ARTHUR D. HOWARD Department of Geology, Stanford University, Stanford, California 94305 Modified Contour-Generalization Procedure As Applied to the Santa Lucia Range, California ABSTRACT tinuously degrading topography. Rapid up- lift, 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 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 3416 A. D. HOWARD 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. 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 GENERALIZED GEOLOGIC MAP SANTA LUCIA RANGE N 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). 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 MODIFIED CONTOUR-GENERALIZATION PROCEDURE, CALIFORNIA 3419 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.