THE OLD EROSION SURFACE IN IDAHO 
A REPLY 
JOSEPH B. UMPLEBY! 
In a recent issue of this Journal? Mr. Eliot Blackwelder criticizes 
adversely the chain of reasoning that led me to the conclusion that 
the old erosion surface in Idaho is of Eocene age. To him the 
evidence seems to point to a much younger age, “‘probably post- 
middle-Miocene’’—an inference which I believe to be incompatible 
with observed facts. 
To account for the valleys now filled in many places with lavas, 
lake beds, and fluviatile deposits principally of Miocene age, Mr. 
Blackwelder suggests three possible explanations. 
One method is the deposition of the sediments in the bottoms of the valleys, 
in essentially their present state, as suggested in the [original] 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 differences of 
resistance to denuding processes. A third hypothesis is that the broad valleys 
occupied by the sediments were excavated and filled before the peneplain 
was made. ; 
I shall endeavor to show that the first hypothesis only accords 
with field evidence. The second hypothesis is inadmissible because 
the old valleys constitute branching systems with members lying 
athwart the structure axes of the region. To derive all of these 
valleys by faulting would be to assume a complexity of fault 
systems in no wise borne out by field observations. Again, some 
of the lakes of Miocene age, as shown by fossil plants, present shore 
lines at elevations 2,800 feet beneath the plateau surface, so that 
even if down-faulting could be admitted the valleys in which the 
lakes existed must have been blocked out prior to the lacustrine 
epoch and hence prior to the Miocene. Therefore if the plateau 
1 Published with the permission of the Director of the United States Geological 
Survey. 
2 Jour. Geol., XX, No. 5 (1912), 410-14. 
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