THE OLD EROSION SURFACE IN IDAHO: A CRITICISM 
ELIOT BLACKWELDER 
University of Wisconsin 
In another part of this volume,t Mr. Joseph B. Umpleby 
describes an old peneplain in the northwestern mountain states, and 
discusses the evidence bearing upon its age. ‘The original planation 
was so nearly completed that comparatively few monadnocks 
were left. The surface truncates folded sedimentary and meta- 
morphic rocks intruded by batholiths of granite, thought to be of 
Triassic or older age. Since it was made, the plain has been lifted 
into a plateau and then intrenched by systems of valleys, some of 
which are as much as 5,000 feet deep. In many places this process 
has completely destroyed the old peneplain, but in some parts of 
central Idaho it has left flat-topped remnants of considerable area. 
By putting together various observed facts, the author reaches the 
conclusion that the peneplain was made during the Eocene period, 
that it was then uplifted and the great valleys excavated during 
the Oligocene, and that in the bottoms of these depressions, lake 
beds were deposited in Miocene time. 
I do not question the identification of the flat-topped remnants 
and accordant summits as parts of an old elevated peneplain, and 
it is evident that, as the author says, the plain was developed after 
the deformation of the strata about the close of the Cretaceous 
period. It does seem to me, however, that the facts given by the 
author himself, and others which may be noted here, lead neces- 
sarily to quite different conclusions regarding the age of the plain. 
The first point in the author’s chain of argument to prove the 
Eocene age of the peneplain is that so-called ‘“‘lake-beds”’ of Miocene 
age were deposited in the valleys excavated in the old plateau after 
it was elevated. It is a fact that continental deposits of various 
ages are now found lining the bottoms of large depressions rather 
generally throughout the Rocky Mountain region, but there are 
t Journal of Geology, XX, No. 2 (1912), pp. 139747- 
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