228 JOSEPH B. UMPLEBY 
surface, thought to be of Eocene age because of its relation to 
surrounding Eocene sediments. ‘The closing stages of the Eocene 
may have been involved in the development of the old valleys, 
although I do not think so, but that erosion continued locally within 
them on into the Miocene and somewhat intermittently, even down 
to the present, is attested by abundant evidence. Such facts and 
possibilities, however, do not appear to me to affect the conclusion 
that the old valleys, here filled and there partially filled, are parts 
of a drainage system which should be considered as Oligocene. 
The latter part of Mr. Blackwelder’s paper is devoted to an 
effort to show that the old erosion surface “‘is much younger than 
Eocene and probably post-middle-Miocene.”’ In the earlier part of 
this reply I have emphasized the fact that the deep erosion valleys 
were locally never filled by many hundreds of feet with Miocene or 
other lavas or sediments, and hence could not have been formed 
until after the old surface of gentle topographic forms had been | 
developed, and not until after it was elevated at least well toward 
its present position. The hypothesis that the valleys were 
developed prior to the planation of the region necessitates that they 
persisted throughout the period of that planation and at its close 
remained as open trenches, some of them 2,800 feet deep, in an 
area which approached closely the base-level of erosion for the 
region. 
Mr. Blackwelder’s principal evidence for his contention that 
the old surface is of late Miocene age is the general folding which 
the Miocene deposits of the region have undergone—a folding which 
he argues would have destroyed the evenness of the plateau surface. 
The folding of the lacustrine deposits has been brought to my 
attention by numerous exposures in Lemhi and Custer counties. 
About Salmon City the lake beds, which, from their thin-bedding, 
must have been laid down in a relatively horizontal position, are 
now inclined in various directions, in most places at angles of about 
to degrees, but locally they dip as steeply as 25 degrees. It is 
noteworthy, however, that the dips, in every instance where I have 
observed them, change direction within short distances. The 
disturbance which folded these lake beds and locally faulted them, 
also folded and faulted the plateau surface, hence my statement 
