INFILLING OF THE VALLEYS 293 



tion, for the reason that at only a few points have I been able to find 

 morainal deposits lying upon the talus slopes in positions where the his- 

 tory of channels due to increased rainfall can be traced. The best of 

 those lie on the west side of the Jefferson river, between Whitelmll and 

 Twin Bridges, Montana. Although the evidence is not as complete as 

 might be desired, it seems to be fairly conclusive as to an increased pre- 

 cipitation before and after the Glacial period, though the amount thereof 

 is not determinable. 



The relation of the glacial deposits to the materials which fill the 

 broad valleys clearly establishes the fact that the valleys were in sub. 

 stantiall}^ their present state before the advent of the ice-streams. The 

 changes which have taken place in post-Glacial time have been very 

 slight. Certainly not the hundredth part, and probably not the thou- 

 sandth part, of the detritus borne in from the hills 'has been laid down 

 since the close of that humid period in which the principal work of ex- 

 cavation was done. If, then, we allow a duration of 100,000 years to 

 have elapsed since the departure of the glaciers frbm the valleys and the 

 reestablishment of the arid conditions as they now exist — in my opin- 

 ion, far too short a time — and suppose that the process of infilling has 

 of late been of average rapidity, we are led to conjecture that some million 

 years have elapsed since the work began. As to the time when the change 

 from normal to abnormal stream-work occurred, we have as yet no trust- 

 worthy evidence, nor do we know that it took place coincidently in all 

 parts of the region. While by no means conclusive, the evidence in 

 general points to the conclusion that the excavation of these vast basins, 

 as well as the process of their refilling, has been substantially accom- 

 plished in the periods since the Cretaceous. 



It may be incidentally noted that the effect of the temporary humid 

 climate during the Glacial episode appears to be marked in the condi- 

 tions of the vegetation of this region. In certain regards the plants, 

 particularly the trees, exhibit a lack of adjustment to the state of the 

 soil such as they now have to meet. The natural way by which plants 

 adapt themselves to arid conditions is by sending tap roots downward 

 to the water level, as in the case of the " beach grass," Carex arenaria, 

 which often sends down such roots to the depth of from 10 to 20 feet 

 below the surface. Other instances may be found in the many tap-root 

 trees. It is a noticeable fact these accommodations to arid conditions 

 appear to be generally lacking in ,the Cordilleran vegetation. It may 

 well be supposed that during the last humid epoch, which may be pre- 

 sumably reckoned as equivalent to the latest Glacial period of the north- 

 ern hemisphere, this region lost the species which had previously been 

 adapted to dry conditions, and that since the aridity was reestablished 



