AN OLD EROSION SURFACE IN IDAHO I4I 
lands extend to the north, and in speaking of them the same 
writer says: ‘Their combined crest-line would form an undu- 
lating plain differing little in elevation in the various parts of the 
Clearwater Mountains. From their westerly margins the moun- 
tains slope rapidly to the lava plateau, which has an elevation of 
A {CIES} WO) BultClO) IMCS a es Along Salmon River the high mountain 
plateau extends farther westward, and its last ramparts overlook 
the great bend of that river, rising 6,500 feet above its water line.”’ 
Mr. Lindgren’s conclusion is that ‘““We must regard this surface 
as the result of erosion. The country was worn down to a com- 
paratively gentle topographic feature, then uplifted and deeply 
dissected by canyons.” 
Mr. Calkins? describes the Coeur d’Alene Range, still farther 
north, as having a “‘general aspect similar to that of a maturely 
dissected plateau.”’ He describes also the Cabinet and Purcell 
ranges in western Montana, in similar terms. East of these, Mr. 
Willis’ recognized a peneplain over the Galton Range, and suggests 
that it may continue eastward over the Livingston and Lewis 
ranges. Northward in British Columbia the Interior Plateau is 
described by G. M. Dawson‘ as an elevated peneplain of Eocene 
age. South of this the writer’ recognized what was thought to 
be the same surface at Republic, Washington. 
The extent of the old erosioned surface can only be outlined in a 
general way because of the many localities where its identity has 
been destroyed and the broad areas which have not been studied 
physiographically. The above citations show, however, that a 
plateau surface cut across greatly disturbed beds extends over 
much of Idaho and into adjoining parts of Montana, Washington, 
and British Columbia. The areas in Idaho are continuous; those 
elsewhere are more isolated, but that all date from the same cycle 
of erosion will appear rather obvious during the later discussion. 
Elevation and preservation.—The combined crest lines of the 
plateau areas in Idaho would form an undulating plain which 
t Waldemar Lindgren, P. P. 27, U.S. Geol. Surv. (1904), 14. 
2 F.C. Calkins, and D. F. MacDonald, Bull. 384, U.S. Geol. Surv. (1909), 14, 19. 
3 Bailey Willis, Geol. Soc. Amer., XII (1901), 340. 
4G. M. Dawson, Trans. Royal Soc. Canada (1890), 13. 
5 J. B. Umpleby, Wash. Geol. Survey, Bull. I (1910), 11. 
