THE OLD EROSION SURFACE IN IDAHO 4Il 
several ways in which such conditions may come about. One 
method is the deposition of the sediments in the bottoms of the 
valleys, in essentially their present state, as suggested in the article. 
Again where weak materials have been down-folded or down- 
faulted between masses of harder rocks, they may be eroded to a 
lowland on account of difference of resistance to denuding processes. 
Cases of this sort are well known in the Colorado park region and 
have been pointed out recently by Davis. A third hypothesis is 
that the broad valleys occupied by the sediments were excavated 
and filled before the old peneplain was made. It is clear that if 
through differential changes of level such filled valleys came to lie 
below grade level for the streams of the planation period, the sedi- 
ments could not be wholly removed, but would be as permanent as 
the most resistant rocks of their surroundings. The Cambrian 
sediments in the Baraboo Valley of Wisconsin illustrate the principle. 
If the author has considered these various possibilities, the paper 
presents no evidence to show that the first hypothesis has any 
advantage in this case over the second or the third. 
The second point made is that the Oligocene period should be 
allowed for the development of the broad valleys in which the 
Miocene sediments are supposed to have been deposited. It may 
be pointed out here that most of the sediments mentioned are 
believed to be late Miocene, according to Osborn’s? recent classi- 
fication, so that it may be permissible to add the earlier part of the 
Miocene period to the time allowed for the process. Furthermore, 
erosion proceeds at such different rates under different circum- 
stances, that it is quite impossible to estimate the amount of time 
necessary for the intrenchment of the Idaho plateau. It may be 
questioned whether the early Miocene epoch would not suffice, or 
why, on the other hand, it might not be necessary to add the 
Eocene to the Oligocene, to account for these valleys. Surely no 
trustworthy determination of the age of the peneplain can be 
attained by allowing a geologic period for a process of unknown 
time requirements. 
ie W. M. Davis, “Front Range in Colorado,” Annals of the Association of American 
Geographers, I, 1912, p. 43. 
2H. F. Osborn, ‘‘Genozoic Mammal Horizons of the West,’ U.S. Geol. Survey 
Bull. 361, 1909. 
