22 



and may result in local horizontal transgressions or regressions of sea level or 

 vertical submergence or uplifting of terraces. 



Tectonic movements may be so slow that they have little influence on 

 geomorphic variability. On the other hand, they may occur as dramatically 

 and suddenly as in the Great Alaskan Earthquake of 1964, which caused 

 upward displacement of 10 m in places, and downward displacement of 2 m 

 in others, over a period of only a few months (Hicks 1972). In general, these 

 movements tend to be greatest at margins of tectonic plates and, in particular, 

 at converging plates. 



Consolidation and dewatering, which cause a reduction in volume, are 

 particularly prominent in areas which experience rapid sedimentation, such as 

 the Mississippi Delta of southeastern Louisiana. Surface and subsurface sedi- 

 ments, which are poorly consolidated and include organic-rich materials, may 

 experience very high rates of compaction. In general, the rates of sediment 

 consolidation decrease with increasing depth. They also decrease with 

 increasing age, especially in environments of recent sedimentation. 



Unloading and loading of the lithosphere by glacial ice and water may also 

 produce vertical and horizontal movements, further complicating the history of 

 relative sea level changes along coastlines. Isostatic depression of the litho- 

 sphere, which occurs with glacial advance over the continents, is proportional 

 to the ratio between the density of ice and that of the mantle (Andrews 1974). 

 Isostatic or crustal rebound follows deglaciation. Rates of uplift, estimated 

 from raised shorelines, may exceed 20 mm/yr (Smith and Dawson 1983). 

 Although much slower rates of isostatic compensation are attributed to ero- 

 sional unloading, these rates may be sustained over longer periods. 



Lithology and Weathering 



The nature of rocks and sediment deposits is of great importance in 

 determining the inherited morphology and the development and modification 

 of coastal features. The lithologic factor having the greatest effect on coastal 

 features is the degree of consolidation. This governs the ability of rock 

 material to be eroded and transported, which, in turn, influences the form and 

 stability of geomorphic elements occurring in the coastal zone. 



Consolidated rock coastlines are often hilly or mountainous, with the 

 exception of reefs, which are generally low-lying. The inherited morphology 

 is usually prominent, with erosional features being more numerous than 

 depositional features. Some geomorphic variability is attributable to rock 

 type, interbedding, jointing, and dip and strike of strata (Figure 7). Temporal 

 geomorphic variability is not as great as on unconsolidated coasts because con- 

 solidated rocks are highly resistant. However, in some places, rocky coasts 

 may be modified by depositional features. These include locations where 



Chapter 2 Relevant Processes and Factors 



