AN OLD EROSION SURFACE IN IDAHO 143 
Evidence from nearby areas.—It is thought that adjoining regions 
afford further evidence as to the age of this surface. Extensive 
deposition is a corollary of profound erosion, hence we should 
expect to find a sedimentary record of the cycle of erosion repre- 
sented by the present plateau surface. The Rocky Mountain 
Fic. 2.—The figure illustrates the distribution of Eocene sediments in the 
Northwest. The horizontal lines indicate areas which have been described as of the 
plateau type. The area of vertical lines is similar but is based on oral communica- 
tions. Adapted from the Geologic Map of North America, with the plateau areas 
added. 
region is known to have supplied vast volumes of sediments during 
the Cretaceous, but not until the Eocene does the distribution of 
sediments bear a significant relation to the present plateau area. 
Fig. 2 illustrates the distribution of the Eocene sediments in the 
