AN OLD EROSION SURFACE IN IDAHO 147 
SUMMARY 
The principal problems discussed in this paper fall under four 
headings, as follows: 
t. An old erosion surface which may prove to be a peneplain, 
but which because of inadequate study is not so defined here, 
extends over much of Idaho and into adjoining parts of Montana, 
Washington, and British Columbia. 
2. The surface is assigned to the Eocene because of the relation 
of Miocene lake beds-to it and because of its relation to Eocene 
deposits. 
3. The Eocene surface forms a valuable datum plane in broad 
areas where time relations between the Algonkian and the Pleisto- 
cene are otherwise obscure. 
4. Itis suggested that the great granitic batholiths of the plateau 
region either initiated or accompanied the initiation of the cycle 
of erosion which resulted in the Eocene surface and hence, that 
they are of late Cretaceous or early Eocene age. 
