30 



much of the geomorphic variability of clastic shorelines and provide informa- 

 tion that may assist in interpreting past processes or in predicting the transport 

 potential of sediments. 



Sedimentary particles show a range in size and are classified according to 

 diameter, assuming the particles are roughly spheroid in shape. These sizes 

 are divided into several major classes, in order of increasing diameter, being 

 clay, silt, very fine to very coarse sand, granule, pebble, cobble, and boulder 

 (Table 1). Generally, clay minerals are cohesive, being held together by 

 electrolytic forces. Coarser sediments, which make up the bulk of coastal 

 sediments, are considered noncohesive or cohesionless. 



The type of sediments found in a location depends on the source and 

 supply of materials and the energy of the environment. If the nature of the 

 source and supply are held constant, coarser sediments are deposited in high- 

 energy environments and produce beaches of steeper slope, whereas finer 

 sediments are deposited in low-energy environments and are associated with 

 beaches of gentler slope. Mixtures of differing populations of sediments are 

 common, including combinations of coarse and fine, clastic and biogenic, and 

 differing source regions. These contribute further to the geomorphic variabil- 

 ity of coasts. 



Sandy coastlines are predominant worldwide. Sand may be supplied by 

 rivers, by adjacent parts of the coasts including beaches, headlands and cliffs, 

 by offshore sources, or by wind. Silt and clay coastal sediments occur in 

 generally lower energy settings such as lagoons and back barriers. Near large 

 rivers (Wells and Coleman 1981a) and in glaciolacustrine settings, silts and 

 clays may dominate coastal materials. 



Pebble and cobble coastlines, called shingle beaches in Britain, are more 

 common in areas of glacial and fluvioglacial sediments (Bird 1969; King 

 1982), in areas where coastal rock formations yield debris of appropriate size 

 (Bird 1969), and in localized areas where rivers deliver coarse materials to the 

 shore. Beaches of granule-sized particles are rare (King 1982). 



Cobbles and pebbles on beaches have a variety of characteristic shapes, 

 including disks, rods, and spheres, which are found in characteristic zones of 

 the beach profile. The distribution of the various sediment shapes on beach 

 profiles is controlled largely by selective sorting (Bluck 1967). Contrary to 

 usual descriptions, marine abrasion appears not to be the predominant cause of 

 the disk shape, in that the largest oblate disks are found near the high tide 

 mark and thus are least worked by the sea. 



Beaches of terrigenous origin generally have quartz as the most abundant 

 mineral, accompanied by varying proportions of feldspar, mica, other light 

 minerals, and heavy minerals. Quartz is dominant because it is the most 

 abundant mineral in the earth's crust, as well as being mechanically durable 

 and chemically inert (Jackson 1970). Pebble and cobble beaches have highly 

 varied mineralogy, depending upon the nature of the source. Clay minerals 



Chapter 2 Relevant Processes and Factors 



