8 trask. PRINCIPLES OF SEDIMENTATION [Ch. 1 



action of some other agent. Roots, however, are direct agents of ero- 

 sion when they grow in cracks and force a part of the rock to fall 

 away. The most profound effect of living material, however, is the 

 action of vegetation in inhibiting the flow of rainwash. Man, because 

 of his various uses of land and the resulting increase in soil erosion, is 

 also an indirect agent. (See Chapters 22 and 23.) 



TRANSPORTATION 



The same six agents of erosion, namely, water, wind, ice, volcanic 

 explosions, gravity, and biologic activity, also control transportation 

 but in a varying degree. With water, the principal factors that affect 

 transport are: turbulence; ratio of settling velocity to lateral motion 

 of water (currents) ; shape, size, density, and quantity of particles; 

 and movement along the bottom by saltation (jumping), rolling, or 

 undermining. (Rubey, 1938; R. D. Russell, 1939; Rittenhouse, 1944). 



Turbulence is non-linear motion of masses of water as the water 

 moves. (See Chapter 3.) Turbulence is characterized by vertical and 

 horizontal eddies which tend to keep particles in suspension. The 

 principal factors that affect transport by turbulence are the quantity, 

 velocity, and temperature of the water that is moving, the load, and the 

 shape or roughness of the surface over which the body of water moves. 

 Thus gradient of stream bed, volume of water, and presence of objects 

 that impede flow are major factors. The quantity and shape of ma- 

 terial already in suspension also influence the amount of additional 

 material that can be transported, because turbulence is related to 

 energy and a given discharge moving over a given slope can transport 

 in suspension only a given amount of material of a given size. 



Debris also is transported along the bottom of a stream. Turbulence 

 may lift particles for a short distance above the bottom, but it may 

 not be strong enough to maintain them in suspension. Consequently 

 they fall to the bottom, only to be lifted again at a later time when 

 turbulence once more reaches a sufficient magnitude. Thus particles 

 jump or roll along the bed in pulses. This process is called saltation. 

 In addition many particles are round and act like ball bearings, causing 

 heavier rock fragments that rest upon them to glide downstream. This 

 process is particularly noticeable in the alluvial fans of the desert. 



In the sea or in a lake many detrital particles are carried a long 

 distance after they enter the sea or lake. Even though turbulence 

 may not be strong enough to maintain the particles in suspension, the 

 mass of water in which the particles lie may move. The particles thus 

 are transported laterally while they settle downward through the mass 



