Ch. 1] EROSION 7 



low depth with respect to width results in a different profile of erosion. 



The principal erosive effect in streams is scour of the banks and 

 the bottom. Erosion in the sea and in lakes is due principally to waves 

 and currents, and to a minor extent to chemical solution. The abra- 

 sion of rocks along the shore or on the bottom in shallow water is 

 caused chiefly by the impact of sand and rock particles carried by mov- 

 ing water. Loose or slightly consolidated sediment may be scoured 

 by wave action or by ocean currents. 



Wind erosion is similar to water erosion. All kinds of earth materials 

 are worn away by impact of particles blown against them by the wind. 

 Constituent particles of loosely consolidated sediments are also plucked 

 as wind blows over them, provided that the wind is strong enough and 

 the sediments are sufficiently dry (Bagnold, 1941). 



Ice flows plastically, much like a viscous body of tar. The essential 

 factors affecting its movement are: the mass of ice; the gradient, par- 

 ticularly the surface configuration over which it flows; and the distri- 

 bution of temperature throughout the ice (Thwaites, 1941). The rate 

 of movement of ice and hence the rate of erosion are accelerated by 

 increased gradient of land and addition of snow to the upper parts of 

 the mass of moving ice. As the plasticity of most solids increases 

 with increase in temperature, it could be argued that ice would flow 

 faster if its average temperature was relatively near the freezing point 

 of water. 



Ice acts as an agent of erosion in the following ways: Rock frag- 

 ments frozen in the ice abrade the rock or soil over which the ice 

 flows. Ice plucks rock fragments as it passes over them. It also 

 pushes masses of rock and earth ahead of it as it moves. Rocks are 

 disrupted by alternate expansion and contraction as the temperature 

 of the ice varies or as the ice melts and refreezes in cracks or crevices. 

 Ice is reported (Blackwelder, 1940) to increase materially in hardness 

 as the temperature decreases, and therefore ice alone, without the aid 

 of rock fragments, can scour soft sediments and rocks. 



Volcanic explosions can disrupt rock fragments as the explosive 

 gases and masses of lava and rock are forced upward by the explosion, 

 but the erosion of material in this way is of minor import, as active 

 volcanoes are relatively few in number. 



Where slopes are steep or smooth, gravity can be an agent of erosion ; 

 but generally it is only a secondary agent, because the stability of the 

 rock fragments must first be upset by some other agency so that frag- 

 ments can become unbalanced and thus slide or roll down the slope. 



Living matter is an important factor in erosion, but its principal 

 effect is indirect, because the living material influences the erosive 



