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



at least in part, to conditions which determine the advance of the talus 

 slopes. Whenever they come against steep rock faces, where the process 

 of erosion is likely to he rapid, the talus climbs to a steeper angle because 

 of the larger supply of detritus. It thus tends the sooner to mask the 

 elevation. Where, on the other hand, the rock slo[)es are gentle, the talus 

 grows less rapidl}'' because of the insufficient su})pl3^ of debris, and there- 

 fore does not advance so ra[)idl3^ on the barrier. Although the general 

 aspect of these slopes when seen from the distance is that of uniformity 

 as regards their u[)per margin, closer inspection shows that they vary 

 considerabl}'' in that regard, the range in some of the valleys being near 

 a thousand feet of elevation. So far as observed, they are generally 

 highest against the more precipitous faces of the mountains at whose 

 feet they lie. 



The measure of the development of the broad valleys will on careful 

 study probabl}^ be found to differ much in the several areas of the Cor- 

 dilleras of this country. There appears on rough inspection to be a dis- 

 tinct increase in the amount of the aggradation as we go southward from 

 the Canadian line, and on the whole a decrease in the amount of it as 

 the region is nearer to the Pacific coast and to the Mississippi valley. 

 In general, the ratio of the infilling seems to be inversel}'' proportionate 

 to the amount of the rainfall that now occurs in that mountain S3'stem. 

 In the Montana district the appearance of the taluses, their occasionally 

 eroded surfaces, and what can be seen of tlfe action of torrential rains 

 suggests that the accumulation of detritus is not going on at i)resent 

 with any great speed, if, indeed, it is not replaced by erosion, while in 

 Arizona the amount of debris at present brought down b}'' the Hood 

 waters is most noteworth}'. Thus in the valle}' of the Williams fork of 

 the Colorado a flood caused by torrential rains lias been observed to cover 

 an area of many thousand acres with debris to the depth of 20 feet or more. 

 On the eastern face of the Cordilleran mountain system in Colorado the 

 aspect of the taluses suggests that they are generally in process of de- 

 struction. The same condition is indicated in the l)elt.of country near 

 the Pacific ocean. 



The most complete effacement of the original valleys appears to have 

 taken place in the region known as the Mojave desert. Here the detrital 

 slopes have risen to near the tops of the ranges. The most extensive 

 benches and ])er]"iaps the broadest of the valle^^s I have examined are in 

 the basin of Hassayam})a river, near Congress junction, Arizona. The 

 least development of the infilling i)rocess, still considerable, is the val- 

 leys of northern Montana. It has been before noted that along the Pacific 

 coast the amount of debris impounded in the vallej'S is relatively small. 



In the region of the Great basin tlie [)henomena of infilling are more 



