148 GEOLOGY 



The gravel rolled along the channel wears even solid rock, ami 

 as the moving stones wear the stream's bed, they are themselves 

 worn by impact both with the bed and with one another, and arc 

 reduced to rounded, water- worn forms. The particles broken off 

 may make grains of sand, or, if very fine, particles of silt or mud. 

 In the course of time the pebbles and cobbles rolled along may be 

 literally worn out. 



The sediment carried in suspension, as well as that rolled along 

 the bottom, wears the rock bed of a stream. The coarser the sedi- 

 ment and the stronger the current, the greater the wear. Contem- 

 poraneous processes are largely concerned in making valleys wide, 

 but the depth of valleys cut in solid rock is chiefly the result of the 

 impact and friction of the sediment in transportation. The gravel, 

 sand, and mud carried by a stream are therefore the tools with 

 which it works. Without them it is relatively impotent, so far as 

 the abrasion of solid rock is concerned; with them, the stream 

 may wear any rock over which it passes. 



Swift and slow streams corrade their valleys differently. The 

 erosion of a swift stream is chiefly at the bottom of its channel. 

 The sluggish stream lowers its channel less rapidly, while lateral 

 erosion is relatively more important. The result is that slow 

 streams increase the width of their valleys more than the depth, 

 while swift streams increase the depth more than the width. It 

 follows that slow streams develop flats, while swift ones do not. 

 Not only is a slow stream more likely to have a flat, and therefore 

 a better chance to meander, but it is more likely to take advantage 

 of opportunities in this line, for a slow stream gets out of the way 

 for such obstacles as it may encounter, while a swift stream is 

 much more likely to get obstacles out of its way. 



Corrosion (Solution, etc.) 



In most cases the solution (and other chemical changes) eff< 

 by a stream is much less important than its mechanical work. 

 Only when conditions are unfavorable for the latter is solution the 

 chief factor in the excavation of a valley. This may be th< 

 where a stream's bed is over soluble rock, such as limestone, and 

 where the stream is clear, or its gradient so low that its current is 



