134 PHYSICAL GEOLOGY 



in suspension is also frequently driven vigorously against the 

 bottom by subordinate downward-moving currents, with 

 similar effect. The tools are themselves worn in the process. 

 Stream-swept stones become rounded (Fig. 123), and their 

 surfaces often have many tiny pits, or depressions, made by 

 the blows they have delivered or received. These characteris- 

 tics have helped to prove that the material of certain rock 

 formations was handled by vigorous streams. 



Rate of wear. The rate at which degrading streams lower 

 their channels depends on several conditions. (1) Weak 

 rocks with soluble cements favor rapid wear, while strong, 

 nonsoluble rocks retard it. Stratified rocks in general prove 

 less resistant than massive rocks. Other things being equal, 

 rocks with numerous joints and cracks are worn faster 

 than others, because these openings are planes of weakness. 

 (2) Rapid streams deal harder blows and more of them than 

 slow ones, and so, other things being equal, wear their channels 

 faster. The velocity of a stream, in turn, depends upon (a) 

 the slope (gradient} of its channel, (6) its volume, (c) its 

 load, and (d) the shape of its channel. Obviously, the 

 steeper the channel and the larger the stream, the greater its 

 velocity. Energy is expended in moving sediment, which 

 otherwise would express itself in greater velocity ; other things 

 equal, a given stream accordingly flows fastest if clear, and 

 slowest if loaded. A stream is retarded by friction with its 

 bed and sides. Crooked channels, with wide, uneven bottoms, 

 occasion great friction, and tend to produce a sluggish cur- 

 rent ; straight channels with narrow and smooth bottoms de- 

 velop less friction, and promote greater velocity. (3) Since 

 the velocity of a stream is decreased as its load is increased, 

 it follows that the force of its blows is also diminished. In 

 other words, the greater the number of tools carried, the 

 greater the number of blows delivered in a given time, but 

 the weaker each blow is ; while, on the other hand, the fewer 

 the tools carried, the fewer the blows delivered in a given time, 

 but the stronger each blow becomes. Clearly, streams wear 



