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Sea Cliffs: Their Processes, Profiles, and Classification

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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 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/93/7/644/3444719/i0016-7606-93-7-644.pdf by guest on 30 September 2021 SEA CLIFFS: PROCESSES, PROFILES, CLASSIFICATION 645 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 differ- ent 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. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/93/7/644/3444719/i0016-7606-93-7-644.pdf by guest on 30 September 2021 646 EMERY AND KUHN 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* .
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