Chapter 1 



DYNAMICS OF SEDIMENTATION 

 Parker D. Trask 



Supervising Geologist 



Division of San Francisco Bay Toll Crossings 



California Department of Public Works 



San Francisco, California 



Geologists and engineers need to understand sediments. They should 

 know what they are like, why they are the way they are, and what 

 factors cause them to change. The object of this paper is to sum- 

 marize the principal features of sediments, emphasizing the processes 

 that lead to their formation or that cause them to change once they 

 have been formed. As sediments are largely a product of the environ- 

 ment in which they are formed, special attention is given to environ- 

 ments of deposition. This chapter, like all other chapters in this sym- 

 posium, is a condensation of a complicated subject. A list of 100 refer- 

 ences is included for benefit of persons desiring further information 

 on particular topics. 



Sediments are aggregates of particles that come to rest in some place 

 after having been transported laterally or vertically for some distance. 

 When first deposited, the particles are unconsolidated or essentially 

 unconsolidated. The geologist calls such deposits recent sediments. 

 With time the sediments consolidate and harden into rock. Such con- 

 solidated sediments are called ancient sediments. During this process, 

 cementing material is deposited in the pore spaces; new minerals form 

 or old minerals grow, thus binding particles to one another; and pres- 

 sure of overburden compresses the sediment, causing the constituent 

 particles to interlock with one another. When rocks weather in place 

 they are altered physically and chemically and eventually are trans- 

 formed into soil. As soils or weathered rocks have many of the proper- 

 ties of recent or unconsolidated sediments, it is convenient to consider 

 them in connection with sediments. Technically they could be called 



* Prepared with aid of a grant from the University of Wisconsin Alumni Re- 

 search Foundation. 



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