THE OLD EROSION SURFACE IN IDAHO 231 
represented by the Eocene beds, and (2) all the more important valleys of the 
area drain westward, and in all probability have done so throughout their 
entire history. This is true of the Rocky Mountain trough, the Purcell trough, 
and the Snake, Salmon, and Columbia river channels. These, together with 
their tributaries, represent perhaps oo per cent of the present dissection of the 
plateau region. If we assume that the old erosion surface is pre-Eocene the 
material derived from these several valleys may be thought to account for the 
narrow fringe of Eocene sediments on the west, but cannot account for the 
incomparably more extensive Eocene beds that lie to the east of the present 
plateau region. 
In conclusion, I cannot agree with Mr. Blackwelder that “‘the 
evidence seems to show that this peneplain is much younger than 
Eocene and probably post-middle-Miocene.” The evidence to me 
seems decisive that the plateau surface of the present day is much 
older than the late-Miocene. If it is other than Eocene, as con- 
cluded in my earlier paper, I believe that it must, because of 
evidence herein set forth, be crowded back toward or into the 
Cretaceous. Individual lines of evidence for assigning the old 
erosion surface of Idaho to the Eocene may not be conclusive but 
since all lines of approach, so far as I have been able to analyze 
them, point without discord’ in the same direction, it seems to me 
that the assignment must be correct, and it is firmly believed that 
the surface does form a “valuable datum plane in broad areas 
where time relations between the Algonkian and the Pleistocene 
are otherwise obscured.” 
«Except the assignment of the Payette formation to the Eocene, as discussed 
above. 
