284 N. S. SHALER — BROAD VALLEYS OF THE CORDILLERAS 



begun, the}" could not have instituted the deposition. We have now to 

 consider the third of these possible sources of the change in conditions, 

 as we shall find it provides a more adequate explanation than the others. 



As is well know^n, in a normal river valley the primary agents of ero- 

 sion deliver to the streams no more broken-up rock than can be con- 

 vej'ed by the torrents to the lower ground and sent away in solution, in 

 suspension, or by working along on the bottom of the rivers. On the 

 substantial completion of this transportative process the access of the 

 eroding waters to the bed rocks depends. In so far as the eroded ma- 

 terial halts in its downward course, the work of erosion is lessened or 

 arrested. In almost all valleys there are local and temporary stoppages 

 of the debris, each of which dimhiishes the cutting action of tlie streams, 

 but these halts in the downward course of the waste commonl}^ are not 

 long enduring, nor do they more than qualify and lessen the erosive 

 work. If, however, from any cause, the amount of debris sent from the 

 heights to the vales is greatly increased or the capacity of the river to 

 bear it away is lessened, then the valley receives more detritus than can 

 be borne away. It is, indeed, evident that in most river valleys, as dis- 

 tinguished from the channels of torrents, the amount of addition of de- 

 tritus required to effect the change from degradation to aggradation is 

 but small, for rivers normally are so adjusted that with the existing slopes 

 they are able to do no more than keep their channels clear. If the}^ cut 

 down more rapidly than the summits descend, the slopes stee[)en and 

 the quantity of debris is increased ; if by excess in the supply of debris 

 the}" cut more slowly, then the divides become lowered more rapidly than 

 the stream beds, the declivities of the torrents are thereby lessened, and 

 the delivery of detritus to the center of the valley goes forward with less 

 rapidity. The question now before us is as to the ways in which this 

 delicately adjusted state of a normal river may be so interfered with that 

 it can no longer keep its way free. 



The most immediate cause of clogging of a valley is to be found in 

 the sudden importation of waste by other than torrent action. Thus 

 glaciers or landslides may sweep detritus from the high country and 

 bring, about the embarrassment of drainage which is noted in the New 

 England district and elsewhere. Again, showers of volcanic ash or more 

 locally lava flows may fill the trough to a great depth. In the field we 

 are considering glacial action has had no significant })artin the accumu- 

 lation of debris in the valleys except it may be in certain limited fields, 

 as in central Colorado. On the other hand, the outbreaks of volcanic 

 materials have undoubtedly done much in the way of accumulating 

 materials which the streams were unable to remove. Where the igneous 

 matter is in the form of lava the blocking of -the valley may be so com- 



