THE OLD EROSION SURFACE IN IDAHO 227 
erosion surface. This is the view which I advanced in my original 
paper and to which I still subscribe. 
The inequal filling of the old valleys, their branching systems 
with members athwart the structure axes of the region, their cross- 
sections, the absence of particular folding or faulting along their 
margins, the shore-line relations of the lakes which occupied some 
of them in Miocene times, the open stretches along the present 
drainage lines—all these features stand opposed to the second and 
third hypotheses but at the same time strongly support the first one. 
The constructive part of Mr. Blackwelder’s criticism is based on 
the third hypothesis; namely, that the old valleys, now in part 
filled with Miocene deposits, were formed before the period of 
profound degradation which resulted in the old erosion surface of 
Idaho. The fact that in many of the valleys the filling material 
never accumulated to within many hundreds of feet (2,800 feet at 
Salmon City and Gilmore) of the present plateau surface, is not 
easy to explain on the basis of this assumption. Widely distributed, 
deep depressions could not have persisted within the area during 
the period of its reduction to gentle topographic forms. 
Mr. Blackwelder also objects to my belief that the old valleys 
were formed during the Oligocene epoch. I am well aware of the 
uncertainties involved in “allowing a geologic period for a process 
of unknown time requirements,” but it does seem that in this case 
time requirements are as well known as is the duration of the 
geologic period involved. The coincidence of the Oligocene epoch 
with the development of the old valleys is not considered as an 
essential part of the constructive argument in my former paper, 
but is rather a corroborative bit of evidence. A considerable time 
must have been consumed in the development of these great 
valleys, especially so since they lie far within the plateau area. If 
the drainage was westward, as is suggested in a later paragraph, the 
streams must have worked headward, at least across the state of 
Idaho. This follows from the fact that the plateau is continuous to 
near the western border of the state. Limits are placed on the 
period of valley development on the one hand by the Miocene 
deposits which occupy them, and on the other by the old erosion 
