INFILLING OF THE VALLEYS 291 



or less obscured by lake deposits. Such lacustrine conditions, though 

 they may be due to other causes, are likely to be brought about in any 

 of these valleys by the irregular subsidence of their floors under the in- 

 fluence of the loads that are brought on them. 



If we were here considering in v. general way the effect of any excess- 

 ive discharge of sediments into a river system in the manner in which 

 it has taken place in the Cordilleran district, we might much extend the 

 subject-matter of this paper. It would be interesting to note that the 

 result of the filling of valleys is to change the effective base line of ero- 

 sion of all the streams that lead down to them, and that the natural 

 completion of the process is the development of an extensive broad up- 

 land with shallow valleys where there was originally a strong mountain 

 topography. In other words, a kind of summit leveling action is then 

 taking the place of the usual baseleveling process. To this condition a 

 large part of the Cordilleran system within the United States is obviously 

 tending. 



Although in the absence of close study of the problem these impres- 

 sions concerning the increase or decrease of the talus slopes can not be 

 considered as more than foundations for inquiry, it may be well to note 

 on what they depend. Where the slopes are continuous and of even 

 surface, with no new-made channels, particularly when they gradually 

 steepen up to the walls of the mountain, there is reason to believe that, 

 while they are subjected to some interstitial decay, they are not sub- 

 jected to much surface erosion. Where, however, the crest line is sharply 

 irregular, and especially where extensive areas of gently sloping bed rock 

 are exposed, it is a legitimate working hypothesis that the upper edge of 

 the talus has been so far eroded that it has lost a part of its height. Here 

 and there in northern Montana low foothills occur which can not be ac- 

 counted for by the structure of the rocks, but seem to have been pro- 

 duced by the lessening in the height of the taluses and the consequent 

 dissection of the newly exposed bed rock. A proper determination of 

 this question as to the growth or shrinkage of the taluses under existing 

 conditions will be difficult to make. 



While the present average state of growth of decay of the deposits in 

 the broad valleys is not determinable, there appears to be good evidence 

 that there has been at least one period of the past, and that near to our 

 day, when for a time these beds were in process of rapid wasting. This 

 is shown by the general occurrence of many wide and often still deep 

 passages through the slopes where the torrents cross them. These exca- 

 vations occur in all the broad valleys I have had a chance to examine. 

 It is clear that the}^ have not been formed by the torrents in their ex- 

 isting volume, for they have no fresh scarps, and those of recent date 



