6 trask. PRINCIPLES OF SEDIMENTATION [Ch. 1 



ing particles from their position of rest, and (2) abrading rocks by 

 impact of moving particles. The size of the particle, the velocity of 

 the transporting agent, and the cohesiveness of the rock or soil are 

 factors in the plucking action. Alternate freezing and thawing and 

 the growth of plants in cracks are agents in the forcing apart of rocks. 

 The size, quantity, hardness, and velocity of the moving particles and 

 of the object that is struck are factors in the abrading action. 



Water is the chief mechanical agent of erosion, but wind, ice, grav- 

 ity, volcanic explosions, plants, and animals also act. Water acts in at 

 least three different ways: (1) when running off the surface of the 

 land before it collects in streams; (2) in streams; and (3) in bodies 

 of essentially standing water, namely lakes and the ocean. Erosion 

 is intimately associated with the ability of the water to transport ma- 

 terial. If the water is fully loaded and is transporting all the material 

 of a given size it can transport, relatively few particles of that size 

 can be picked up; or, to be more precise, the net erosion of such 

 particles is small. However, when the water is underloaded, erosion 

 is effective. The erosive power, that is, the ability of water to pick 

 up particles, depends upon the energy relationships of the moving 

 water, particularly the turbulence. Turbulence is discussed in the next 

 section. 



Water moving over relatively steep surfaces of land before it reaches 

 streams acts in some ways as a thin, flat sheet (Paige, 1912). The 

 thickness and continuity of the sheet depend upon the average slope 

 of the land, the amount of rain that has fallen, the rate of infiltration 

 of water into the soil, and the interference of grass, plants, and other 

 obstacles with its free movement. If the obstacles to downward flow 

 of the water are numerous, relatively little material is eroded, as, for 

 example, in humid regions where plants are plentiful or in areas of 

 highly pervious soil which absorbs the water readily. Sheet erosion is 

 best developed in arid or semi-arid regions, and it results in a topog- 

 raphy that comprises relatively steep slopes above and more gentle 

 slopes below, where the water spreads out over a fan-shaped area be- 

 fore collecting in a stream or some body of standing water. In humid 

 areas the fan-shaped areas usually are less well developed. 



The steep slopes above seem to be related to the rate of weathering 

 of the rocks and the permeability of the soil. The flatter slopes below 

 represent an equilibrium profile determined by the average load of 

 sediment that the water is carrying. If the water is underloaded it 

 will cut; if overloaded it will deposit. The same fundamental prin- 

 ciples of transport and erosion apply to this water moving in a sheet 

 as to water flowing in streams or in the ocean, but the generally shal- 



