. The present conglomerate and the older one both consist largely of pebbles and boulders of a still older complex, which will be called the Coast complex. On the summit and eastern flank of the Santa Lucia range occur strata which are much younger than the Fran ciscan formation. They constitute a group whose members bring the record down to late Neocene (Pliocene) time. The sequence thus briefly indicated includes (1) the development of the Coast complex; (2) profound erosion of the Coast complex; (3) deposition of the Franciscan conglomerate, sandstone, and shale; (4) orogenic movements, which resulted in deformation of the Franciscan forma tion ; (5) erosion of the Coast complex and Franciscan rocks, which is partially represented in later sediments; (6) evolution of the present mountain system and coastal front.

♦ The Geology of Carmelo bay. University of California, Bull, of the Dept, of Geology, vol. i, p p . 1-50.

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The history is a long one, extending from possibly the Paleozoic period to the present, and a very complex one, comprising the phenomena of sedimentation, erosion,metamorphism, orogeny,and perhaps epeirogeny.

P r e -F ranciscan H istory

COAST COMPLEX The Coast complex consists of crystalline rocks, chiefly metamorphic, including marble, quartzitic schists, mica-schists, gneisses, and granite. The granite is intruded in the others and did not share their experience of profound metamorphism. The marble and schists constitute a sedi mentary series. The gneisses may in part or whole have had an igneous origin. The whole has suffered deformation in a deep zone of flow, re sulting in structural and mineralogical changes which to a great extent obscure the original characters of the rocks. Fairbanks * has described the series somewhat in detail under the name “ basement complex.” This term is aptly applied, as the Coast complex is the intricate mosaic at the base of the geologic column in the Coast ranges. The age of the Coast complex can only be vaguely guessed. It is long pre-Cretaceous; it may probably be Paleozoic. The former statement follows from the age of the succeeding Franciscan rocks, which are either Eo-Cretaceous or Jurassic. The Paleozoic date is an inference on the uncertain grounds of lithologic and structural similarity to the meta morphic complex of the Sierra Nevada. The oldest rocks of the Coast complex were sediments, either calca reous muds or quartzose sands. The region of their accumulation was therefore marine, but not so remote from land as to escape terrigenous deposits. Land was probably near, since the sediments were buried to that depth which conditions deformation-by-flow and recrystallization, and zones of such great accumulation of sediment are usually shore zones.

PRE-FRANCISCAN EROSION

Pebbles of gneiss and granite from the Coast complex chiefly compose the basal Franciscan conglomerate. These rocks had become surface rocks at the inception of the Franciscan epoch. The elevation of the folded and schistose gneisses from the zone of deformation-by-flow in volved orogenic or epeirogenic movements vertically of great amount, and corresponding erosion of the mass. Thus in some pre-Cretaceous epoch a land area succeeded to the sea in this region, and in Franciscan time its shore closely approximated the position of the present shore.

♦ Review of our knowledge of the geology of the California Coast ranges. Bull. Geol. Soc. Am., vol. 6, pp. 7&-82.

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The sediments resulting from the erosion of that land have not been discovered; only the fact of a great hiatus in the record is obvious at the unconformity.

F ra nciscan H istory

FRANCISCAN STRATIGRAPHY

The basal conglomerate of the Franciscan series is a very coarse shore deposit composed of pebbles and boulders up to two or more feet in diameter in a sandy matrix. The materials were derived from the im mediately adjacent Coast complex upon which the conglomerate was deposited. The strata exposed in the mountain slope are strikingly like the deposits which constitute the beach at the base of the cliffs, and to realize the aspects of the Franciscan coast the observer needs only to consider those of the present coast. The Franciscan was a bold and rocky shore, against which broke powerful waves (see plates 26 and 27). The stratigraphy of the Franciscan formation has not been ascertained in any of its outcrops. It comprises several lithologic varieties with a great range of character, from the coarse basal conglomerates to fine de posits, partly organic. The mass of the formation is sandstone and shale. A minor portion consists of radiolarian chert and limestone. In the section at Slate Springs the thickness exposed is, according to Fairbanks, about 1,500 feet, and grades from the basal conglomerate westward and upward into arenaceous black shales, with interbedded thin layers of sandstone. FRANCISCAN GEOGRAPHIC CONDITIONS

In the presence of the Pacific, one does not question that it was the ocean whose waves wrought the Franciscan shore. The dip of the strata, though nearly vertical, is toward the Pacific, and bears out the immediate inference that it was the adjacent ocean in Franciscan time. Such was probably the fact; but as in later epochs a water body existed east of this position, apparently with a land area west of it, it should not be too confidently assumed that this Franciscan shore was certainly the then Pacific coast of the continent. It may have been the eastern coast of a western land. NATURE OF DEFORMATION

The structure of the Franciscan series is complex and among the associated formations characteristic. Deformation has been accom plished chiefly by fracture, with minor folding. The shales are generally plicated and broken at acute bends. The sandstones are broken and faulted. The cherts are crushed. In many instances the sandstones

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BULL. GEOL. SCC AM. VOL. 1 1, 1899 PL. 26

COAST NEAR SLATE SPRINGS, CALIFORNIA In the foreground are masses of basal conglomerate of Franciscan age. The sea-cliffs are cut in fossiliferous black shale (Franciscan) and extend to the Pleistocene terrace. The terraced profile of the distant tree-topped hill extends to about 1,200 feet above sea on 24 September 2021 by guest Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/11/1/417/3383257/BUL11-0417.pdf

BULL. GEOL. SOC. AM VOL. 11, 1899, PL. 27

COAST AT SLATE SPRINGS, CALIFORNIA Showing structure of the Franciscan strata and sea-cliif cut in Pleistocene terrace FRANCISCAN AND POST-FRANCISCAN HISTORY 4 2 1

have been crushed into small fragments and recemented by silica, forming a lithologic variety which is in some degree characteristic of the forma tion. At Slate Springs the formation has been tilted into a nearly vertical attitude by northeast-southwest stress, and a distinct series of fractures and faults has been produced by a stress making a large angle with that which produced the general dip. The complexity of the cross folding and faulting in the Franciscan series has prevented geologists from unraveling the detailed structure. The problem is apparently not more difficult than that presented in the Marquette iron region of lake Superior, and will yield to patient’investi- gation. We shall then know whether there is one or several strata of radiolarian chert and limestone, and be able more adequately to under stand the oscillations of sealevel which resulted in the variations of stratigraphy.

P o st-F r a n c is c a n H is t o r y

CRETACEOUS LAND

Elsewhere succeeding the Franciscan in normal section occur several well known formations, namely, the Knoxville and Chico of the Creta ceous period, the Tejon (Eocene), and the Monterey (Miocene). Among these formations are several unconformities of greater or less significance and more or less widely extended occurrence. But in the region under discussion, the northern Santa Lucia range, several of the formations be-

PACIFIC SANTA LUCIA RANGE SAN ANTONIO VALLEY SANTA LUCIA PEAK

F i g u r e 1 Generalized Section of Santa Lucia Range and San Antonio Valley. Taken from Gamboa point to Santa Lucia peak. The existence of the normal faults is inferred, not actually observed.

tween the Franciscan and the Monterey are missing, and in the geologic record there is a corresponding hiatus which is significant of profound erosion. At Gamboa point, a few miles south of Slate Springs, the Fran ciscan formation occurs next to the ocean, extending from the shore up to an elevation of approximately 3,000 feet upon the spurs of the Santa Lucia range. The upper portion of the range east of this belt of Fran ciscan rocks is composed of the ancient Coast complex. Gneiss and marble constitute the summit at Twin peak and Cone peak, 4,700 and

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5,000 feet above the sea respectively. The spurs immediately east of the crest and almost equaling the highest points in elevation consist of a fine basal conglomerate of pebbles from the Coast complex and grade rapidly eastward and stratigraphically upward into sandstone and shale. These strata are very distinct from those of the Franciscan series out cropping a mile or two farther west. They are distinctly younger and of comparatively simple structural character. In the opinion of Doctor Fairbanks they are of Miocene age, since there is no unconformity be tween these formations and the characteristic shales of the Monterey formation overlying them. Some obscure fossils collected by Doctor Fairbanks and Doctor Palache from this locality have not been carefully determined. In this article the strata will accordingly be described as Miocene (?) provisionally.

MIOCENE (?) STRATIGRAPHY

Near the summit of the range the Miocene (?) strata have a steep north eastern dip, and descending eastward into the valley of the San Antonio river one traverses a section of three distinct lithologic members in as cending order. The lowest is the basal conglomerate already referred to as resting on the Coast complex, and continuous with it is an arena ceous formation consisting chiefly of coarse sandstone with some inter bedded shale. This formation is roughly estimated to have a thickness of 800 to 1,000 feet. These sandstones are followed by several thousand feet of dark gray shale with occasional interbedded sandstone layers, and these in turn by several hundred feet of sandstone, which is coarse, ferruginous, and at certain horizons conglomeratic. This stratigraphic sequence presents a simple record of three steps: The first, a transgres sion of a sea upon an eroded surface of the Coast complex; the second, represented by the shales, a considerable subsidence of the sea bottom with concurrent deposition of fine material; the third, agencies of trans portation, so invigorated as to once more contribute the coarse sands and pebbles to the sediment.

GEOGRAPHIC CONDITIONS OP THE MIOCENE (?)

A more detailed interpretation of the Miocene (?) stratigraphic record may be suggested as follows: In striking contrast to the very coarse con glomerate at the base of the Franciscan formation is the comparatively fine basal conglomerate of the Miocene (?) formations. The former con sists of pebbles many inches in diameter—rolled fragments of fresh rocks. The other contains few pebbles as large as an inch or two across, and includes numerous subangular bits of feldspar and quartz from decayed rocks. Both conglomerates were derived from the Santa Lucia gneisses

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and granites. The evidence is equally direct that the Franciscan coast was abrupt and the Miocene (?) coast was plain. It also appears that waves beat heavily on the Franciscan cliffs and lapped gently on the Miocene (?) beaches. The proportions of the water bodies thus sug gested are as the Pacific ocean and the bay of San Francisco. The arenaceous formation which succeeds consecutively the basal conglomerate of the Miocene (?) group is of small volume and arkose composition. It is the product of erosion of a surface possessing only moderate elevation and relief. The considerable mass of gray-black shale overlying the lower sand stone is sediment such as commonly gathers in estuaries similar to that which now deposits in the southern portion of San Francisco bay. If it be true, as will presently be suggested, that the Miocene (?) waters were in this district bounded on the west by a land, then the geographic place of this mud deposit may have resembled a larger San Francisco bay. The great thickness of the formation accumulated during and in consequence of equivalent subsidence. The carious brown sandstone and conglomerate overlying the shale are in part probably not immediately derived from those crystalline rocks which originally furnished the sand and pebbles, although, in so far as the formation is an arkose, it does represent direct degradation of decayed rocks of the Coast complex. The conglomerate, however, is composed of pebbles of hard, enduring minerals, thoroughly rounded and worn, which apparently have been more than once transported from place to place of deposit. Together with the quartzose portion of the sands, they may be detritus of a coastal conglomerate or of alluvial cones. In contrast with the preceding sediment they indicate either more vigorous subaerial transportation or more energetic wave and cur rent work, or both. By adequate study the significance of this member and of the two preceding it may be fully deciphered. The structure of these Miocene (?) formations is that of a flexed mono cline, the dips from Cone peak being toward the northeast and ranging from 70 degrees near the crest of the range to 5 in the eastern foothills. The attitude is suggested in the sketched section, figure 1. Some dis tant views indicated that gentle synclines were formed by southwestern dips about the head of San Antonio valley, but the observations were not conclusive.

RELATION OF THE MIOCENE (?) AND MIOCENE STRATA

That range which has thus far in this paper been called Santa Lucia is the western of two ranges which bear the name. The western extends adjacent to the coast from point Sur to Morro bay (90 miles), where it

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turns eastward north of San Luis Obispo. A shorter range lies parallel with and 10 to 15 miles east of the western. Between them is the double valley of San Antonio and Nacimiento rivers. The eastern supports Santa Lucia peak (5,967 feet), the highest in either range. In this article it will be distinguished as the Santa Lucia Peak range. Having descended over the Miocene (?) sandstone and shale on the eastern flank of the Santa Lucia range, the observer reaches the terraced flats of the San Antonio valley. Strata outcropping here and there through later gravels lie at low northeastern dips. East of the valley rises the Santa Lucia Peak range, its foothills composed of white shale characteristic of the Monterey (Miocene) formation. It also dips north east, overlying the upper sandstone of the Miocene (?) group. Imme diately east of the foothills rises the mass of the eastern range, composed of the ancient coast complex. The structural relation of the Monterey shale to these oldest rocks of the district forcibly suggests normal fault ing, but the fault was not demonstrated by observation of the contact. The course of the upper San Antonio valley is along the general strike of the Miocene rocks. At a certain point, however, the stream turns abruptly northeastward, flows through a short gorge in a low range of hills, and, entering a broad valley, resumes its course southeastward. The upper valley is opened in soft Miocene shale; the gorge, is cut in crystallines of the Coast complex; the valley beyond it is developed in shale and sandstone which are of lower Miocene (?) pr Miocene age. The basal member of the Miocene (?) group in this section is either wanting or is much finer, much less conspicuously a conglomeratic sand stone than it is 3 or 4 miles farther west. The upper coarse sandstone and conglomerate of the western outcrops also appears to be replaced by shale in part, if not entirely. Widespread gravel deposits generally obscure the strata in the lower San Antonio valley, but such outcrops as were observed were of shale. This apparent change in sediments takes place in a short distance (as is the case in a water body of moderate ex panse) and offshore from a land which lay to the west; hence the in ference that the coarser Miocene (?) sediments were derived from a western land beyond the crest of the Santa Lucia range. If so, that land is now submerged beneath the Pacific.

PINE MOUNTAIN SECTION; FRANCISCAN TO PLIOCENE

Where it was crossed in the vicinity of Cone peak the western Santa Lucia range has been described as exhibiting Miocene (?) strata on the eastern slope, rocks of the Coast complex in the summit and below on the west, and Franciscan at the western base. Thirty-four miles south east of Cone peak the range was crossed a second time from Nacimiento

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BULL. GEOL. SOC. AM. VOL. 11. 1899, PL. 28

VIEW FROM PINE MOUNTAIN TOWARD PASO ROBLES, CALIFORNIA Showing synclinal mountain of Pliocene and Miocene strata PINE MOUNTAIN SECTION 4 2 5

valley over Pine mountain to San Simeon. Franciscan strata with in- trusives are the oldest rocks exposed in this section. Their outcrops extend from the Pacific to the eastern base of Pine mountain, a width of 10 miles or more. Overlying the Franciscan on the east, in the valley of the Nacimiento, are coarse conglomerate beds which closely resemble the upper conglomerate of the Miocene (?) group. White shale of the Monterey (Miocene) succeeds them and is in turn followed by the chalky San Pablo (marine Pliocene) strata. The Miocene and Pliocene* for mations, including the conglomerate, are involved in an extensive synclinorium. The structure was obvious in the outcrops passed over en route and was traceable at a distance in consequence of the individuality of the Miocene (?) conglomerate and especially of the white San Pablo formation. A feature of the distant landscape east of Pine mountain is an isolated synclinal height in which San Pablo strata are maintained probably more than 2,500 feet above sea (see plate 28) The magnitude of this synclinorium argues for its original develop ment by simple subsidence before it was exaggerated and rendered com plex by compression. Such a subsidence has already been inferred from the Miocene (?) formations. The broad facts of structure not only sup port the previous inference, but also define the areal extent of subsidence, inasmuch as the axis of the synclinorium trends with the axis of the original depression, northwest-southeast, and the anticlinorium of the western Santa Lucia range presumably coincides with a district which did not share or lagged behind the downward movement. From the character and distribution of strata east of the Santa Lucia range and from their broad structural relations, it thus appears that the geographic conditions of Miocene and Pliocene time included a bay or strait northeast of a western land.

P l io c e n e to P r e se n t

PHYSIOGRAPHY OF THE COASTAL SLOPE The western Santa Lucia range in the stretch of 60 miles from near point Sur to Pine mountain carries no strata younger than the Miocene (?) sand stones. If the district was submerged at any time subsequent to the Miocene (?), whatever sediments formed have been eroded. The history is therefore to be read in the physiographic aspects, of which the western slope of the range bears the more legible features. To the writer’s eye the Santa Lucia range presents two distinct stages of topographic development. An earlier is recorded in the aspects of

* Identified by Doctor Fairbanks.

LXI—B u l l . G e o l . Soc. Am., V o l . 11, 1899

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higher slopes, including acute peaks, sharp ridges, and wide ravines. A later is suggested in the lower slopes by broad spurs, canyons, steep scarps, terrace remnants, and terraces, the sequence leading down to the present ocean level. The aspect of the higher slopes, above 3,200 feet, more or less, above sea, is that of advanced maturity. All forms are acutely developed. Rock masses are sharply distinguished according to resistance to erosion. There are practically no areas of gentle slope which might be considered as remnants of an earlier topographic surface. At a certain elevation—that is, probably not far from 3.200 feet above sea—a great number of summits, ridges, and spurs fall into a level with the horizon. Nearly flat surfaces, which, though of moderate extent, are nevertheless inconsistent with existing conditions of energetic cor- rasion, survive here and there. This expanse of elevations is interpreted as evidence of an early lowland, a hilly lowland possibly, possibly a plain, above which the greater heights stood as monadnocks. Below this certain elevation of approximately 3,200 feet the slope of the range toward the Pacific is comparatively gentle down to 2,000 feet or less above sea. Long spurs jut south westward, descending at easy grades, or from bench to bench by steps. Between the spurs deep wide- gaping ravines are sunk, V-shaped. Approaching the ocean they become canyons. This description applies particularly to the spurs abreast of Gamboa point and to the Devils canyon. At 2,000 feet above sea, or thereabouts, the spurs end, some in prominent knobs, others in terrace remnants (see plate 29), and thence to the ocean the descent is distinctly steeper than it is from above. Locally for 1,500 feet the slope is probably as steep as 35 degrees. No lower spurs jut oceanward, but where canyons trend parallel to the coast narrow divides separate them from the Pacific. On these divides terrace remnants are preserved, as may be seen at Posts, between Big Sur valley and the ocean, at an elevation of 800 to 900 feet, and at the mouth of Devils canyon, 1,000 to 1,150 feet above sea. Other less conspicuous bits of terrace hang here and there on the steep slope. The final descent to the sea is locally by a precipice against whose foot the surf dashes. Elsewhere a terrace intervenes, 20 to 80 feet above sea, affording a strip of pasture land which may attain a width of 300 yards. This lowest terrace bears a heavy deposit of boulders and gravel, a con glomerate probably of Pleistocene age. Its face is usually too steep to climb from the beach (see plate 27). As a working hypothesis, the writer offers the following reading of this physiography of the Santa Lucia range. The site of the Santa Lucia range was submerged in early Miocene (?) time and may have been so throughout later Miocene epochs, but a land area lay west of it. During

BULL. GEOL SOC. AM. VOL. 11, 1899, PL. 29

TERRACES AT GAM30A POINT, CALIFORNIA Approximately 2,000 feet above sea Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/11/1/417/3383257/BUL11-0417.pdf by guest on 24 September 2021 INTERPRETATION OF COASTAL PHYSIOGRAPHY 427

some late Miocene or early Pliocene episode there developed an eleva tion such that the higher hills stood between several hundred and 2,000 feet above the then sealevel. They are now represented by the peaks 4.000 to 5,000 feet above sea, which have been described as monadnocks. During a long episode of constant sealevel, erosion produced a broad low land that still may be recognized in the general approximation of sum mits and spurs to a uniform height of perhaps 3,200 feet. Renewed, uplift, amounting to 1,200 feet or more, led to partial dissection of the lowland plain and established the upper courses of the present ravines on the western slope. The streams discharging at a level which is now 2.000 feet above sea may have entered the ocean not far beyond, or at some distance west of the present coast. The features of the upper por tion of the Santa Lucia range above 2,000 feet are apparently character istic of subaerial development only. Benches of marine origin have not been detected, and the range may not have been a coast range during its earlier history. Farther elevation of the range to its present altitude involved an uplift of 2,000 feet more. In referring to this movement as a later stage of development in comparison with the earlier evolution, there is no inten tion of describing it as a simple episode, but as the topographic features belonging to it are distinctly youthful, whereas those of the higher part of the range are mature, there are two distinguishable movements. Among the features of the later stage, attention is especially drawn to the great scarp which rises from the ocean., Not everywhere equally bold, this scarp is most conspicuous south of the Devils canyon'at Gamboa point, but in many remnants, which are only a little less modified than the gentler slopes, it still retains steeps of 30 to 35 degrees, and even of 40 degrees. On these the development of gullies and landslides gives abundant evidence of the process of recession which is reducing the slope. At the base sea cliffs here and there mark the opposite effect of the attack of the waves. Among the various possible types of scarps there are but two whose genetic conditions are consistent with the environment of this one. They are the sea cliff and the fault scarp.* To distinguish them on this coast it is desirable to describe certain types of coastal slope which appear to differ in conditions of development.

THREE TYPES OF COASTAL SLOPES Three types of coastal slopes may be distinguished between Santa Barbara and San Francisco, according to the domina'nt condition of modeling which determined the present aspect of the surface above the

* Gilbert: Monograph I, U. S. Geological Survey.

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immediate coastline. A broad lowland borders the ocean about Santa Barbara, extending from the shore to the steeps of the adjacent coast range. Although elevated from 40 to 200 feet or more above sea, although traversed by incised streamways, although built up by alluvial cones from the mountain ravines, and in general modified by various subaerial agents, the lowland represents a plain of submarine origin. It is a sec tion of sea-bed elevated slightly above sea. In the development of its dominant slope marine conditions controlled, whereas deformation and erosion only subordinately affected its present attitude and detail. This is the first of the three types referred to. Prom Port Harford to point Buchon the San Luis range stretches near the coast. At its foot is a marked marine terrace, which fronts the im mediate shore with sea cliffs from 20 to 80 feet in height and in places attains a width of half a mile. Being a wave-cut bench topped by coarse conglomerate, probably of Pleistocene age, this terrace belongs to the first type of coastal slope already described. From the back of the terrace the mountains rise 2,000 feet or more in spurs, between which are deep V-shaped ravines. The horizontal contours of the relief are those of softened maturity. The spurs are rounded, the ravines are acute. The contour curves are convex toward the sea and form sharply réentrant angles landward. Seen against the sky the profiles of the spurs are in agreement with the maturity of contours in general, but in detail they are marked by slight terrace facets. Traced horizontally these terraces are so continuous and nearly level that there is no doubt they are terraces of wave erosion. The most conspicuous one is approx imately 700 feet above sea. The history of this coastal slope of the San Luis mountains is one of subaerial erosion, transient submergence, and emergence. The dominant activity by which it was sculptured was erosion. Marine erosion was a very subordinate influence. Deforma tion has had no effect upon its form, except as it has modified the con ditions of the other two activities. This is the second of the three types of coastal slopes. A third type is thought to be exemplified on the coast from point Sur to Piedras Blancas, and perhaps most strikingly at Gamboa point. The steep front which this coast presents oceanward has been described. Considered in a broad way, its horizontal contours are gentle curves, which are concave seaward and meet in prominent angles that are capes. The shoreline on the accompanying map illustrates this form, which, however, in detail is modified by the canyons cut across the scarp. Landslide benches and scarps and remnants of marine terraces are subordinate features of the slope. As has already been stated, this steep may have been either a sea-cliff or a fault scarp.

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Considered as a sea-cliff, this scarp must be supposed to have been cut upon a gentler slope during its elevation from a submarine position. The work accomplished by the waves would much exceed that recorded upon the coastal front of the San Luis range, which lies nearly contigu ous and exposed to the same attack. Marked terraces might be looked for as records of intermittent elevation, whereas only discontinuous and very local benches occur. The execution of such a task of cliff-cutting would require long time, the lapse of which should be recorded in the forms of subaerial sculpture, but the youthful aspects of the scarp and of the canyons traversing it do not bear out the inference. Thus the in terpretation of this coastal slope as a modified sea-cliff is in several re spects not in accord with facts. Considered as a fault scarp, the feature is more readily understood, although the evidence of faulting is not conclusive. Steepness is an original character of a fault scarp, and may in this case be supposed to have limited the effect of wave erosion and to have been favorable to landslides. Thus the subordinate occurrence of marine terraces and the dominant concave contour characteristic of landslip scarps are explained. The canyons in which the mature ravines of the higher ranges terminate seaward are cuts in a coastal margin rapidly raised above baselevel, after the manner of a fault block elevated relatively to the downthrown block. The uniform trend of the coast for 60 miles, coterminous with the steep scarp, bears the character of a fault line. The test of this interpretation of the physiographic aspects may be the determination of the fault where it runs ashore. Northwestward it passes possibly east of point Sur, and thence beneath the ocean. The writer did not observe the geologic rela tions near point Sur. Southeastward the trend of the supposed fault strikes the shore near Piedras Blancas. The white rocks, which are de scribed by the Spanish name, are of Miocene shale, and form a small' area west of the Franciscan formation that extends from the coast across Pine mountain. This isolated patch of Miocene, Piedras Blancas, has the position of a downfaulted mass. The fault, not having been traced on the ground, can not, however, be said to be demonstrated. The weight of physiographic and geologic evidence is clearly in favor of a structural origin for the bold scarp of the coast, at least from near Posts to Arroyo San Carpoforo, and it is therefore considered to consti tute a third type, in the genesis of which deformation was the dominant activity, while erosion and marine sculpture played subordinate roles.

INCIDENTS IN THE SCARP-GROWTH

The rise of this scarp from ocean level was interrupted by a decided pause, which is marked in a group of marine terraces covering a space

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of 100 feet or more in altitude, and occurring at Posts 800 to 900 feet, at Devils canyon possibly 1,000 to 1,100 feet, and at point Buchon approx imately 700 feet above sea. Their elevation at Posts is read from con tours of Coast Survey chart number 5476. The other altitudes are based on aneroid readings of uncertain value. These terraces are more- strongly marked and more generally present than any above or below them, and they have been correlated by Doctor Fairbanks with a gen eral elevation of hiil summits about San Luis Obispo and throughout an extensive area of the adjacent district. Along the base of the scarp extend stretcher of terrace now elevated 40 to 80 feet above sea, which are covered with heavy gravel deposits and are tentatively referred to the Pleistocene. No more definite date has yet been determined for them. Between the monadnock, Cone peak, and the marine bench now being cut in the base of the scarp, there is thus a physiographic record of the uplift of the Santa Lucia range. The history covers Pliocene and Ple istocene time to the present. Marked features of the physiography are the high monadnocks, the upper dissected plain at 3,200 feet above sea, the lower dissected plain and correlated terraces at 700 to 1,000 feet above sea, and the Pleistocene terrace; but the most striking physio graphic feature is the great faultscarp, if such be its true character, dividing the upraised mountain block of the Santa Lucia range from the downthrown mass of the western land, which is submerged beneath the Pacific. Future observers will trace out relations between the various physio graphic features and the marine and fresh-water deposits of post-Miocene age. The sedimentary and physiographic records are both fairly con tinuous and legible in the interior valleys as well as along the coast, and will yield a connected history of the Coast ranges when studied in detail with adequate maps.

S u m m a r y

The outline of the history since the earliest geologic conditions re corded is summarized in the accompanying diagram; but one salient fact needs here to be emphasized: Along this farthest Pacific margin of the continent the present coast occupies a position across which coasts of earlier ages have repeatedly migrated, as land or sea for the time being prevailed. The student of continental growth finds no evidence of permanent encroachment of continent or ocean either upon the other. The student of mountain growth, however, finds an example of the gen eral law that the site of an existing mountain range has been occupied in past ages by successive generations of mountain ranges.

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E x p l a n a t i o n o p F i g u r e 2

Conventions used in diagram.—Time

is represented in sequence from left 3 2 to right. As the total lapse of ages represented by the record is un known and the relative durations of successive ages are indeterm inate, there is no definite time scale; but that portion of pre- Franciscan time represented was probably much longer than post- Franciscan time. The time scale is therefore much smaller at the left than at the right of the diagram. Vertical ordinates represent posi tions in the earth’s superficial crust from a mile above sealevel to 6 miles below sealevel. Sealevel is arbitra rily taken as a fixed datum. ¡Sequence of sea and land.—The upper curve represents from age to o age the varying relation of the rock a a> surface to sealevel in the chosen o5 *o locality. Below sealevel the posi S?» tion of this line results from the 3W amount of subsidence, less the thickness of deposits. Above sea level its position is determined by the amount of uplift, less the effect of erosion. The values indicated are to be taken qualitatively, not quantitatively. The following is the interpretation of the several lettered sections : a-b. Submergence ; sedimentation <<5 ® corresponding to the strata ofthe Coast complex; com prising epochs probably of great duration and of com plex physical changes; probably closed by orogenic movements thatresulted in folding of the Coast com plex in the zone of defor mation by flow, and were connected with granitic in trusions. P resen t

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b-c-d. Emergence; erosion which sufficed to lay bare metamorphic schists and granite of the Coast complex, so that these rocks entered the basal con glomerate of the Franciscan series at the time d. d-e. Submergence; sedimentation corresponding to the Franciscan series, of late Juratrias or early Cretaceous date; closed by movements which gave rise to deformation of the Franciscan series in the zone of deformation by fracture. e-f-g-h. Emergence; erosion following the initial uplift and probably other move ments corresponding to unconformities among Cretaceous and Eocene formations of adjacent sections. h-i. Submergence; sedimentation corresponding to the formations of Miocene (?) and possibly later Miocene (Monterey) time, limited at an indeterminate date by the initial rise of the present Santa Lucia range, but continued into Pliocene time in the adjacent eastern district. i-j. Emergence; corresponding to the initial rise of the Santa Lucia range; approximately 1,800 feet. j-k-l. Pause in uplift, resulting in erosion to general lowland with monadnocks. l-m-n. Renewed uplift (1,200 feet ± ) and erosion, with development of high level terraces, now 2,000 feet ± above sea at Gamboa point. n-o. Later elevation of the Santa Lucia range, a rise of 2,000 feet, probably asso ciated with development of coastal fault scarp. This later movement may have been complicated by or succeeded by oscillations of level of several hundred feet, apparently recorded in the San Luis Obispo district. Path of Coast complex stratum.—At the right of the diagram at o is represented a point near Cone peak, 5,000 feet above sea, where there is an outcrop of marble and gneisses of the Coast complex. The lower curve represents the path of these sediments from their place of deposition in the Paleozoic (?) sea across the zones of the earth’s crust to their present position. a-p. Subsidence; which carried the strata of the Coast complex down to the zone of deformation by flow. p-q. Uplift, accompanied by movements of compression and granite intrusion. q-r. Subsidence, corresponding to burial beneath Franciscan sediments. r-g-h. Uplift accompanied by deformation of Franciscan formation and volcanic activity, probably distributed in several episodes with quiescent inter vals. At g the particular strata of the Coast, complex which now out crop at o are assumed to be exposed and to form part of the surface upon which the Miocene (?) sediments were deposited.

BULL. GEOL. SOC. AM. VOL. 11, 1899, PL. 30

MOUNT KTAADN, MAINE As seen from the morainic wall which encloses one of the ponds near the entrance to the South basin. North basin is on extreme right, behind the ridge