142 JOSEPH B. UMPLEBY 
reaches its maximum elevation of about 10,000 feet along a course 
through Gilmore and Challis. Northwestward it grades off to 
8,500 feet in the north part of Lemhi County, and thence to about 
7,000 feet in the Clearwater Mountains. This elevation is also 
common in northwest Montana, but westward in the Colville 
Mountains and on north in the Interior Plateau the summits are 
about 5,000 feet above sea. 
Faulting and folding have affected the plateau area of central 
and eastern Idaho since its last elevation, but through all, the 
integrity of the old surface has persisted in a remarkable degree. 
Local prominences above the general level, though not character- 
istic of the region, occur. Some of these are undoubtedly erosion 
remnants, but others probably involve faulting, and some may be 
due to folding. In western Montana the old surface appears from 
the literature to be far less perfectly preserved. In northeastern 
Washington and in British Columbia it is also preserved imper- 
fectly. Here there is a remarkable accordance of summit levels, 
but no large plateau remnants, such as those in Idaho, have been 
described. 
AGE OF THE SURFACE 
Evidence from the area.—After the last general elevation of the 
region great valleys were developed, and in these extensive lake 
beds accumulated during the Miocene period. Such deposits 
occur at Salmon, Idaho,t in western Montana,? at Republic, 
Washington,’ and at various places in the Interior Plateau of 
British Columbia.‘ 
Allowing the Oligocene for the development of the broad valleys 
occupied by the Miocene lake beds, the old erosion surface is 
pre-Oligocene. On the other hand, it cuts all the older formations 
of the region including the granite, which is post-Triassic. Thus 
from evidence within the plateau region the old erosion surface 
is pre-Oligocene and post-Triassic. 
«J. B. Umpleby, Bull. U.S. Geol. Surv. (now in manuscript). 
2 Earl Douglass, Mont. Univ. Missoula, Mont., 27 pp., 4 plates, 1899; Carnegie 
Mus. Annals, V (1909), 159-65. 
3 J. B. Umpleby, Bull. I, Wash. Geol. Surv. (1910), It. 
4G. M. Daavson, Trans. Royal Soc. Canada (1890), 14. 
