Ch. 1] WATER 9 



of moving water. Transportation of this type is a statistical phenome- 

 non. For given size distribution of sediment and for given velocity 

 and depth of water, the average size of particles that are transported 

 decreases progressively with distance of transport, but the range in size 

 of particles found in a mass of water under any given conditions varies 

 about a mean grain size, and the average deviation in size of particle 

 from that mean grain size varies according to the conditions of trans- 

 port. The subject of transport in the sea is still not well understood, 

 but it is of such vital importance that it is now being attacked actively 

 by oceanographic institutions. 



Transport of sediment by wind follows essentially the same laws as 

 does transport by water, except that, because of its low density com- 

 pared with water, air is a less effective agent of transport (Bagnold, 

 1941; Loess symposium, 1945). However, particles are also blown 

 through the air or moved along the ground by the same principle of 

 saltation that applies to water. 



Ice transports material either within or on top of the ice or by 

 shoving masses of earth and rock ahead of it as it moves. Transporta- 

 tion by volcanism is a question of the force of the explosion and the 

 direction and strength of the prevailing wind. Fine ash can be car- 

 ried many miles, as the Katmai explosion in Alaska indicated. The 

 distance that objects will be transported under the influence of gravity 

 is governed by the angle and smoothness of the slope along which the 

 material moves, the coefficient of friction, and the amount (mass) of 

 material that moves. Organisms are minor agents of transportation. 

 Even man moves relatively little material compared with nature. 



DEPOSITION 



The most important factor in sedimentation is deposition. Once the 

 sediments have accumulated in their final resting place, their general 

 nature is fairly well formulated. Subsequent changes, for most sedi- 

 ments at least, alter their characteristics relatively little. The proc- 

 esses of deposition are complex and the products are manifold. The 

 same six agents that affect erosion and transportation likewise control 

 deposition. 



Water 



The processes of deposition in water are essentially a question of 

 energy, place, and time. If the energy available to move constituents 

 that are in the act of being transported decreases, some of the particles 

 can no longer be transported and they come to rest. The environment 



