Sea Cliffs: Their Processes, Profiles, and Classification

Sea cliffs: Their processes, profiles, and classification

K. O. EMERY Woods Hole Océanographie Institution, Woods Hole, Massachusetts 25431 G. G. KUHN Scripps Institution of Oceanography, La Jolla, California 92093

ABSTRACT sion and retreat of sea cliffs are in prospect towns were established on coastal plains or because of projected regionally wetter and deltas, sea cliffs rarely are involved in urban General concavity or convexity of sea- stormier climate, rising sea level, and in- or industrial development. Even where cliff profiles is controlled by relative rates of creased human activities. urban growth encroaches upon sea cliffs, erosion by marine and subaerial processes, the ruggedness of the cliffs tends to cause as well as by positions of more resistant INTRODUCTION them to be avoided except for residential strata in the cliffs. Profiles supplemented by purposes. on-site examination can establish the activ- Sea cliffs are steep slopes that border Lighthouses, whose visibility requires place- ity and dominance of erosional processes ocean coasts; similar features border lakes ment at the tops of sea cliffs, historically and indicate changes in regimen. A sharp and other small bodies of water. Their have been moved landward or have been angle at the sea-cliff base generally indicates steepness causes them to be so narrow in rebuilt at intervals to avoid destruction by active marine erosion, whereas a smooth plan that they make up only a very small sea cliff retreat. During the past few curve at the base means that subaerial ero- area compared with the areas of the pla- decades, sea cliffs have increasingly become sion may dominate. Talus shows absence of teaus, hills, or mountains that they separate sites for houses, with attendant risks. How- marine erosion. Studies of profiles can be from the ocean. Nevertheless, sea cliffs are ever, limited industrial and mass residential useful for estimating stability for residences, ubiquitous, occurring along ~ 80% of the value has caused sea cliffs to receive little railroads, and highways at the top, face, and ocean coasts of the Earth (Fig. 1) and at all geological study compared with the consid- base of sea cliffs. Generally increased ero- latitudes. Because most coastal cities and erable research that is devoted to cycles of

Figure 1. Distribution of coasts that consist mainly of sea cliffs (black), as opposed to coasts that consist mainly of beaches, mud flats, coral reefs, mangroves, and other shores unbacked by sea cliffs. Even many coasts that are not indicated as having sea cliffs do have some a few metres high (from Isakov, 1953, and personal observation). Other maps, such as one by McGill (1958), are more concerned with the entire coastal zone (both continental shelf and near hinterland) than with the presence or absence of sea cliffs.

Geological Society of America Bulletin, v. 93, p. 644-654, 8 figs., July 1982.

644

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growth and retreat of beaches. Many stu- 1947; Byrne, 1963; Zeigler and others, 1964; are relict from a former 2-m sea level (Jut- dies of sea cliffs have concentrated only Rudberg, 1967; Shepard and Wanless, 1971; son, 1949b; Shepard and others, 1967; Gill, upon their plan, or map, form with respect Sunamura and Horikawa, 1972; Kuhn and 1973; Trenhaile, 1974; Robinson, 1977a). to stages of morphological development Shepard, 1979, 1980; Kuhn and others, Although we know of no classification of and classification of coasts, as early illus- 1980; Emery and Kuhn, 1980; Sunamura, in entire sea cliffs as complete units such as trated by the work of Johnson (1919, 1925). press; and many others), or engineering mea- attempted in our analysis, several authors During the past few decades, sea cliffs have sures to stabilize the sea cliffs (many, but have used general sea-cliff profiles to illus- received increasing attention from engineers with limited scope and publication). Nearly trate the different effects of marine and sub- directed toward reducing damage by mass all studies are fairly local and intended to aerial processes (Guilcher, 1954; Steers, movements associated with highway and clarify some particular aspect of sea cliffs— 1948, p. 67, 1962; Bradley, 1958; Ottmann, residential siting. This attention, however, mostly relative to marine erosion. Even 1965; Zenkovich, 1967, p. 298; Sunamura has been restricted to local individual sites though the above list is incomplete, it shows and Horikawa, 1972; Tinsley, 1972; Kaplin, rather than broad comparisons between the wide interest attached to at least certain 1973; Palmer, 1973; Trenhaile, 1974; Rob- regions. aspects of sea cliffs. inson, 1977b). We use such profiles as a The present study considers the descrip- After local description, the next stage of major basis for our classification. tion and classification of active sea cliffs investigation usually is that of classification In some ways, our thinking about the according to their profiles. Previous related that takes into account differences in char- profiles , of sea cliffs is derived from much investigations have specialized on the na- acteristics given by the descriptions. For earlier investigations of the profiles of ture of marine erosion at the bases of the shore platforms at the base of sea cliffs, the stream valleys and their interfluves. The cliffs (Wentworth, 1938-1939; Emery and interest in broad classification appears to concavity or convexity of the land topog- Foster, 1956; Revelle and Emery, 1957; have increased in recent years, particularly raphy on homogeneous materials is deemed Flemming, 1965; Gill, 1967; Kaye, 1967; to resolve the question of whether the plat- to be controlled by relative rates of Sanders, 1968; Sunamura and Horikawa, forms are produced at present sea level or erosion at the axis of the main stream, and 1972; Sunamura, 1975, 1977; Robinson, 1977a; McGreal, .1979), or development of shore platforms (Bartram, 1926, 1935; Jut- son, 1949a; Gill, 1967, 1972; Wright, 1970; (A) (B) (C) Hills, 1971, 1972; Bradley and Griggs, 1976; homogeneous resistant at top resistant at bottom Sunamura, 1978), or presence of earlier cycles of sea-cliff erosion (Davis, 1912; Cot- ton, 1951, 1969; Fleming, 1965; Bird, 1969), or rates of sea-cliff retreat and production of detrital sediments (Shepard and Grant,

Active Inactive

talus boulders^

M < SA

Figure 3. Matrix of active sea-cliff profiles to be expected from bedrock of three different limiting degrees, of homogeneity with respect to relative erodibility at bottom and top, and of four different major degrees of relative effectiveness of marine (M) versus subaerial (SA) erosion. It assumes that sea cliffs are cut into pla- teaus and are near steady state equilibrium. Diagonal lines.denote Figure 2. Idealized stages in geological history of a. sea cliff. resistant beds.

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erosion of the sides by rivulets, sheet wash, cliffs are in homogeneous soft materials and teristics are generally effective and correctly and creep (Gilbert, 1877, p. 120-123, 1909; thus are simple and uninteresting. described in the next few paragraphs even Davis, 1892, 1930; Fenneman, 1908; Law- though some of them still are subjects of son, 1932; Cotton, 1952; King, 1953). Sim- PROCESSES AND CLASSIFICATION investigation. ilar concavity and convexity of homoge- Profiles of active sea cliffs appear to be neous sea cliffs can be attributed to relative Sea cliffs undergo three main stages: (1) controlled by two major agents and three rates of erosion by subaerial and marine active—cliffs that consist of bedrock ex- major characteristics of the rocks being agents. In fact, measurement and mathe- posed by their continuous retreat under the eroded. The two agents and their processes matical modeling of lakeshore cliffs and influence of both marine and subaerial are marine erosion and subaerial erosion. river bluffs in near-homogeneous and un- agents and processes; (2) inactive—cliffs Their control of sea-cliff profiles is more a consolidated glacial till and flood-plain sed- that are mantled, especially along their function of their relative effectiveness than iments show a gradual transition from bases, by a cover of talus having slopes of their absolute effectiveness. Marine erosion sharp-angled cliffs to broadly sigmoid about 25° to 30° and commonly supporting is accomplished at the base of the sea cliffs slopes that develop after cessation of ero- land vegetation, including trees; and (3) by abrasion, biological activity, solution by sion by waves and currents. Examples of former—cliffs that have been removed from ocean water, and quarrying of blocks. such studies are increasing (Strahler, 1950; the influences of marine processes so that Abrasion is materially increased by sedi- Schiedegger, 1961; White, 1966; Brunsden subaerial erosion rounds the crests and pro- ment (mainly sand and pebbles) carried in and Kesel, 1973; Hirano, 1975; Nash, 1980), vides material for stream deposition beyond suspension. Relatively fast marine erosion but the results are transferrable only to the bases (Fig. 2). The active stage is far produces oversteepening of the lower part those sea cliffs that are no longer influenced more complex than the others and is the of the cliffs (even undercutting or notching, by marine processes. Moreover, the studies main subject of this study. In order to reach as is common in limestone) that leads to do not address the original form of the new fields for investigation, we must assume rock falls, slumps, and other kinds of mass active cliffs, perhaps because the model that the agents, processes, and rock charac- movements. Subaerial erosion takes the

B» B-» B-a B-a B-a B-a B-a B-a B-a B-a B-a — B~a B-« B-a C-b C-b C-b C-b C-b c-b C-b C-b Cb C-b C-b — — —

NOT NOT

EXPOSED EXPOSED ""¿"\ I*

. NOT EXPOSE<>SEDD ) 5 5 [

B MEASURED STRATIGRAPHIC SECTIONS

AGUA HEDIONDA LAGOON BATIQUITOS LAGOON

O KM. 2 /'i I— —i A BCDEF6 H IJK L M NO

Figure 4. Profiles and stratigraphy of sea cliffs measured by Kuhn at intervals along Carlsbad State Beach, southern California. Most of these sections were measured during March and April 1978 immediately after rainstorms when the runoff was concentrated in newly constructed drain pipes that debouched at more or less regular intervals along the top of the sea cliff. Section consists of compact Eocene shale (Santiago formation, 5), unconformably overlain by Pleistocene marine sands, 4; and in turn by alluvium (Fluvial sands, 3; mud-flat clays, 2B; and sand-flat sands, 2A), topped by uncemented dune sands (IB) and iron-cemented dune sands (IA).

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SAN ONOFRE STATE PARK

Figure S. Coast of San Diego County in southern California. A. Coastline showing areas of sea cliffs (black). B. Subdivisions according to active marine erosion (A), inactive marine erosion (I), and no cliff erosion (N) at flood plains, lagoons, and river mouths. C. Representative measured cliff profiles at typical sites and their classification according to Figure 3.

form of gullying and rainwash at the ground surface and of slumping and other mass movements (Sharpe, 1938) induced by ground water that weights, overpressures, and expands certain clay minerals in a form of lubrication. At high latitudes; frost wedg- ing is a major factor, bringing down large blocks as well as grains. In a general way, marine erosion tends to make the base of the sea' cliffs sharply angular, whereas subaerial erosion tends to make the top broadly convex upward with short-termconcavities associated with local runoff, mass movements, or. notching by ground water. Wher- ever human construction occurs, far-reaching effects are caused by overloading, undercutting, grading and. removal of stabi- lizing soil and dune cover (including-plants POINT LOMA whose roots help to bind the material), and, perhaps most important, additions of water. The characteristics of the rocks that are most important in shaping sea cliffs- are B

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Figure 6. Profiles of sea cliffs world-wide obtained from color transparencies projected to uniform height and with ocean to left; broad horizontal line denotes height reached by waves; bedding indicated where evident; height of cliff is estimated in metres: A -a. 1. Islet off Capri, Italy, 100 m. 2. Northeastern coast Hawaii, Neogene lava flows, 300 m. 3. Perce Rock, Gaspe, Canada, vertical Devonian limestones, 100 m (from D. W. Johnson). 4. Holyhead, northwestern Wales, gneiss-schist, 15 m (from J. A. Steers). 5. Guam, Quaternary limestone, 60 m (from F. P. Shepard). A-b. 1. Hartland Quay, southwestern England, folded Mississippian flysch, 100 m. 2. Cape of Good Hope, South Africa, Paleozoic sandstones, 75 m. 3. Taguan Bay, Guam, Quaternary limestone, 30 m. 4. South side Grand Cayman Island, Caribbean Sea, Miocene limestone, 20 m. 5. South side Gibraltar, dipping Paleozoic limestones, 500 m. A-c. 1. Island east of Oman, Miocene limestone, 100 m. 2. Northwestern Las Palmas, Canary Islands, Neogene volcanic rocks, 60 m. 3. Lands End, southwestern England, jointed granite, 20 m. 4. Koko Head, Oahu, Neogene lavas, 150 m. 5. Janum Beach, northeastern Guam, Neogene limestone, 15 m. A-d. Cape Canaveral, Florida, dunes, 3 m (from D. W. Johnson). 2. Wellington, New Zealand, 200 m (may be partly fan or talus). 3. St. Bees Head, northwestern England, Carboniferous sedimentary rocks, 100 m (from J. A. Steers). B-a. 1. Punta Camales, Baja California, Mexico, Pleistocene terrace alluvium, 100 m. 2. Birchington Cliffs, south side Thames Estuary, England, soil over Cretaceous chalk, 25 m (from J. A. Steers). 3. Table Head, Cape Breton, Canada, Carboniferous sandstone over shale, 15 m (from D. W. Johnson). 4. Nauset Light, Cape Cod, Massachusetts, macadam pavement over glacial till, 10 m. B-b. 1. Reculver Cliff, south side Thames Estuary, England, soil over Paleogene limestone, 15 m (from J. A. Steers). 2. Scituate, Massachusetts, sod over glacial till, 10 m (from D. W. Johnson). B-c. 1. Cabo Colnett, Baja California, Mexico, Neogene lavas atop Cretaceous sediments atop lavas, 150 m. 2. Wellfleet, Cape Cod, Massachusetts, hardpan over glacial till, 30 m (from R. Siever). 3. Table Mountain, Cape Town, South Africa, Paleozoic sandstones, 200 m. 4. Ensenada, Baja California, Mexico, Neogene lava over tuff, 15 m (from F. P. Shepard). B-d. 1. Santa Barbara, California, soil over Pleistocene terrace alluvium, 10 m (from H. R. Wanless). 2. Gower, Cornwall, England, Devonian sandstone, 50 m (from J. A. Steers). 3. Winthrop Great Head, Boston, Massachusetts, soil over glacial till, 30 m (from D. W. Johnson). 4. Highland Light, Cape Cod, Massachusetts, glacial sands over till, 30 m (from D. W. Johnson). C-a. 1. Southwestern end Guadaloupe Island, Mexico, Neogene lavas and tuffs, 400 m. 2. Birling Gap, Dover, England, Cretaceous chalk, 30 m (from J. A. Steers). 3. Murawai, New Zealand, Miocene tuff, 10 m. C-b. 1. Southeastern end Guadaloupe Island, Mexico, Neogene lavas and tuffs, 400 m. 2. Shell Beach, northern California, Mesozoic gneiss (from F. P. Shepard). 3. Cwm yr Eglus, southwestern Wales, Silurian sedimentary rocks, 40 m (from J. A. Steers). C-c. 1. Scientist Cliffs, Patuxent, Maryland (Chesapeake Bay), Miocene shale, 5 m. 2. south end San Lorenzo Island, Mexico (Gulf of California), Miocene tuffs, 40 m. 3. Cape Thompson, Alaska, Permian dolomite, 300 m. 4. Culver Cliff, Isle of Wight, England. Carbonifer- ous shale over greensands, 50 m (from J. A. Steers). C-d. 1. Yaquina Bay, Oregon, Miocene shales over sandstones, 15 m (from J. V. Byrne ). 2. Cape Thompson, Alaska, solifluction mass atop Permian dolomite, 15 m. 3. Martha's Vineyard, Massachusetts, soil over two(?) glacial tills, 8 m (from C. A. Kaye). 4. Teignmouth, southwestern England, Permian sandstone, 7 m.

homogeneity, structure, and local pre-cliff resistant strata at the top (for example, cliffs cliff instability (Emery, 1960, p. 20, 312; topography. Many sea cliffs are homogene- capped by a thick bed of iron-cemented Zenkovich, 1967, p. 297-316; Shepard and ous, consisting entirely of just one kind of sand or gravel or by a lava flow). Still others Wanless, 1971, p. 260, 276, 358, 369), but rock or unconsolidated sediment whose re- contain several resistant layers. they are not the subject of this classification sistance to weathering and erosion is nearly Structure of the rocks being eroded because they are very site specific. On the uniform from the bottom to the top of the .relates here to the major directions of struc- other hand, beds that have slid become cliff. For some cliffs, the homogeneous tural weakness with respect to the free face fragmented and less resistant than the origi- material (alluvium, poorly cemented eolia- of the sea cliff. The structural directions nal seaward-dipping strata to further sea- nite, or glacial till) can be eroded easily, may be those of stratification, joints, or cliff erosion (somewhat like glacial till), and whereas for others, it can be fairly uniform faults accentuated in some regions by dikes, their further erosion can be part of this but resistant rock (such as unweathered veins, or foliation. Where these discontinui- classification. granite, thick basalt flows, or dense meta- ties dip landward, they produce little insta- The nature of the topography being morphic types). Some sea cliffs have their bility of the sea cliff, but where they dip eroded at the sea cliff controls the initial most resistant rocks at the base, with much oceanward, they can become the soles of profile landward of the cliff. Simplest is less resistant rock above (for example, cliffs large landslides that cause the final slope of cliffing of a flat plateau, but probably most having well-consolidated ancient sedimen- sea cliffs to approach the dip of the structu- sea cliffs are cut into land forms that are tary or other rocks overlain by alluvium or ral weaknesses. Such landslides are the most sloping and irregular through former ero- glacial till). Other sea cliffs have their most spectacular and far-reaching results of sea- sion by streams or glaciers or shaping by

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1/ 2/ 31 4/ 5,

A-a B-a C-a

A-b B-b C-b

A-c B-c C-c

A-d B-d C-d

volcanic activity or previous landslides. sion interrupted by climate change is being developed, and it will be illustrated by This variety merely means that sea-cliff ero- resumed (Rudberg, 1967), first by removal examples. This deductive approach is sion is destroying land forms that were pro- of talus. The period of lapse in activity can deemed simpler than that of reproducing duced by entirely different sets of agents be a few to hundreds of millions of years large numbers of sea-cliff profiles and arriv- and environments. In simpler circum- where sea cliffs are being exhumed (rare), or ing at the models by induction, and it takes stances, previous topography produced by it can be a few years to centuries where cliff- advantage of long past experience of the an earlier cycle of sea-cliff erosion inter- ing is being renewed. authors in viewing and thinking about sea rupted by land movement is being exhumed A model illustrating the most common cliffs. The model (Fig. 3) is in the form of a (Wood, 1962; Cotton, 1969), or marine ero- variations in profile of active sea cliffs was matrix whose lateral divisions are for differ-

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ent degrees of rock homogeneity, and whose Closely spaced profiles and detailed strati- development of most of these sea cliffs is vertical divisions denote relative effective- graphic sections were measured at mostly undocumented or poorly documented (to ness of marine versus subaerial erosion. For 100- to 200-m intervals along a coastal unit our knowledge), and our classification of simplicity, the model assumes that the struc- about 5 km long where alluvium lies uncon- them is less certain than of those in Figures ture is not important and that the region formably atop compact Eocene shale (Fig. 4 and 5. Nevertheless, the profiles of Figure being eroded is a plateau. Readers can infer 4). The sequence illustrates variations of the 6 serve to show the presence throughout the the different sea-cliff profiles caused by matrix class B-a because of the presence of a world of sea cliffs that represent all twelve oceanward-dipping zones of structural cemented sand layer at the top of the cliffs, classes of the matrix (Fig. 3). Variations weakness, the presence of resistant rocks at but it also illustrates class C-b because of within any given class result partly from mid-cliff height, and the erosion of hilly or the resistant shale at the base of the cliffs. intentional inclusion of sea cliffs that have a mountainous topography. The model ig- Other representative classes of the matrix wide range of heights and rock types. Gentle nores the presence or absence of low-shore are illustrated in a longer (120-km) stretch slopes of two high profiles in matrix class platforms and nips (intertidal undercuts), of coast in southern California (Fig. 5). A-d may indicate inclusion of some former considering them as transient features pro- These cliffs range from inactive (where talus sea cliffs now highly modified by subaerial duced by marine erosion in shaping the masks the cliff base), to classes A-a to A-d erosion and deposition, as in Figure 2. entire sea cliff. (where the cliff consists of fairly homogene- For many reasons, one cannot expect sea ous sediments), to C-c (where subaerial ero- CHANGES IN PROFILES cliffs to have the same profiles throughout a sion is about equal to marine erosion), and large region, particularly after a change of to C-d (where human-induced subaerial Changes in the shapes of active sea-cliff marine conditions. For example, if relative erosional processes at present are more profiles can be caused by both natural and sea level rises, low sea cliffs respond and effective than marine erosion). Seven differ- human-induced events. For example, slid- reach a new equilibrium more quickly than ent classes are presented in Figure 5. ing of sea cliffs in Oregon (Byrne, 1963), do high ones. Similarly, sea cliffs attacked Some classes of the matrix do not occur southern California, and elsewhere is more by large waves should respond more quickly or have not been identified in southern Cali- frequent during winter months of higher than those having smaller waves—for ex- fornia. Examples of them are included in precipitation and higher ocean waves than ample, opposite coasts of Japan and of Eire. profiles of Figure 6 compiled on a world- during the rest of the year. Water-table Even with stable sea level, differences in wide basis from photographs mostly by measurements at Encinitas (Fig. 7) recorded exposure to large waves at the front and Emery, but supplemented with others from an irregular drop between 1967 and 1972, back of large cliffed coastal embayments publications of several geologists. Historical with few landslides in the nearby sea cliff. may produce active sea cliffs at the fronts and periodically inactive ones at the more UJ sheltered backs of the embayments (exam- > ples exist in the semicircular bays of the HI -I Palos Verdes Hills near Los Angeles, 330 - o around Japan, and elsewhere). 300 - oc H 270 - PROFILES OF LU EXISTING SEA CLIFFS 2 240 - O N 210 - Tests of the matrix (Fig. 3) come from its UJ application to existing sea cliffs. Three lev- 180 - Q. BORING no. 3 els of application were used. Most detailed il o 150 - II BORING no. 7 • in treatment is a short section of coast for II BORING no. 8 • which stratigraphy and profiles were mea- Z 120 - II O sured in the field, and influences of storm 90- li I- LANDSLIDE/BLOCKFALL 1 waves, climate, and humans upon positions < OCCURRENCE and profiles of the cliffs are known for sev- 60 eral decades. Second is a longer coast hav- I- 30- ing more varied stratigraphy and profiles O 3 o- ~r 1 1 r——i r~—i 1— 1 r T T but known in less detail. Third are examples 1967 1969 1971 1973 1975 1977 1979 of sea cliffs throughout much of the world, 1968 1970 1972 1974 1976 1978 1980 even though we know little about local cliff history. YEAR Examples for the first and second levels Figure 7. Water tables in three borings at Encinitas, California (location of Encinitas of application are from southern California, shown in Fig. 5), and dates of landslides or block falls from nearby sea cliffs. Borings are 50 where knowledge has been accumulated by to 70 m apart, 10 to 26 m deep, and 20 to 70 m from edge of sea cliff. (Data from Self both authors and especially by Kuhn. Realization Fellowship, Encinitas.)

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Figure 8. Rainfall measurements and estimates for southern California. A. Rainfall at Julian, 1880-1980. B. Combined-core varve thickness index for Santa Barbara Basin expressed in standard unit form. Generally negative unit values from 1824 to 1859 reflect bioturbation. (From Soutar and Crill, 1977, Fig. 7, p. 1166.) C. Standardized tree-ring indices for Pseudotsuga macrocarpa big cone spruce trees along steep slopes of Santa Ana Mountains (elevation 1,214 m). (From Douglas, 1973; 1976, p. 188.) D. Winter sea "surface" temperature 5 m below the surface at La Jolla (reconstructed from tree-ring data). (From Douglas, 1976, p. 188.)

Note the sharp drop in water table that fol- lowed a large slide very near the three borings in 1969-1970. Beginning in 1973, extensive urbanization inland of the sea cliff accompanied by watering of lawns, effluent from cesspools and septic-tank leach lines, and other water losses produced a marked rise of water table and more frequent landslides. A similar high correlation of high water tables and slides was reported by Hutchinson (1969) from his studies in England. Climate is known to undergo long-term variations of wet to dry (Fig. 8), cold to hot, and calm to stormy periods. Probably most such variations simultaneously affect the rates of erosion by both marine and subaerial processes. The record of rainfall in southern California is complete for only the past century, during which there were periods of about 6 years' length superimposed upon longer periods of perhaps 50 years (Fig. 8A). During the next previous century (1780-1880), only exceptionally wet or dry years were recorded, and so we have attempted to estimate rainfall during that century. The most direct evidence is from thicknesses of varves and tree rings. Sedi- ments on the floor of the Santa Barbara Basin off southern California contain varves whose main component is detrital silt and clay brought by rivers in flood. The 1960 1980 thicknesses (Fig. 8B) correspond at least I roughly to the rainfall that produced the to carry the cyclic pattern of tree-ring thick- 1924-1940 and 1949-1972, obtaining trans- floods. Tree rings tend to be thicker during nesses in western North America back to fer functions that allowed estimates for wetter years, and those for big cone spruce the year 1385 A.D. Douglas (1976) com- water temperatures (Fig. 8D). Because (Pseudotsuga macrocarpa) of the Santa pared his own tree-ring data back to 1671 warm water in the region is linked fre- Ana Mountains in southern California are A.D. with measurements of regional aver- quently with increased rainfall, Figure 8D shown in Figure 8C. In a different and less age/ocean temperature at 5-m depths off corresponds reasonably well with Figures sophisticated study, Ganus (1977) was able southern California and Baja California for 8A and 8B.

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Abundance of pelagic fish and atmos- plunging cliffs (whose bases are in deep the presence of a talus cover. Active sea pheric dust from large volcanic eruptions water; Cotton, 1952) and completely sub- cliffs have exposed bedrock whose vertical are less closely linked with local rainfall. merged cliffs and terraces (Emery, 1960, p. variations in steepness reflect (1) the degree Past abundances of pelagic fish were ob- 34-38; and many others). World-wide low- of homogeneity of the materials that are tained by Soutar and Isaacs (1974) from ering of sea level caused by removal of being eroded, and (2) the relative roles of counts of fish scales in the same Santa Bar- water during glacial epochs generally is marine erosion versus subaerial (including bara Basin cores on which varve thicknesses slower, as is rise when the glaciers melt. In ground-water) erosion. The activities and were measured. Their plots exhibit a general the very long term are changes in sea level relative roles of marine versus subaerial ero- correlation of more fish with more rainfall. resulting from variations in the width and sion are controlled by tectonic changes in A last indicator is that of dust veil in the height of mid-ocean ridges caused in turn by land level, eustatic changes of sea level, atmosphere produced by large volcanic changes in rates of sea-floor spreading (Vail variations of climate and weather, and local explosions within human experience or and others, 1977). Probably sea cliffs form- human activities. recorded in sediments. Dense veils reflect ed rapidly but were not as high as at present Tectonism, sea-level changes, and cli- sunlight, usually reducing the temperature during the time that sea level rose from at matic changes are not in phase with one at the Earth's surface (Lamb, 1970; Bray, least 100 m to about 5 m below the present another; moreover, even climates do not 1974, 1978; Ninkovich and Donn, 1976; level between 15,000 and 5,000 yr ago (a change simultaneously at different latitudes Stommel and Stommel, 1979) and possibly mean rate of~10 mm/yr). The erosion and regions. Therefore, we cannot expect increasing general storminess. Although probably slowed but produced the present that sea cliffs everywhere are simultane- these dust-veil effects are widespread, they higher sea cliffs when the rate of sea-level ously active or inactive. This means that sea do not necessarily affect southern Califor- rise subsequently declined to a mean of only cliffs are in different phases of activity from nia; moreover, they occur at irregular and about 1 mm/yr. Present tide-gauge records region to region. Even a single given region unpredictable intervals. appear to indicate renewed rapid rise of such as southern California contains both Taking the qualitative data in Figure 8 perhaps 10 mm/yr during the past decade active and inactive sea cliffs, and, of course, into consideration, one can detect some (Emery, 1980); if true, this change may sea cliffs have profiles that differ because of general parallelism of indicators of past explain the observations by Bird (1980) and stratigraphic and structural differences in rainfall for southern California. The periods others that the world's sand beaches appear the rocks that are being eroded. Classifica- 1883-1892, 1934-1945, and 1978-present to be retreating, and it foretells the onset of tion of sea cliffs in just a small region (such had unusually high rainfall and runoff. a new epoch of sea-cliff erosion and retreat as shown by Figs. 4 and 5) involves more Large waves at sea during these periods at rates much faster than experienced by generalizations than can be accepted for were accompanied by substantial retreat of modern civilization and engineering prac- engineering purposes. However, the onset sea cliffs that destroyed railroad tracks and tice. of changed conditions of climate and sea coastal roads in the 1880s; oceanfront lots, Numerous human contributions to level that are regional in scope can be houses, and trains were destroyed in the changes in sea-cliff profiles lead to increased expected to affect the activity of most sea 1930s and 1940s; and railroad trestles, piers, erosion by both marine and subaerial pro- cliffs in a region in spite of differences in and houses were lost in the late 1970s (Kuhn cesses. Damming of rivers has reduced the homogeneity of the rocks and materials. and Shepard, 1979, 1980). The intervening contribution of sediment to the ocean, nar- On the basis of less information than is periods 1842-1883 (except for 1851, 1862, rowing beaches and increasing wave erosion available in Figure 8, Baker (1948) success- 1867, and 1873), 1892-1934, and 1945-1977 of sea cliffs. Erosion has partly been coun- fully predicted the onset of the latest had generally lower rainfall, less runoff, terbalanced by local construction of sea drought period, and Ganus (1977) predicted lower ground-water tables, and probably walls and riprap barriers. Home construc- a coming wet period in southern California. smaller waves. At least, the later dry periods tion atop the cliffs and on their faces also We concur in the belief that a wet period of were times of greater stability of sea cliffs, has increased subaerial erosion through at least several years is approaching and except where human construction was espe- construction of storm drains, fences, and that it can cause drastic increases in sliding cially active. As a result, the sea cliffs gener- stairways; removal of cover; oversteepen- of sea cliffs, especially because it is to be ally consisted of freshly exposed rock and ing; overloading; and both accidental and augmented by faster rise of sea level and strata during the wet periods, and they purposeful release of water onto and into increased human use of cliff tops. became covered with talus during the dry the cliff material. Only partial compensa- We hope that this analysis of sea cliffs periods. tion can be achieved by local provision of may help geologists recognize the agents Another natural change in regimen re- drains and gutters; in fact, many examples and processes that influence different coasts sults from relative changes of sea level. are known of actual increased local erosion and help engineers identify causes and Examples abound of sea cliffs uplifted far caused by such protective measures. devise remedies needed for local and tem- above the reach of the ocean, whereby porary stabilization of sea cliffs. Even more, marine erosion is completely replaced by CONCLUSIONS we hope that the general public will realize subaerial erosion and deposition. Raised that sea cliffs are inherently unstable before terraces are produced in this way. Converse- Analysis of profiles of sea cliffs allows investing life savings in homes on the cliffs. ly, depression of the land has produced ready identification of inactive sea cliffs by In the end, of course, the sea cliffs belong to

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the ocean, and the ocean eventually will Oregon, in Clements, T., ed., Essays in forms to past sea levels: Borea, v. 1, p. 1-25. reclaim its property. marine geology in honor of K. O. Emery: 1973, Rate and mode of rétrogradation on Los Angeles, California, University of South- rocky coasts in Victoria, Australia, and their ern California Press, p. 11-33. relationship to sea level changes: Borea, v. 2, ACKNOWLEDGMENTS Cotton, C. A., 1951, Atlantic gulfs, estuaries, p. 143-171. and cliffs: Geological Magazine, v. 88, Guilcher, A., 1954, Morphologie littorale et sous- This work was not specifically funded, p. 113-128. marine: Paris, France, University Press, 216 but appreciation is due the Ocean Industry 1952, The erosional grading of convex P- and concave slopes: Journal of Geography, Program of Woods Hole Oceanographic Hills, E. 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