III. SEDIMENTATION 



21. BEACH AND NEARSHORE PROCESSES 



PART I. MECHANICS OF MARINE SEDIMENTATION 



R. A. Bagnold 



(Foi' list of symbols and definitions see p. 550) 



1. Definition, Origin and Relevant Physical Properties 



A sea sediment, as distinct from native rock or from a coral accretion, may 

 be defined as solid granular matter heavier than the sea-water, the separate 

 pieces or "grains" of which, irrespective of their size, are or were at one time 

 loose upon the sea bed or are in the process of falling through the sea. 



Strictly, sedimentation denotes the process by which the grains of a sediment 

 attain their ultimate resting places, or become "deposited". Since this process 

 in general extends over a period of time during wliich the grains are being 

 moved from place to place, sedimentation in this sense is inseparable from 

 sediment transportation. But the term sedimentation is often used in the 

 rather different geological sense to denote the study of the composition of 

 existing sedimentary deposits in their ultimate static state, by mechanical, 

 chemical and biological analysis, for purposes of classification. Here again, 

 liowever, any such classification which excludes from consideration the dynamic 

 process by which the composition has been brought about must remain to some 

 extent arbitrary. 



Grains of sea sediment may be classified as follows according to their origin. 



a. Terrigenous. 



(i) Introduced by rivers and by polar glaciers, 

 (ii) introduced by winds, 



(iii) eroded by the action of sea waves on shores, 

 (iv) disintegrated as the debris of marine avalanches. . 



b. Cosmic. Introduced through the atmosphere as dust from extra-terri&rial 

 sources. 



c. Biogenous. Created as the debris of marine organisms. 



d. Hydrogenous. Created as mineral nodules by crystallization from solution, 

 e.g. manganese nodules. 



e. Volcanic. Introduced as products of submarine vulcanism. 



The size of sediment grains ranges from that of boulders, originated as a(i) 

 and (iii) above, to microscopic particles. Both the size and density affect the 

 dynamic process of transport in or by the sea-water. The effective density, ps, 



[MS received October, 1960] 507 



