THE WORK OF RUXXIXG WATER. 



127 



a stream i.t any other position. But in the case of inchnecl strata a 

 new element enters into the prob- 

 lem. AVhen the stream flows par- 

 allel to the strike, the valley 

 which is in process of deepening 

 is not smik vertically, but is 

 shifted more or less in the direc- 

 tion of the dip (Tig. 107). This 

 is called monodinol shifting. The 

 result is that there is a constant 

 tendency to midermine Tsap) the 



vallev blUi7 on the down-dip side, Fig- 106. — Diagram to illustrate the various 



and this process of sapping T^dlL relations a stream may sustain to the 



.. . . 1 outcrops of mclmed lavers of rock. 



accordmg to its rate, accelerate 



the growth of the vahey, especially in width. ]\Ionoclinal shifting 

 is favored by the presence of a hard layer (H), as shoT^ii in Fig. 107, 

 if this stratum is the bed of the stream. 



In the second and third cases mentioned above, the only difference 



is in the angle at which the cmTent strikes the outcropping edges of 



f£ a h layers and laminse. The mechanical 



advantage is T\ith the stream which 



flows T^ith the dip. In the fomth 



and fifth cases something ^ill depend 



on the angle which the stream's 



Fig. 107.— Diagram to iUustrate mono- com^se makes with the Strike. In 



clinal shifting. The valley abc, as aU these cases, as in those where the 



seen in cross-section, becomes dch, as ^^^^^^ ^^^ vertical, much will depend 



the stream lowers its channel. . ^ 



on the thickness and re "istance oi 



the layers and on the strength of the currents concerned. 



The Inf-uence of Climate. 



Climate has both a direct and an indirect effect on erosion. Its 

 direct influence is tlirough precipitation, evaporation, changes of tem- 

 peratui'e, and wind; its indirect, tlirough vegetation. Like decli^dty 

 and rock structure, climate does not affect all elements of erosion equally. 



The chief elements of climate are temperature, moisture, and at- 

 mospheric movements; the principal factors which influence it are 

 latitude, altitude, distance from the sea^ direction of prevailing -^-inds, 

 and topograpliic relations. 



