24 



Unconsolidated or poorly consolidated materials are found in low-lying 

 coastal plains and deltaic complexes. Along coasts of unconsolidated 

 materials, large amounts of sediment typically are available and morphologic 

 variations can occur rapidly. Relict geomorphic features are readily altered in 

 this environment. Depositional features are likely to be more numerous than 

 erosional features. A detailed description of coasts composed of uncon- 

 solidated sediments follows in a section on sediment character. 



Both lithology and jointing influence the resistance of a rocky coastline to 

 weathering and erosion. Additional factors are susceptibility to weathering 

 (mechanical or chemical), hardness of the constituent minerals and cementa- 

 tion, the nature and density of voids, and climatic conditions. Mechanical 

 weathering involves the breakdown or disintegration of rock without any sub- 

 stantial degree of chemical change. Examples are processes such as tempera- 

 ture change, crystallization by salt or frost, wetting and drying, changes in 

 overburden, and organic activities. Chemical weathering involves the decom- 

 position or decay of minerals because of hydration and hydrolysis, oxidation 

 and reduction, solution and carbonation, chelation, and/or biochemical 

 changes. 



In a given area, weathering or erosion may occur at different rates. 

 Various rock types may be more or less susceptible to erosion by waves, 

 tides, and currents. As a result, differential weathering and erosion may 

 produce uneven coastlines where headlands are formed of resistant materials 

 and bays occur in less resistant materials (Shepard and Grant 1947) 

 (Figure 8). Small-scale effects of differential weathering cause rocks to have 

 uneven pitting or surface characteristics. Coastal configuration at differing 

 scales may also be influenced by terrestrial agents that cause erosion, such as 

 running water, ice, wind, and groundwater. 



Organisms 



Marine organisms may play either a destructive or constructive role in the 

 formation of coastal sediment deposits. Some organisms, including species of 

 algae, mollusks, echinoids, worms, and sponges have the ability to bore into 

 rock for protection against predators or to obtain anchorage on the bottom. 

 The resulting weakening and breakdown of the rocks makes them more 

 susceptible to wave erosion. Physical and chemical weathering generally 

 break down and dissolve organically produced or biogenic materials more 

 rapidly than clastic sediments. 



Biological activity also plays a constructive role in the formation of coastal 

 sediment deposits. Although biogenic materials are more easily destroyed 

 than terrestrial elastics, new sources are continually being provided. Many 

 organisms that inhabit the submerged part of the coastal zone contain hard 

 parts, usually composed of calcium carbonate. Upon the death of the 



Chapter 2 Relevant Processes and Factors 



