AN OLD EROSION SURFACE IN IDAHO: ITS AGE AND 
VALUE AS A DATUM PLANE’ 
JOSEPH B. UMPLEBY 
THE EROSION SURFACE 
Evidences of a Former Erosion Cycle 
Correlation and Extent 
Elevation and Preservation 
AGE OF THE SURFACE 
Evidence from Within the Area 
Evidence from Nearby Areas 
THE EOCENE SURFACE AS A DATUM PLANE 
AGE OF GRANITIC INTRUSIONS SUGGESTED BY THE EOCENE SURFACE 
SUMMARY 
THE EROSION SURFACE 
Evidences of a former erosion cycle—A plateau surface has long 
been recognized in Idaho, and over much of the state it has been 
described as a feature of erosion. The observations of the writer 
have been confined to an area of about 5,000 square miles in the 
eastern part of that great highland mass known as the Salmon 
River Mountains, but the literature shows that a description of 
this area is applicable to much of the state. 
The Salmon River Mountains are characterized by deep canyons 
separated by even-crested divides, which here and there widen out 
into broad flats. Within the region there is not only a striking 
accordance of summit levels; there is a general continuity of level 
summit areas. Plateau remnants several square miles in extent 
are not uncommon. One of these is Poverty Flat near Challis in 
Custer County. It is a comparatively level tract of about 25 
square miles, and occurs at an elevation 9,600 feet above sea (Fig. 1). 
Bordering it are narrow valleys as much as 5,000 feet deep, but 
beyond them high, even-crested ridges and occasionally flat areas 
continue to the horizon in every direction. The rocks forming 
this flat are steeply tilted schists, slates,‘and quartzites, the latter 
1 Published by permission of the Director of the U.S. Geological Survey. 
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