286 N. S. SHALER — BROAD VALLEYS OP THE CORDILLERAS 



to the extent of a hundred feet or more, the air is drawn down after it. 

 The most important effect of the coating of vegetation is in protecting 

 the surfiice from temporary torrents. An ordinarily firm mat of roots, 

 even those of the grasses, w^ill usually defend the earth from the heaviest 

 streams that flow over it during the hrief period when it is called on to 

 withstand such an attack, while if not thus protected the earth would 

 be excavated to the depth of several feet; furthermore, even where by 

 chance a torrential rain manages to sweep a ])art of the protecting plants 

 awa}'', their debris is pretty certain to so far embarrass the work of the 

 torrent that its load of detritus is likel}^ to be in large part arrested on 

 the wa}^ to the main river. It is, indeed, safe to say that the erosive 

 eff'ect of a given amount of water descending a torrent-making slope 

 diminishes in a ver}' rapid ratio with the increase in the thickness of the 

 mat of vegetation. A virgin forest of the Appalachian type will, if its bed 

 be fairly dr}^ detain a rainfall of as much as 3 inches falling in, say, one 

 hour, and distribute the discharge over a day or more. Even the pre- 

 cipitation of the heaviest " cloud-burst" of the Cordilleras would be so 

 far hindered in its downward movement that it would have little de- 

 structive effect. 



Where, as in nearly all parts of our Cordilleras, local rainfalls not in- 

 frequently occur to an amount of several inches an hour the scanty 

 vegetation that prevails in that area is incompetent, in any large meas- 

 ure, to restrain erosion. The result is that a slieet of water sweeps down 

 the steeps with such volume and speed that the loose material up to 

 fragments a foot or more in diameter are swei)t into the torrent channels 

 or, if the discharge be into a broad valley, far out on its slopes. I have 

 never had an opportunity to observe a good example of these interest- 

 ing floods, but from the accounts I have had of them from trustworthy 

 observers and from an examination of their effects I am satisfied that 

 the amount of debris which is by their action discharged into the broad 

 valleys is very great, and that the process is one that has hardl}^ any 

 likeness in regions of considerable and unifortn rainfall. 



Although the contribution of debris to the detrital slopes of the val- 

 leys, as may be readily seen in the field, comes to them in part directly 

 from the action of torrential rains, carrying the debris out onto their 

 surfaces, the larger portion of the eroded material is delivered by way 

 of the permanent torrent channels and dis])osed as detrital fans at their 

 mouths. If the points of discharge of these torrents were ])ermanently 

 fixed, the debris thus brought into the main valleys would be heaped 

 into a series of these fans, while in the spaces between the gulches there 

 would be only the waste brought down from the steeps that face the 

 valley — debris which did not pass through the torrent channels. In 



