10 trask. PRINCIPLES OF SEDIMENTATION [Ch. 1 



in which the particles are being transported profoundly affects the 

 types of sediment that are deposited, primarily because of the effect 

 of the configuration of the basin of deposition upon the movement of 

 the transporting agent, be it water, air, or ice. Diastrophism, or move- 

 ment of the crust of the earth, is a critical factor, for diastrophism not 

 only influences the shape of the environment of deposition, but it also 

 affects the configuration of the land which supplies the debris that is 

 deposited. Time is a factor because the rate of movement of particles 

 determines whether the particles will be picked up, moved, or de- 

 posited. Furthermore, sediments, once they have been deposited, 

 change in the course of time (diagenesis) . 



The three factors — energy, place, and time — are intimately inter- 

 related. The principal source of energy comes from streams, currents, 

 wind, volcanic explosions, or from material that has acquired potential 

 energy by having been placed in a higher position than it once was, 

 by evaporation, convection, turbulence, or diastrophism. The effect 

 of environment is treated in a later section. The influence of time, 

 particularly the rate of change, is complicated. The principal effect 

 of time, especially with respect to deposition by water, is the rate of 

 change in velocity, ds/dt. To this also should be added the change in 

 direction and the rate of change in direction. In other words, deposi- 

 tion is strongly influenced by turbulence when the interval of time is 

 short and by changes in average velocity when the time is not short. 



Turbulence affects the size of particle that is laid down, that is, the 

 size distribution of the sediments (grading curve). If the average ve- 

 locity of the water is fairly constant and fluctuations in turbulence are 

 not too great, well-sorted sediments are deposited. Particles small 

 enough to stay in suspension for a significant interval of time are trans- 

 ported away from the locus of deposition, and the particles that settle 

 to the bottom are comparatively well graded in size. Consequently a 

 relatively large proportion of the particles deviate in size slightly from 

 the average particle deposited. The energy relations that govern 

 transport are too complicated to discuss here, but, even though the 

 average deviation in velocity of water is proportionately the same 

 with respect to the average velocity of the water, the degree of sorting 

 of the constituents (uniformity coefficient of the engineers) is not the 

 same. Slowly moving water seemingly leads to poorer sorting than 

 fast-moving water. Silts are rarely as well sorted as sands. 



The size distribution of the constituent particles, because of its de- 

 pendence upon the motion of the water in which the sediments accumu- 

 late, therefore is an index of the mode of deposition. If one process is 

 operating, the size distribution is more likely to be symmetrical than 



