44 



Lithified Coasts: Cliffs and Platforms 



Rocks and sediments that are semi-consolidated or consolidated may form 

 vertical or steep cliffs that constitute a marked break in slope between the 

 hinterland and the shore. If marine processes are given adequate time, the 

 cliff may be fronted by a gently sloping shore or intertidal platform where 

 debris may rest (Figure 17). The slope and recession rates of the cliff faces 

 depend on a number of terrestrial and marine processes, as well as the 

 geotechnical properties of materials comprising them, including grain size and 

 degrees of consolidation. Recent volcanic or loosely consolidated Quaternary 

 sediments show the greatest erosion rates (Sunamura 1983). 



Marine processes remove cliff materials directly by attack at the cliff base, 

 and indirectly by undercutting, causing failure or mass wasting of the overly- 

 ing rock. The debris produced by these processes may then be transported by 

 a variety of marine processes. Resulting accumulation, erosion, or cliff 

 modification depends on die relationship between the relative rates of supply 

 and removal at the shoreline (Pethick 1984). 



If marine processes are capable of removing slope debris much faster than 

 the rate of debris supply, then, in most cases (depending upon the structure 

 and lithology of the rock) the slope will retreat parallel to itself. When the 

 supply of debris far exceeds the capacity for removal, basal debris will 

 accumulate at the angle of repose (the maximum angle of slope at which loose 

 cohesionless material rests). Between these extremes, a variety of slopes 

 ranging from debris angle of repose to vertical faces are possible. 



Typically, profiles of steeply sloping shorelines are highly reflective, 

 whether the materials are consolidated or unconsolidated. Currents are shore- 

 parallel and unidirectional, and bars are generally absent. Longshore sediment 

 transport rates are high, and most materials are moved away by currents. 

 Thus, along cliff coasts, the rates of supply must generally exceed those of 

 lower slope coasts to produce beaches. 



As with beaches, lithified coasts show irregular shorelines. Headlands and 

 bays may be related to the submergence of a hilly or mountainous topography 

 by a rise in sea level. Differential erosion and weathering may also lead to 

 the development of alternating headlands and bays on rocky cliff coasts. Once 

 formed, the presence of prominent headlands on rocky coasts influences waves 

 and tides, sediment dispersal and deposition, and shoreline evolution. 

 Headlands influence refraction, causing wave ray convergence. Recent 

 research has shown that headlands may protrude into tidal flows, causing tidal 

 eddies, and in some cases providing a stagnation zone where offshore shoals 

 can form. These, in turn, can alter the wave energy environment, creating 

 nonuniform wave attack, and altering tlie spatial patterns of cliff erosion 

 (Carter 1988). 



Chapter 3 Variable Coastal Features 



