Modified Contour-Generalization Procedure As Applied to the Santa Lucia Range, California

ARTHUR D. HOWARD Department of Geology, Stanford University, Stanford, California 94305

ABSTRACT tinuously degrading topography. Rapid uplift, on the other hand, may provide an almost Contour generalization is an established unmodified initial landform. These considera- procedure for restoring dissected landscapes to tions, however, are not of vital concern in the predissection forms. The modified generaliza- present procedural discussion. tion procedure herein proposed dispenses with The generalization procedure involves the most side-slope contours, leaving upland sur- projection of contours across valleys from spur faces suspended at their individual levels. In to spur, which results in pictorially eliminating addition, sequential cartographic patterns the valleys. Generalization is subjective in permit ready recognition of surfaces, and that it requires judgment in (1) the selection hachures emphasize abrupt peripheral slopes. of an appropriate contour interval for general- Although the primary purpose is to discuss ization, (2) the size of valleys to be eliminated, the proposed procedure, the bearing of the and (3) the extent to which deviations from final map on the history of the Santa Lucia item 2 may be made. Thus, rectilinear valleys Range is briefly considered. The origin of the and small, aligned valleys may warrant reten- high-level surfaces has not yet been deter- tion if the possibility of faulting is important mined, although fragmentary evidence sug- to the problem. Carelessly planned generaliza- gests they are probably fluvial. The multiple tion may lead to incorrect portrayal of the surfaces were presumably developed in response number, attitude, and areal extent of upland to changes in the elevation of base level. surfaces and result in errors in interpretation. Simultaneous development of the surfaces due I have prepared a generalized contour map to differential lowering of rocks of varying of the coast range belt of middle California resistance, independent of base level, is refuted from a point about 100 mi north of San by the similarity of the rock assemblages Francisco to a point some 150 mi south. The truncated by the different surfaces. The high scale of the generalized map is 1:250,000; the surfaces in the northern part of the range are contour interval is 250 ft. The Santa Lucia found to be sloping, rather than horizontal as Range is used to illustrate the procedure previously supposed. Tilting of the northern employed. area seems indicated. At their elevated southern It should be noted that the Santa Lucia ends, these surfaces hang above considerably Range is already included on the 1:250,000 lower terrain. Here, the relative horizontality Santa Cruz and San Luis Obispo topographic of at least one extensive surface suggests simple maps prepared by the Army Map Service, vertical uplift. U.S. Army Corps of Engineers (1955b, 1955a). The generalization of contours in these maps, INTRODUCTION however, is obviously designed for best Generalization of contours is intended to cartographic presentation and retains too reveal the pristine nature of a landform or much dissection for maximum effectiveness in landscape prior to dissection. In many in- geomorphic analysis. stances, of course, it is invalid to assume that The extreme dissection of the Santa Lucia erosion was negligible prior to completion of a Range makes it especially difficult to envision landform or landscape. Erosion during ex- elemental forms, particularly ancient erosion ceedingly slow uplift may result in a con- surfaces. It is in such situations that generaliza-

Geological Society of America Bulletin, v. 84, p. 3415-3428, 7 figs., October 1973 3415

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tion proves a useful tool. It often facilitates Between the Sierra de Salinas and the main field work by bringing into focus significant portion of the range to the west is a broad, geomorphic features and revealing problems o.; northwest-trending lowland largely drained by origin and interrelations. Carmel River. West of Carmel Valley, the The first map offered in the present artick main part of the range is a sprawling mass with represents maximum possible generalization deep valleys and relatively narrow ridges. The of the Santa Lucia Range. In this map, all ridges attain their maximum elevation along valleys have been eliminated. A series of four the line of high peaks previously noted. experimental generalization maps follow. The One of the major valleys of the Santa Lucia last of these, enhanced by hachures and Range, Arroyo Seco, transgresses the regional sequential patterns, is believed to represent trend (see Fig. 3). The valley rises near the best the upland surfaces of this complicated western border of the range and trends gener- tectonic block and hence most likely to result ally eastward into Salinas Valley near Green- in meaningful interpretation. field. TOPOGRAPHY GEOLOGY The Santa Lucia Range extends about 140 The regional trend of the: Santa Lucia Range, mi from Monterey Bay in the northwest to as well as its internal grain, is geologically con- beyond San Luis Obispo Bay in the southeast trolled (compare Figs. 2 and 3). Faulting may (see insert map of Fig. 1). The northwest 65 be directly responsible, at least in part, for mi of the range is the subject of this article the abrupt eastern and western margins of the and includes the highest elevations, culminat- range. The northwest-trending interior valleys, ing in Junipero Serra Peak at an elevation of however, and the transverse valley of Arroyo 5,862 ft (see Fig. 3). Junipero Serra is one of a Seco, are presumably largely due to erosion line of high peaks that trends northeasterly along faults rather than to modern offsets. across the range. Southeast of this line of peaks, This is suggested by the accordance of ridge the topography descends steeply to con- crests over relatively large areas. siderably lower elevations. The coastal portion The major part of the range herein con- of the lower terrain is a broad, dissected, linear sidered is underlain by Mesozoic or earlier upland with only a few summits reaching or metamorphic rocks and by Mesozoic sedimen- exceeding 3,500 ft. Inland from this upland, tary and granitic rocks (Fig. 2). The meta- the terrain includes broad valleys and irreg- morphic complex consists of high-grade meta- ular basins; only a few summits exceed 2,000 ft. sedimentary rocks, including quartzites, In the southern two-thirds of the mapped gneisses, schists, amphibol.ites, and marbles, area, the shoreline parallels the regional trend with interbedded granitic sheets. Large bodies of the range, and the coast consists of an almost of granite appear as lensoid masses with bound- continuous, precipitous slope locally dropping aries generally concordant with the foliation 4,000 ft in approximately 2 mi. In the northern of the surrounding metamorphic rocks (Comp- third, however, the shoreline changes to a ton, 1966). The Jurassic-Cretaceous rocks of northerly direction and cuts obliquely across the Franciscan Formation include sandstone, the range. Here valleys and spurs alternate. claystone, sedimentary breccia, greenstone, The interior of the range is dominated by and chert, as well as scattered ultrabasic intru- northwest-trending ridges and valleys. The sive bodies. The Franciscan rocks are tightly ridges, however, are irregular and discontinu- folded. The Upper Cretaceous rocks consist ous; and the topography bears little resem- of unmetamorphosed conglomerate, sandstone, blance to classical Appalachian ridge-and- siltstone, and claystone in lenses and thick valley topography. layers; these and the Tertiary rocks are only The Sierra de Salinas, the asymmetric ridge moderately folded. west of the Salinas River between Chualar and The Tertiary formations, which include both Greenfield (see Fig. 3) forms a more or less marine and nonmarine sedimentary rocks, distinct unit in the northern part of the range. underlie much of Carmel Valley from its Elevations in the Sierra approach 4,500 ft. The mouth to its headwaters and form much of slope facing northeast is a precipitous scarp the low country southeast of the line of high overlooking the Salinas Valley, which is less peaks mentioned earlier. They consist of the than 200 ft above sea level. usual array of sedimentary types and con-

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/10/3415/3433234/i0016-7606-84-10-3415.pdf by guest on 29 September 2021 north half of Santa Lucia Range, California. All M = Monterey Bay; SO = San Luis Obispo Bay; valleys eliminated. In insert map, stippled area rep- SM = Santa Maria.

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LEGEND QUATERNARY AND PLIO- PLEISTOCENE

Alluvium. Sond. Terroce deposits. TERTIARY

Marine and non-marine sedimentary rocks

UPPER CRETACEOUS

Marne sedimentary rocks

JURA-CRETACEOUS

Franciscan formation. Sedimentary rocks. Ultra- basic intrusives

PRE - CRETACEOUS

lilMWil Metamorphic rocks

MESOZOIC N

Granite

J- Figure 2. Geologic map of northern Santa Lucia Range. Generalized from San Luis Obispo and Santa Cruz sheets of Geologic Atlas of California (1958b, 1958a).

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siderable amounts of strongly lithified por- upland surface between 4,000 and 5,000 ft cellanite and opaline chert of Miocene age. with considerable northwest-southeast extent Numerous eroded, downfaulted, and down- in the westerly half of the range but with only folded slivers of Tertiary and Mesozoic sedi- a blunt peninsulalike projection extending mentary rocks contribute to the internal toward the east. This surface drops off abruptly topographic grain. Very few of the ridges, along the entire coastal side of the range and however, consist of a single rock unit. The locally along the inland side but descends more surface of Junipero Serra ridge, for example, gradually elsewhere. Over large areas the truncates a variety of igneous, metamorphic, descent from 4,000 to 3,000 ft appears gentle and sedimentary rocks. in contrast to the slope from 3,000 to 2,000 ft. The metamorphic and granitic rocks termi- If, on the other hand, one uses the alternative nate abruptly just south of the belt of high generalization in which Carmel Valley is pre- peaks that includes Junipero Serra. The con- served, the slope between 4,000 and 3,000 ft tact is mapped in part as a complicated fault appears steep over large areas. Except for boundary (Fig. 2), but steep flexuring of the these very general observations bearing on the resistant basement complex is also indicated presence of an upland surface and the slope (Compton, 1966). The narrow coastal upland of its margins, the gross generalization map south of the high peaks is composed largely of offers little opportunity for meaningful geo- Franciscan sedimentary rocks with scattered morphic analysis. ultrabasic intrusive bodies. The lower area to the east is largely underlain by Upper Creta- EXPERIMENTAL GENERALIZATION ceous sedimentary rocks on the west and MAPS Tertiary sedimentary rocks on the east. In Figure 3, each of the rectangles defined by the grid of 15' latitude and longitude lines MAXIMUM GENERALIZATION MAP represents a 1:62,500 topographic quadrangle Figure 1 is a maximum generalization map map. Each map was generalized with a 250-ft of the Santa Lucia Range prepared with a contour interval, and all valleys less than 3 mi contour interval of 1,000 ft. The small, heavy, long (3 in. at the original map scale) were arcuate lines represent the principal spur ends eliminated. The generalized quadrangle maps which, together with longer linear segments of (all or portions of 11 maps) were then reduced present contours, provide the control for the to 1:250,000 scale, and the set was assembled generalized contours. Except for the steep to fashion Figure 3 prior to reduction. What marginal slopes of the range, the generalized seemed to be appropriate generalization at contours, consisting of both the heavy solid 1:62,500 proved too detailed when reduced lines and the heavy dashed lines, are speculative to 1:250,000; the terrain still appeared too and subject to alternative interpretations. One dissected for a clear picture of upland surfaces. notable example has been indicated by the Figure 4 represents further generalization of light interrupted lines for the 3,000- and Figure 3 in which all but major valleys and 4,000-ft contours: Instead of crossing directly valleys whose trend or other characteristics ap- from the west to the east side of the range in pear to have geomorphic significance have the northern third of the map, the contours been eliminated. The dotted lines represent the are redrawn to follow spur ends, represented valleys of Figure 3, most of which were elimi- by heavy dots, extending southward into the nated in this version. Figure 4 now reveals a range interior. Note that this modification number of upland surfaces; at least one does preserves the broad north-trending lowland not appear to be horizontal. This is the large between the main mass of the range and the remnant, 12 mi long and 6 mi wide, just north- Sierra de Salinas block. Similar modifications west of the center of the range. Almost the of the generalized contours are possible in the entire surface descends gradually to the north- lower third of the map. Obviously, the appear- west. Some geomorphologists will be satisfied ance of the generalized map will depend on the with the portrayal in Figure 4 and may find breadth of the gaps one feels justified in Figures 5 and 6 unnecessary. bridging. It may be appropriate at this time to justify Gross generalization, as in Figure 1, may selection of the relatively large 250-ft contour severely limit interpretation of geomorphic interval. It is reasonable to ask whether so history. The map appears to indicate a broad large an interval might not in itself give an

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/10/3415/3433234/i0016-7606-84-10-3415.pdf by guest on 29 September 2021 Figure 3. First stage generalization. Each of the of the generalized maps was then reduced to 1 ¡250,000, 1:62,500 topographic maps within the 15' grid lines and the collection assembled to provide the present was generalized with a 250-ft contour interval. All map, which has been further reduced in printing, valleys less than 3 mi long were eliminated. Each

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Figure 4. Second stage generalization. This map the largest valleys, and a few smaller ones of possible represents further generalization of Figure 3. The structural significance, have been eliminated. dotted lines are valleys of Figure 3. Note that all but

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erroneous impression of broad upland surfaces. confused by the extension of a single sloping If, however, one examines the above-mentioned surface across two or more patterns. The de- large upland surface just northwest of the scending peripheral scarp and the absence of a center of the range in Figure 5 or 6, it will be scarp between adjacent patterns indicate the apparent that, even if we added 50-ft contours, continuously sloping surface. the appearance of this gently sloping upland Remnants of lower surfaces appear below surface would not be fundamentally changed. the high upland, but they are too small and If preliminary examination of the original scattered to justify attempts at correlation, maps had indicated the possibility that the at least on the basis of elevation alone. upland surface consisted of a series of flat or To the north and northwest of the high sloping benches stepping down to the north- upland is an extensive flat surface with its edge west, we would have employed auxiliary at about 2,000 ft in the east but decreasing to hachures, supplementary contours, or some 1,750 ft in the northwest. For convenience, other device to indicate this. In generalization, this will be referred to as the 2,000-ft surface; therefore, a contour interval should be selected it has correlatives directly across Carmel Valley to provide as accurate a portrayal of restored to the east. Theoretically, this extensive surface surfaces as possible without unnecessary clut- should correlate with one of the terraces along tering, provided that provision is made for the coast, but intervening gaps and the frag- inclusion of important details. mentary nature of the high coastal benches Figures 5 and 6 involve further elimination make such correlation hazardous. One coastal of irrelevant detail and the addition of certain bench shown in Figure 6 has its outer edge at embellishments designed to provide more vivid about 2,500 ft. This remnant, however, may portrayal of the upland surfaces. In Figure 5, represent only the higher, inner part of a ter- most of the side-slope contours have been race, the lower distal part of which was long deleted. Figure 6 involves two embellishments. ago removed by erosion. Conceivably, this One is the use of hachures to represent the remnant could correlate with the 2,000-ft steep, marginal slopes of upland surfaces. surface inland, but map study alone provides Contours are still retained, however, where the no basis for such correlation. Below the 2,500-ft limiting slopes are moderate or gentle. The bench is a more extensive lower one with its second embellishment is the use of sequential outer edge at approximately 1,000 ft. This patterns for ready identification of elevations lower bench continues with only short inter- and slopes. A separate design is used for each ruptions around the northwest end of the 1,000-ft interval, and the texture of the design range on both sides of Carinel Valley. Like the is finer for each successively lower 250-ft large upland remnant and the extensive 2,000- interval. ft surface, the 1,000-ft surface in the Carmel Valley lowland slopes to t'ne northwest. Along It would have been helpful if the same pat- the coast, however, the elevation of the outer tern, in a continuous series from coarse to fine, edge of the 1,000-ft bench locally rises toward could have been used for the entire range of the north. The significance of this must await elevations. The present map, however, would detailed investigation. A single remnant of a have required 17 textural grades of a single 750-ft coastal bench appears in Figure 6. Field pattern: the coarsest pattern would have been study indicates that other remnants are present too coarse for small areas and the finest would but too small to appear on the map. have appeared as a gray tone after reduction. Furthermore, in such a long gradational series, The asymmetric Sierra de Salinas block dis- it would be difficult to differentiate surfaces plays a relatively broad restored surface be- at slightly different elevations. tween 4,000 and 4,250 f:. Fragmentary rem- Figure 6 appears to lend itself well to iden- nants of surfaces at this level appear elsewhere tification of upland surfaces and to determina- in the range, but the intervening distances do tion of regional slopes, topographic relations, not justify attempts at correlation. Below the and possible correlations. Consider, for ex- summit surface is a more extensive one between ample, the high remnant just northwest of 3,000 and 3,500 ft. Both surfaces are developed the center of the range in Figure 6. The surface on the highly deformed metamorphic com- pattern reveals a northwesterly slope, and the plex, and there is no reason to attribute the marginal hachures indicate a relatively steep ';wo levels to differential erosion. A very large peripheral descent. The reader should not be flat surface at the same elevation as the lower

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/10/3415/3433234/i0016-7606-84-10-3415.pdf by guest on 29 September 2021 Figure 5. Third stage generalization. Slope contours of Figure 4 have been eliminated except where they emphasize steps in topography.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/84/10/3415/3433234/i0016-7606-84-10-3415.pdf by guest on 29 September 2021 Figure 6. Fourth stage generalization. Upland in patterns without intervening scarps and display surfaces are represented by patterns, and abrupt slopes continuous peripheral scarps which lose elevation in by hachures. Sloping surfaces show sequential changes the same direction.

of the two Sierra de Salinas surfaces appears far of significant tilting in this area. Vertical to the south inland from the village of Lucia. movements, however, are presumably indi- Correlation on the basis of altitude alone is cated by these widespread erosion surfaces far entirely unjustified, however, not only because above any possible eustatic rises of sea level. of the great distance but because of the strong Whether these high surfaces are marine, sub- probability that the range north of here has aerial, or composite is at present not estab- been strongly tilted. lished. That they were not formed simul- The surfaces beveling the crest of the Sierra taneously by differential erosion of rocks of de Salinas block suggest that the higher part different resistances to erosion is indicated by of the present westerly slope with its extensive lack of correspondence between rock type and exposures of basement rocks may be an ex- elevation of remnants (Fig. 7). humed surface stripped of overlying weak Trask (1926) postulated an upland surface deposits. As for the lower flats at 2,000 and of low relief in the northern part of the Santa 1,000 ft on both sides of Carmel Valley, they Lucia Range and suggested that the surface are close enough to justify correlation. They was developed and uplifted in Pleistocene time. were presumably developed after faulting, Compton (1966) found that the high surface beveling, and further erosion of the Sierra de beveled Pliocene-Pleistocene folds and faults, Salinas block. The northwesterly slope of and that in one place at least, bore "relict these surfaces suggests tilting, erosion by the stream gravels." Christensen (1965) concluded Carmel River and its tributaries, or both. from a gross generalization map that the Santa The restored surfaces in the southern part Lucia was vertically uplifted. His map, how- of Figure 6 vary considerably in size. The large ever, fails to reveal the indications of tilting remnant between 3,250 and 3,500 ft is about brought out by the more detailed maps pre- 16 mi long and more than 6 mi across. Except sented here. for a narrow fringe around the high peak Stratigraphic evidence indicates comparable immediately to the north, it has no counter- tilting in pre-Pleistocene time. Miocene shal- parts to the east where the terrain is con- low-water marine sedimentary rocks, which siderably lower. This extensive surface steps up are exposed up to the 4,000-ft level in the abruptly to a narrow flat surface at 3,750 ft, Santa Lucia Range, have been penetrated by and this in turn steps up to the summit surface drilling to depths of 3,000 ft below sea level at 4,500 to 4,750 ft. To the northwest, the first along the shores of Monterey Bay to the north extensive surface below the upland summit is (Starke and Howard, 1968). Pliocene shallow- at 3,500 ft, not 3,750 ft as to the south. This water deposits are similarly displaced, but to suggests a northwestward slope similar to that a lesser degree. of the large upland northwest of the center of Recent and continuing deformation is sug- the range. gested by folded and eroded alluvial fans The 3,500-ft surface is limited by a marginal (Gribi. E. A., 1964, personal commun. cited scarp which connects this surface to a 3,250-ft by Compton, 1966) and by faulted stream remnant north of an intervening rise. Again a terraces (Snetsinger, 1962). regional northwestward slope from the higher The small scale and large contour interval parts of the range seems indicated. Because the of the generalized maps presented here make surfaces in these higher areas hang above the meaningful inferences on the number, attitude, low country to the south, intermittent tilting and correlation of the coastal terraces impos- and erosion of the northern area is suggested. sible. Many remnants are very small and are at The Geologic Atlas sheets (Jennings, 1958; elevations that fall within the contour interval Jennings and Strand, 1958) show local faulting of the generalized maps. Map analysis of these along this topographic boundary (Fig. 2). The terraces prior to field investigation requires topographic break, however, may be in part generalization maps of large scale and small due to differential erosion inasmuch as strong contour interval as well as other map-analysis flexuring of the basement complex has brought procedures presently under way. weaker rocks to low levels in the southern area (Compton, 1966). SUMMARY South of the high peaks, the great extent A modified contour-generalization procedure and consistency in elevation of some of the has been presented using one of the California restored surfaces seem to negate the possibility coast ranges as an example. In this procedure,

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Figure 7. Geology of surface remnants. Note that extreme south bevel the same folded formation, the most of the surfaces in the north, in spite of differences Franciscan. Variation in rock resistance, without the in elevation, truncate identical rock assemblages. Note, influence of a changing base level, cannot explain the too, that considerable areas of the two surfaces in the development of these surfaces.

surface remnants are isolated by eliminating daily helplul in areas of recent and compli- most side-slope contours; hachures are used for cated tectonic activity although the refine- steep, marginal slopes; and sequential patterns ments may be unnecessary in other areas. For are added to facilitate identification and cor- the most part, the generalized map draws relation of surfaces. Such maps may be espe- attention to problems and, in some instances,

suggests possible explanations. In investigation Compton, R. R., 1966, Analyses of Pliocene- of large areas, it seems advisable to experiment Pleistocene deformation and stresses in north- with several degrees of generalization in one ern Santa Lucia Range, California: Geol. Soc. or more small areas before embarking on America Bull., v. 77, p. 1361-1380. regional generalization. Jennings, C. W., compiler, 1958, San Luis Obispo sheet, in Geologic atlas of California: Califor- The generalized map of the Santa Lucia nia Div. Mines and Geology, scale, 1:250,000. Range reveals a number of high-level surfaces. Jennings, C. W., and Strand, R. G., compilers, These surfaces are independent of rock type, 1958, Santa Cruz sheet, in Geologic atlas of indicating development under base-level con- California: California Div. Mines and Geology, trol with intermittent uplift. The north- scale, 1:250,000. westward slopes in the high northern part of Snetsinger, K. N,, 1962, Late Tertiary and Quat- the area suggest tilting during uplift. The ernary history of the lower Arroyo Seco, essentially horizontal attitude of the surfaces Monterey County, California [M.S. thesis]: in the south, however, presumably indicate Stanford, Calif., Stanford Univ., 36 p. uniform uplift. Starke, G. W„ and Howard, Arthur, 1968, Poly- genetic origin of Monterey submarine canyon: Geol. Soc. America Bull., v. 79, p. 813-826. ACKNOWLEDGMENTS Trask, P. D., 1926, Geomorphogeny of the northern part of the Santa Lucia Coast Range, I am indebted to Peter W. Birkeland, California: Am. Jour. Sci., v. 12, p. 293-300. William C. Bradley, Robert R. Compton, U.S. Army Corps Engineers, 1955a, San Luis Charles S. Denny, Ben M. Page, and Ernest Obispo Sheet: U.S. Army Map Service Map I. Rich for review of the manuscript and many NI 10-3, scale, 1:250,000. helpful comments. 1955b, Santa Cruz Sheet: U.S. Army Map Service Map NJ 10-12, scale, 1:250,000. REFERENCES CITED

Christensen, M. N., 1965, Late Cenozoic deforma- MANUSCRIPT RECEIVED BY THE SOCIETY JANUARY tion in the central Coast Ranges of California: 29, 1973 Geol. Soc. America Bull., v. 76, p. 1105-1124. REVISED MANUSCRIPT RECEIVED MAY 1, 1973

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